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5 ways to Add a Property to object in JavaScript

To demonstrate adding new properties to JavaScript object, we will consider the following example of an employee object:

There are multiple instances to consider, which are as follows:

  • Static property names : Addition of “id” property to an employee object.
  • Dynamic property names : Include property based on the value of “customProp” variable.
  • Adding properties from another object : Add location property from a person object to an employee object.

1. “object.property_name” syntax

The dot notation is the simplest way to access/modify the properties of a JavaScript object. A new property can be initialized by the following syntax:

In the below example, we are creating an “id” property with the value 130 for the employee object.

Further, adding properties for nested objects follows a similar syntax:

Below, the “country” property is added to a nested object which is the value of the “location” parent property in the employee object.

This approach is most ideal for the following instances:

  • The property name is a static value and needs to be initialized manually.
  • Property names don’t include special characters like extra space, etc.

2. Access property though “object[‘property_name’]”

The syntax based on the square brackets is an alternative approach with similar capabilities and avoids most of the disadvantages of the dot operator. The syntax for adding new property using square brackets is as follows:

Square bracket syntax offers the following advantages over traditional dot operator:

  • When the property name is based on a dynamic variable . For example, the property name is retrieved from API calls, user inputs, etc.
  • The property name string value contains special characters like extra space, etc.

In the below example, the “Last name” property cannot be directly added using dot operator syntax due to the presence of space characters . Hence, we are able to accomplish the creation of “Last name” property using square bracket syntax.

The nested objects are accessed using a series of multiple square bracket syntax or alternatively, a combination of dot operator and square brackets can also be used.

3. Create new property using Object.defineProperty()

JavaScript Object class also provides native “ defineProperty() ” method to define new properties for an object. In addition to specifying the value of the new property, “ defineProperty() ” also allow configuring the behavior for the property .

The generic syntax for “defineProperty()” is as follows:

In the below example, we define “id” property for the employee object with a value of 130 and writable as false . Hence, subsequent changes in the value of the id property are discarded. Read more about Object.defineProperty from developer.mozilla.org/Object/defineProperty .

4. Spread operator syntax “{ …object, property_name: property_value } “

The spread operator allows the creation of a soft copy of an object with existing properties. The Spread operator followed by an inline property definition allows the addition of new properties. Further, the properties of another object can be added by using the spread operator multiple times.

In the below example, we create a copy of the employee object combined with location and id properties. Next line, “id” object is added to employee object using the spread operator .

5. Assign properties using Object.assign()

The “Object.assign()” method allows properties of the source object to be added to the target object . By defining all the new properties which need to be added to the source object, we achieve the addition of properties to a target object.

In the below example, we add the “id” property with value 670 to the employee object using Object.assign.

Finally, to conclude the summary of the methods to add properties to objects is as follows:

  • Dot method syntax is ideal for static property values.
  • Square bracket syntax works best for dynamic values from external API, user input, etc.
  • Object.defineProperty() is used when the property’s writability, getter, setters, etc needs to be configured.
  • In object/array formatter functions and when properties from another object are included, the spread operator and Object.assign() are more ideal.

Related Articles

How to Remove Property from a JavaScript object

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Properties are the most important part of any JavaScript object.

JavaScript Properties

Properties are the values associated with a JavaScript object.

A JavaScript object is a collection of unordered properties.

Properties can usually be changed, added, and deleted, but some are read only.

Accessing JavaScript Properties

The syntax for accessing the property of an object is:

The expression must evaluate to a property name.

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JavaScript for...in Loop

The JavaScript for...in statement loops through the properties of an object.

The block of code inside of the for...in loop will be executed once for each property.

Looping through the properties of an object:

Adding New Properties

You can add new properties to an existing object by simply giving it a value.

Assume that the person object already exists - you can then give it new properties:

Deleting Properties

The delete keyword deletes a property from an object:

or delete person["age"];

The delete keyword deletes both the value of the property and the property itself.

After deletion, the property cannot be used before it is added back again.

The delete operator is designed to be used on object properties. It has no effect on variables or functions.

Nested Objects

Values in an object can be another object:

You can access nested objects using the dot notation or the bracket notation:

Nested Arrays and Objects

Values in objects can be arrays, and values in arrays can be objects:

To access arrays inside arrays, use a for-in loop for each array:

Property Attributes

All properties have a name. In addition they also have a value.

The value is one of the property's attributes.

Other attributes are: enumerable, configurable, and writable.

These attributes define how the property can be accessed (is it readable?, is it writable?)

In JavaScript, all attributes can be read, but only the value attribute can be changed (and only if the property is writable).

( ECMAScript 5 has methods for both getting and setting all property attributes)

Prototype Properties

JavaScript objects inherit the properties of their prototype.

The delete keyword does not delete inherited properties, but if you delete a prototype property, it will affect all objects inherited from the prototype.

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In this article, we will try to understand how to add properties to an object as well as how to add or remove properties from an object in JavaScript .

Before we actually go and see the addition and removal of properties from an object let us first understand the basics of an object in JavaScript.

  • An object in JavaScript is a collection of properties.
  • A single property in a JavaScript object is actually the association between the name (or key ) and a value.
  • An object can contain a different number of properties which is further having different names as well as values.

Syntax: By using the following syntax one can easily create an object with a different number of properties.

Now that we have understood the basic details associated with the object in JavaScript, let us see some examples to add properties to an object as well as how to remove properties from an object.

Adding/Removing Properties from an Object:

add property from object javascript

  • As depicted in the above pictorial representation, we can easily add or remove several properties from an object in JavaScript by following certain methods or techniques.
  • For adding any property, one could either use object_name.property_name = value (or) object_name[“property_name”] = value .
  • For deleting any property, one could easily use delete object_name.property_name ( or)   delete object_name[“property_name”] .

here are several methods that can be used to add and remove properties from objects.

  • Use dot notation or bracket notation
  • Using the Object.assign() method
  • using Object.defineProperty() and Object.defineProperties()
  • Using the spread operator (ES6)

Approach 1: Use dot notation or bracket notation

In this approach to add properties to an object, you can use either dot notation or bracket notation. and To remove properties from an object, you can use the delete keyword with either dot notation or bracket notation.

Example 1: In this example, we have an object person with initial properties: name, age, and occupation. We then use both dot notation and bracket notation to add properties (city, email, hobby, and gender) to the object, Next, we use both dot notation and bracket notation to remove properties (age, occupation, hobby, and gender) from the object

Approach2 : Using the Object.assign() method

The approach involves using Object.assign() to merge properties from one object into another, facilitating property addition. To remove properties, a new object is created using the spread operator.

Example 2: In this example, we use Object.assign() to add properties from one object to another, and then remove specific properties to create a new object without those properties.

Approach 3: Using Object.defineProperty() and Object.defineProperties()

Using Object.defineProperty() and Object.defineProperties() provides fine-grained control over property addition and removal, and it allows us to create a new object with desired properties selectively.

Example 3: In this example, we use Object.defineProperty() to add a property with attributes, and Object.defineProperties() to remove specific properties, creating a new object.

Approach 4: Using the spread operator (ES6)

In this approach we use the spread operator to add properties from one object to another, facilitating property addition. To remove properties, a new object is created selectively excluding specified properties.

Example: In this example, we add properties from additionalInfo to person, we use the spread operator (…). The spread operator allows us to merge the properties from additionalInfo into person and we use the spread operator to create a new object newPerson without the age property from the original person object. The … spread operator creates a shallow copy of the properties.

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Add Property to an Object in Javascript (6 ways)

This article demonstrates easy ways to add a property in the javascript object using different methods and various example illustrations. We will also see how to add new properties dynamically to javascript objects after their creation.

Table of Contents:

  • Add Property to a Javascript Object Dynamically using Object. defineProperty()
  • Add Property to a Javascript Object
  • Add Property to a Javascript Object using eval()
  • Add Property to a Javascript Object using Spread Operator
  • Add Property to a Javascript Object using Comma Operator
  • Add Property to a Javascript Object using map()

Add Property to Javascript Object Dynamically using Object.defineProperty()

Javascript’s Object.defineProperty() method is static and adds a new property to an object directly or modifies an existing property. This method returns the modified Object.

Add the property { city: ‘Santiago’ } to an object { personFirstName: ‘George’, personLastName: ‘Smith’, dateOfBirth: “Nov 14 1984”}

Frequently Asked:

  • Javascript: Copy an array items to another
  • Javascript: loop through an array of objects
  • JavaScript: Check if String Starts with LowerCase
  • Javascript Check If an Object Is Null or Undefined

Explanation:-

  • Here, in the above code we are creating a function that will be able to add a property to any object.
  • _object -> name of the object for which we need to add a new property
  • _propertyName -> is the name of the property
  • _propertyValue -> value to be assigned to the property.
  • Within the function the code makes this new property writable, enumerable and configurable and hence can be added dynamically after the object creation.

Add Property to Javascript Object

Add property to javascript object using eval().

Javascript’s eval(string) function is used to evaluate the javascript code represented as a string. It evaluates the expression passed in the function as a parameter.

Note that eval() has some security concerns, and therefore, if you are planning to use it for the values supplied by the user, then, in that case, avoid using this function.

Add Property to Javascript Object using Spread Operator

Javascript’s Spread syntax(…) allows an iterable to be expanded in places where zero or more arguments or elements are expected.

Add Property to Javascript Object using Comma Operator

Add property to javascript object using map().

Javascript’s map() method is used to create a new array with the elements resulting from applying the provided function on the calling array elements.

I hope this method helped you to add a new property to an existing javascript object. Good Luck!!!

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How to add a property to an object in JavaScript

To add a new property to a JavaScript object:

  • You either use the dot ( . ) notation or the square bracket ( [] ).
  • In dot donation, you use the object name followed by the dot, the name of the new property, an equal sign, and the value for the new property.
  • In square bracket notation, you use the property name as a key in square bracket followed by an equal sign and the value of the new property.

A JavaScript object is a collection of key-value pairs called properties. Unlike arrays , objects don't provide an index to access the properties.

You can either use the dot ( . ) notation or the square bracket ( [] ) notation to access property values.

The simplest and most popular way is to use the dot notation to add a new key-value pair to an object:

Alternatively, you could also use the square bracket notation to add a new item:

As you can see above, when you add a new item to an object, it usually gets added at the end of the object.

To learn more about JavaScript objects, prototypes, and classes, read this article .

✌️ Like this article? Follow me on Twitter and LinkedIn . You can also subscribe to RSS Feed .

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JavaScript: 5 ways to add new properties to an existing object

This succinct, straightforward article will walk you through 5 different ways to add new properties to an existing object in JavaScript. We’ll write code with the modern feature of the language, such as arrow functions, spread operator syntax, etc. Without any further ado, let’s get started.

Dot Notation

Using dot notation is the simplest way to add new properties to an object. You just write the object name, followed by a dot (.), followed by the property name.

Bracket Notation

Just write the object name, followed by a pair of square brackets [] , and inside the brackets, you write the property name as a string. This is useful when the property name is not a valid identifier or when it is stored in a variable.

Object.defineProperty()

This is a static method that allows you to add a property to an object with more options, such as making it read-only, enumerable, or configurable. You pass 3 arguments to this method: the object, the property name, and an object with some descriptors for the property.

If you try to modify the new property hobby , you will end up with an error:

Spread Operator

This is an operator that allows you to create a copy of an object and add or modify some properties at the same time. You write three dots … followed by the object name, and then you write any additional properties inside curly braces {} .

Object.assign()

This is another static method that allows you to copy the properties of one or more objects into another object. You pass at least 2 arguments to this method: the target object and one or more source objects. The properties of the source objects will be copied to the target object.

This tutorial ends here. Happy coding & have a nice day!

Next Article: JavaScript: 3 Ways to Check if an Object is Empty

Previous Article: The Modern JavaScript Objects Cheat Sheet

Series: Working with Objects in JavaScript

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Add a property to an object in JavaScript

Add a property to an object in JavaScript

in this post, We will discuss how to add a property to an object in JavaScript. The objects are key-value pairs. We’ll cover different ways to add a property into the object.

How to Add a Key/Value pair to an Object in JavaScript

There are many ways to add key-value pairs to an Object in JavaScript.

We can add a key/value pair to an Object in JavaScript by the following ways:

  • Using dot (.) notation
  • Use the bracket [ ] notation
  • Using the Object.assign() method

Using Spread operator

  • Using third-party library

Using dot notation

A simple approach is to use the dot notation with an assignment operator to add a property to an existing object.

The syntax is:

Simple Example of Dot:

Using bracket notation.

We can add the objects and keys using the bracket notations.

When a property’s name is unknown in advance or if it contains an invalid variable identifier, the dot notation will not function.

Simple Example of Bracket []:

Using object.assign() method.

The Object.assign() method can copy properties from a source object to a target object. Properties in the sources overwrite the target object’s properties if the same key is found in both objects.

The assign() method allows the user to add multiple key-value pairs within the same code line conveniently.

The Spread operator is also used to merge the properties of two objects into a new object. This approach does not overwrite the source object with the properties of the given object for common keys.

Using Underscore/Lodash Library

There are Underscore and Lodash libraries that offer several utility methods to add properties to an existing object.

You can use any of the .merge, .assignIn (alias .extend), .assign, or _.defaults method.

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Conditionally Add Property to Object in JavaScript

add property from object javascript

Introduction

In the world of JavaScript, objects are king. They allow us to store, manipulate, and access data in a structured and efficient manner. But what if we want to add a property to an object based on certain conditions? This might seem like a simple task, and luckily, it is! But how can we do it in a more compact or modern way?

In this Byte, we'll explore how to conditionally add a property to an object in JavaScript using various methods, starting with the humble if statement and moving on to the more modern spread syntax.

Conditionally Add a Property to an Object

As you'll see in the following sections, JavaScript provides multiple ways to conditionally add properties to an object, each with its own strengths.

Using an if Statement

The simplest way to conditionally add a property to an object is by using an if statement. This is a simple approach that even beginners in JavaScript should be familiar with.

Let's say we have an object representing a book, and we want to add a publisher property, but only if the book is not self-published.

The object will now have the new property:

As you can see, the publisher property was added to the book object since the book was not self-published.

Using the Spread Syntax

The spread syntax, introduced in ES6, provides a more concise way to conditionally add properties to an object. It's a bit more advanced, but once you get the hang of it, you'll find it incredibly useful.

Here's how we can accomplish the same task as above using the spread syntax:

This will output the same result as before:

The spread syntax ( ... ) is used to unpack elements from an object or array into a new object or array. In this case, we're using it to create a new object that contains all the properties of the original book object, plus the publisher property if the book is not self-published.

Using Object.assign()

The Object.assign() method is another way we can conditionally add a property to an object in JavaScript. This method works by copying all properties from one or more source objects to a target object. It then returns the target object.

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We use the ternary operator to conditionally assign either the new property or null . If null is sent to .assign() , then no property is added.

When you run this code, you should get:

Again, the publisher property is now added.

Use Cases for Conditional Property Addition

You'll likely run into situations where you might want to conditionally add properties to an object in JavaScript, especially if you want to reduce lines of code and make it more compact.

For instance, you might be creating an object based on user input. Some fields might be optional, so you only want to add them to the object if the user has provided a value.

Another use case could be when working with APIs. Sometimes, you might want to add certain properties to your request object based on specific conditions, like filters. For example, if you're making a request to a search API, you might want to add a sort property, but only if the user has chosen to sort the results.

Here's how you might use conditional property addition in the context of user input:

In this code, the age property will only be added to the user object if the age variable has a truthy value.

In this Byte, we've explored how to conditionally add properties to an object in JavaScript using various methods, like the spread operator or Object.assign() method, and we've also discussed some use cases for this technique.

Conditional property addition can help you create more flexible and dynamic code, whether you're dealing with user input or working with APIs.

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add property from object javascript

Javascript add property to object

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Let us see three ways in javascript to add a property to an object. We will also see how we can use some of the methods to dynamically create the property key as well.

These methods are the dot notation , square bracket notation , and using the spread operator .

You can also update already created properties with these methods instead of adding them.

Method 1: Dot notation

One of the simplest method to add properties to object is the dot notation. The only downside for using this method is that you already need to know the property key before adding it, this way doesn't allow dynamic addition of the property key.

Method 2: Square bracket notation

This method allows you to create new properties normally or with the additional advantage that the property key can also be passed on dynamically from a variable besides the value.

Method 3: Spread operator

Spread operator allows us to destructure objects and utilizing that we can create a new object with the additional property we want to create. The advantage of this method is that it creates a shallow copy of the original object for us and also allows dynamic keys for the object.

These are some of the ways to add properties to objects, they all have different advantages ranging from simplicity to dynamic key values and creating shallow object copies.

Hopefully, you find different usage for all these methods in your coding journey 🚀.

How to Add Property to an object in JavaScript?

JavaScript Course With Certification: Unlocking the Power of JavaScript

Object in JavaScript is a collection of properties and these properties are basically key-value pairs. Properties define the characteristics of an object in JavaScript and these can be changed, deleted, and even added dynamically even after the object has been created. Some of the ways to add property to object JavaScript include: using dot notation, using bracket [ ] notation, using defineProperty() method, using spread operator, and using Object.assign() method.

Introduction

To start with, let us revise and go through the concept of objects and their properties in brief. Object in JavaScript is a collection of unordered properties and these properties are basically key-value pairs that define the characteristics of an object. Just like JavaScript variables, object names & their properties are case-sensitive. Let us take an example of an object in JavaScript where we will define some of the properties inside it.

Here, we have created an object named car with brand and year as its properties with assigned values. Now, if we have to access these properties of car object, we can simply use dot notation or square-bracket notation. Note that if we try to access properties of an object which are unassigned, then it will return undefined and (not null).

Have you ever wondered that if it is possible to add more properties to an existing object that has already been created? The answer to this question is yes, we can do so and can add properties to an object in JavaScript dynamically even if the object has already been created.

There are many methods and ways by which we can add properties to an existing object in JavaScript and we will discuss all of them in detail as we move through the course of this article.

  • Using dot notation (.)
  • Using square brackets notation [ ]
  • Using Object.defineProperty() method
  • Using Object.assign() method
  • Using spread operator syntax

Using Dot Notation (.)

One of the simplest ways to add, access, or modify properties of an existing object in JavaScript is by using dot notation.

Here, object denotes existing object name, and to add property to object JavaScript, we are using the dot operator. new_property is basically the key and new_value is the value assigned to this new property that is to be added inside the object.

Note that this dot notation will not work for the cases where the property key name includes special characters like spaces, dashes, digits, etc.

Example to Demonstrate the use of Dot Notation in JavaScript

In the below JavaScript example code, we have created an object named obj with some initial properties defined inside it.

Now to demonstrate the use of dot notation, we are adding two new properties with assigned values inside obj: obj.roll_no = 121 and obj.secondName = 'Jain' with the help of the dot operator.

In the end, we are printing the object obj to check newly added properties (key-value pairs) inside it.

Note that with the help of the dot operator, we can also change the existing properties of an object. Example: obj.name = 'Raj';

As we can observe, new properties have been added dynamically using dot notation.

Note that if we try to use any special character or numeric digit in the property name while adding using dot notation, it will generate a syntax error.

Using Square Brackets Notation [ ]

As we have discussed above, there are some disadvantages while working with dot notation in cases where the property name is invalid variable identifier (say all digits, having spaces, special characters).

To handle such cases, we can imagine object as an associative array and can use square bracket notation to add new properties inside an object.

Also in case of dynamic variables where property names are retrieved from user inputs or API calls, this square bracket notation is helpful over dot notation in JavaScript.

Here, object denotes existing object name, and to add property to object JavaScript, we are using square brackets notation. new_property is basically the key and new_value is the value assigned to this new property that is to be added inside the object.

Example to Demonstrate the Use of Square Brackets Notation in JavaScript

In the below JavaScript example code, we have initially created an object named obj with some defined properties. Clearly, as we already know now that using square brackets notation also, we can access, add and even modify the existing properties.

To start with, we modified existing property of object obj i.e, obj['state'] = 'Mumbai'. We then created an empty nested object (obj.city = {}) within obj. We can also access nested objects using square bracket notation just like accessing a 2D array.

We have also showcased the combinational use of the dot operator and square brackets to access nested objects.

Output: As we can observe that the object obj has been displayed here with modified 'state' property and a newly added nested object 'city' with name and street properties.

Taking another example to understand the advantages of square brackets notation, here we have an empty JavaScript object obj and to add properties inside it, we are running for-loop.

The property name is an expression that has to be evaluated on each loop iteration depending on the value of 'i' (when i==1, obj['num1'] = 1;), clearly helpful in case of dynamic property names.

Using Object.defineProperty() Method

JavaScript object class provides the defineProperty() method and using this, we can modify an existing object or add property to object JavaScript.

This method returns the modified object and also allows us to control or configure the behavior of the properties.

Here, the configuration consists of majorly two properties which we can manually set: enumerable and writable.

If writable is set to false, it means that we can't modify or set a new value for this particular property of object whereas enumerable denotes that the value has to be retrieved dynamically using either for-loop or user-input, etc. Let us understand this theory practically seeing example codes.

Example to Demonstrate Use of defineProperty() Method in JavaScript

In the below JavaScript example code, we have initially taken an empty object named obj. To add properties inside it, we have used Object.defineProperty() method to add 'id' property to obj and set its configuration property - writable as 'false'.

Next, we tried modifying the 'id' property of obj but due to the writable configuration which was set to false, there is no effect on the value of object 'id'.

Using Object.assign() Method

There is another interesting way by which we can add property to object JavaScript i.e, by using the Object.assign() method. This method allows us to add properties of one source object to another target object. We can just define all those properties inside the source object that we need to add to an existing target object.

Note that the properties inside the target object will be overwritten by the source object’s properties if the same key name is found in both the objects.

Here, the Object.assign() method of JavaScript has been used and source & target denotes that the properties of the source object will now be added or assigned to the target object as well.

Example to demonstrate the use of Object.assign() Method in JavaScript

In the below JavaScript example code, we have two objects student and info with already defined properties inside them. Now our task is to add properties of the info object to the student object and for this, we can use Object.assign() method by passing student and info as parameters respectively.

Using Spread Operator (...)

There is another unique way to add property to object JavaScript by using the spread operator. Spread operator (...) creates a copy of the existing object with all its properties and therefore, using inline property definition, we can further add more properties to that object. Let us look at the syntax to understand this in a better manner!

Here, ...object virtually creates a copy of existing properties of object, and using inline definition, we can add more properties (property1, property2....) to the existing object.

Also, the spread operator allows us to merge the properties of two objects into a new object.

Note that this approach of using the spread operator to merge two objects to form a new one, does not overwrite existing properties of the object if common keys are found in both source and target objects.

Example to Demonstrate the use of Spread Operator Method in JavaScript

In the below JavaScript example code, we have an object named obj with already defined properties inside it. We will now check the functionality of the spread operator using which we can add more properties inside obj.

We will use the spread operator on object obj and using inline definition, we will add some more properties ('nationality', 'gender') inside obj. To check the resultant object, we have displayed the object values in the end.

  • Properties define the characteristics of an object in JavaScript and these can be changed, deleted, and even added dynamically after the object is created.
  • Just like JavaScript variables, object property-names are also case sensitive.
  • using dot notation**
  • using bracket [ ] notation**
  • using defineProperty() method**
  • using spread operator**
  • using Object.assign() method**
  • Dot notation method works ideally for static property name values without any invalid identifier.
  • Square bracket notation works best when values are to be retrieved by user input, for-loop, or by API calls dynamically to set property names.
  • Object.defineProperty() method is useful when we need to control or configure the behavior of properties of an existing object in JavaScript.
  • Spread operator syntax method is useful when we want to assign or add values from the target object to the source object by making a copy of the actual given object.
  • We can also pass the target object and the source object as parameters to the Object.assign() method to add or assign the values of one object to another object.
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Add a property to an object in JavaScript

This post will discuss how to add a property to an object in JavaScript.

1. Using dot notation

A simple approach is to use the dot notation with an assignment operator to add a property to an existing object. The syntax is: object.property = value .

Download    Run Code

2. Using bracket notation

The dot notation won’t work when the property’s name is not known in advance or the name is an invalid variable identifier (say all digits).

To handle these cases, one can think of an object as an associative array and use the bracket notation. The syntax is: object['property'] = value .

  Note that both the dot notation and bracket notation will overwrite the object’s property with the given property when the specified key is already found in the object. To illustrate, consider the following code, which overwrites the age property of the person object.

3. Using Object.assign() function

The Object.assign() method can copy properties from a source object to a target object. Properties in the sources overwrite the target object’s properties if the same key is found in both objects.

4. Using Spread operator

Alternatively, you can use the Spread operator to merge the properties of two objects into a new object. This approach does not overwrite the source object with the properties of the given object for common keys.

5. Using jQuery

With jQuery, you can use the jQuery.extend() method, which merges the contents of the second object into the first object. If objects have keys in common, this approach overwrites the source object with the given object’s properties.

Download Code

6. Using Underscore/Lodash Library

Both Underscore and Lodash libraries offer several utility methods to add properties to an existing object.

With the Lodash library, you can use any of the _.merge , _.assignIn (alias _.extend ), _.assign , or _.defaults method. Alternatively, if you prefer the Underscore library, you can use the _.extendOwn (Alias: _.assign ) or _.defaults method.

All the above-mentioned methods overwrite the source object with the given object’s properties if they have any key in common, except the _.defaults method, which silently ignores the common key.

That’s all about adding a property to an object in JavaScript.

Remove a property from an object in JavaScript
Verify if a JavaScript object has a certain property
Initialize an object in JavaScript

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There’re a few ways to add properties to an object in JavaScript. One way is to add a property using the dot notation:

We added the foo property to the obj object above with value 1.

We can also add a property by using the bracket notation:

It does the same thing as the first example, but we can have invalid property identifiers in the string.

So, we can write something like:

Remember: 'foo-bar' isn’t a valid identifier, but we can add it as a property.

Object.defineProperty

We can also use the Object.defineProperty function:

We can have more control over how the property will act with this method. In addition to setting the value with the value property, we can also set it to be writable with the writable property, and enumerable with the enumerable property.

Enumerable means that it’ll be retrieved or looped through with Object.keys or the for-in loop.

Writable determines if we can set a new value for the property.

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How to Add Property to an Object in JavaScript [5 Ways]

Here are five ways to add a property to a JavaScript Object:

  • Using “dot(.) notation”
  • Using “square brackets”
  • Using “Object.defineProperty()” function
  • Using “Object.assign()” function
  • Using  “Spread operator”

Method 1: Using dot(.) notation

Visual representation.

Method 1 - Using the dot(.) notation

Method 2: Using Square Bracket Notation

Method 2 - Using Square Bracket Notation

Square bracket syntax is required if a property name contains spaces or other special characters or includes the keyword reserved. Otherwise, errors will be your result.

Method 3: Using Object.defineProperty()

Object.defineProperty() method can be “used to add a new property to an object” .

Method 4: Using Object.assign()

Object.assign() method is used to copy values from one or more source objects to a target object. It can also be used to add a new property to an object. It returns the target object.

Method 4 - Using the Object.assign() function

Method 5: Using Spread Operator (…)

The spread operator (…) allows an iterable such as an array or string, to be expanded where zero or more arguments or elements are expected. When used with an object, it copies all properties of an object into a new object.

Method 5 - Using Spread Operator (...)

Krunal Lathiya is a seasoned Computer Science expert with over eight years in the tech industry. He boasts deep knowledge in Data Science and Machine Learning. Versed in Python, JavaScript, PHP, R, and Golang. Skilled in frameworks like Angular and React and platforms such as Node.js. His expertise spans both front-end and back-end development. His proficiency in the Python language stands as a testament to his versatility and commitment to the craft.

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That doesn’t work for me, in typescript. I get this error TS2339: Property ‘value’ does not exist on type …

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The Object type represents one of JavaScript's data types . It is used to store various keyed collections and more complex entities. Objects can be created using the Object() constructor or the object initializer / literal syntax .

Description

Nearly all objects in JavaScript are instances of Object ; a typical object inherits properties (including methods) from Object.prototype , although these properties may be shadowed (a.k.a. overridden). The only objects that don't inherit from Object.prototype are those with null prototype , or descended from other null prototype objects.

Changes to the Object.prototype object are seen by all objects through prototype chaining, unless the properties and methods subject to those changes are overridden further along the prototype chain. This provides a very powerful although potentially dangerous mechanism to override or extend object behavior. To make it more secure, Object.prototype is the only object in the core JavaScript language that has immutable prototype — the prototype of Object.prototype is always null and not changeable.

Object prototype properties

You should avoid calling any Object.prototype method directly from the instance, especially those that are not intended to be polymorphic (i.e. only its initial behavior makes sense and no descending object could override it in a meaningful way). All objects descending from Object.prototype may define a custom own property that has the same name, but with entirely different semantics from what you expect. Furthermore, these properties are not inherited by null -prototype objects . All modern JavaScript utilities for working with objects are static . More specifically:

  • valueOf() , toString() , and toLocaleString() exist to be polymorphic and you should expect the object to define its own implementation with sensible behaviors, so you can call them as instance methods. However, valueOf() and toString() are usually implicitly called through type conversion and you don't need to call them yourself in your code.
  • __defineGetter__() , __defineSetter__() , __lookupGetter__() , and __lookupSetter__() are deprecated and should not be used. Use the static alternatives Object.defineProperty() and Object.getOwnPropertyDescriptor() instead.
  • The __proto__ property is deprecated and should not be used. The Object.getPrototypeOf() and Object.setPrototypeOf() alternatives are static methods.
  • The propertyIsEnumerable() and hasOwnProperty() methods can be replaced with the Object.getOwnPropertyDescriptor() and Object.hasOwn() static methods, respectively.
  • The isPrototypeOf() method can usually be replaced with instanceof , if you are checking the prototype property of a constructor.

In case where a semantically equivalent static method doesn't exist, or if you really want to use the Object.prototype method, you should directly call() the Object.prototype method on your target object instead, to prevent the object from having an overriding property that produces unexpected results.

Deleting a property from an object

There isn't any method in an Object itself to delete its own properties (such as Map.prototype.delete() ). To do so, one must use the delete operator.

null-prototype objects

Almost all objects in JavaScript ultimately inherit from Object.prototype (see inheritance and the prototype chain ). However, you may create null -prototype objects using Object.create(null) or the object initializer syntax with __proto__: null (note: the __proto__ key in object literals is different from the deprecated Object.prototype.__proto__ property). You can also change the prototype of an existing object to null by calling Object.setPrototypeOf(obj, null) .

An object with a null prototype can behave in unexpected ways, because it doesn't inherit any object methods from Object.prototype . This is especially true when debugging, since common object-property converting/detecting utility functions may generate errors, or lose information (especially if using silent error-traps that ignore errors).

For example, the lack of Object.prototype.toString() often makes debugging intractable:

Other methods will fail as well.

We can add the toString method back to the null-prototype object by assigning it one:

Unlike normal objects, in which toString() is on the object's prototype, the toString() method here is an own property of nullProtoObj . This is because nullProtoObj has no ( null ) prototype.

You can also revert a null-prototype object back to an ordinary object using Object.setPrototypeOf(nullProtoObj, Object.prototype) .

In practice, objects with null prototype are usually used as a cheap substitute for maps . The presence of Object.prototype properties will cause some bugs:

Using a null-prototype object removes this hazard without introducing too much complexity to the hasPerson and getAge functions:

In such case, the addition of any method should be done cautiously, as they can be confused with the other key-value pairs stored as data.

Making your object not inherit from Object.prototype also prevents prototype pollution attacks. If a malicious script adds a property to Object.prototype , it will be accessible on every object in your program, except objects that have null prototype.

JavaScript also has built-in APIs that produce null -prototype objects, especially those that use objects as ad hoc key-value collections. For example:

  • The return value of Object.groupBy()
  • The groups and indices.groups properties of the result of RegExp.prototype.exec()
  • Array.prototype[@@unscopables] (all @@unscopables objects should have null -prototype)
  • import.meta
  • Module namespace objects, obtained through import * as ns from "module"; or import()

The term " null -prototype object" often also includes any object without Object.prototype in its prototype chain. Such objects can be created with extends null when using classes.

Object coercion

Many built-in operations that expect objects first coerce their arguments to objects. The operation can be summarized as follows:

  • Objects are returned as-is.
  • undefined and null throw a TypeError .
  • Number , String , Boolean , Symbol , BigInt primitives are wrapped into their corresponding object wrappers.

There are two ways to achieve nearly the same effect in JavaScript.

  • Object.prototype.valueOf() : Object.prototype.valueOf.call(x) does exactly the object coercion steps explained above to convert x .
  • The Object() function: Object(x) uses the same algorithm to convert x , except that undefined and null don't throw a TypeError , but return a plain object.

Places that use object coercion include:

  • The object parameter of for...in loops.
  • The this value of Array methods.
  • Parameters of Object methods such as Object.keys() .
  • Auto-boxing when a property is accessed on a primitive value, since primitives do not have properties.
  • The this value when calling a non-strict function. Primitives are boxed while null and undefined are replaced with the global object .

Unlike conversion to primitives , the object coercion process itself is not observable in any way, since it doesn't invoke custom code like toString or valueOf methods.

Constructor

Turns the input into an object.

Static methods

Copies the values of all enumerable own properties from one or more source objects to a target object.

Creates a new object with the specified prototype object and properties.

Adds the named properties described by the given descriptors to an object.

Adds the named property described by a given descriptor to an object.

Returns an array containing all of the [key, value] pairs of a given object's own enumerable string properties.

Freezes an object. Other code cannot delete or change its properties.

Returns a new object from an iterable of [key, value] pairs. (This is the reverse of Object.entries ).

Returns a property descriptor for a named property on an object.

Returns an object containing all own property descriptors for an object.

Returns an array containing the names of all of the given object's own enumerable and non-enumerable properties.

Returns an array of all symbol properties found directly upon a given object.

Returns the prototype (internal [[Prototype]] property) of the specified object.

Groups the elements of a given iterable according to the string values returned by a provided callback function. The returned object has separate properties for each group, containing arrays with the elements in the group.

Returns true if the specified object has the indicated property as its own property, or false if the property is inherited or does not exist.

Compares if two values are the same value. Equates all NaN values (which differs from both IsLooselyEqual used by == and IsStrictlyEqual used by === ).

Determines if extending of an object is allowed.

Determines if an object was frozen.

Determines if an object is sealed.

Returns an array containing the names of all of the given object's own enumerable string properties.

Prevents any extensions of an object.

Prevents other code from deleting properties of an object.

Sets the object's prototype (its internal [[Prototype]] property).

Returns an array containing the values that correspond to all of a given object's own enumerable string properties.

Instance properties

These properties are defined on Object.prototype and shared by all Object instances.

Points to the object which was used as prototype when the object was instantiated.

The constructor function that created the instance object. For plain Object instances, the initial value is the Object constructor. Instances of other constructors each inherit the constructor property from their respective Constructor.prototype object.

Instance methods

Associates a function with a property that, when accessed, executes that function and returns its return value.

Associates a function with a property that, when set, executes that function which modifies the property.

Returns the function bound as a getter to the specified property.

Returns the function bound as a setter to the specified property.

Returns a boolean indicating whether an object contains the specified property as a direct property of that object and not inherited through the prototype chain.

Returns a boolean indicating whether the object this method is called upon is in the prototype chain of the specified object.

Returns a boolean indicating whether the specified property is the object's enumerable own property.

Calls toString() .

Returns a string representation of the object.

Returns the primitive value of the specified object.

Constructing empty objects

The following example creates empty objects using the new keyword with different arguments:

Using Object() constructor to turn primitives into an Object of their respective type

You can use the Object() constructor to create an object wrapper of a primitive value.

The following examples create variables o1 and o2 which are objects storing Boolean and BigInt values:

Object prototypes

When altering the behavior of existing Object.prototype methods, consider injecting code by wrapping your extension before or after the existing logic. For example, this (untested) code will pre-conditionally execute custom logic before the built-in logic or someone else's extension is executed.

When modifying prototypes with hooks, pass this and the arguments (the call state) to the current behavior by calling apply() on the function. This pattern can be used for any prototype, such as Node.prototype , Function.prototype , etc.

Warning: Modifying the prototype property of any built-in constructor is considered a bad practice and risks forward compatibility.

You can read more about prototypes in Inheritance and the prototype chain .

Specifications

Browser compatibility.

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

  • Object initializer

add property from object javascript

Create a form in Word that users can complete or print

In Word, you can create a form that others can fill out and save or print.  To do this, you will start with baseline content in a document, potentially via a form template.  Then you can add content controls for elements such as check boxes, text boxes, date pickers, and drop-down lists. Optionally, these content controls can be linked to database information.  Following are the recommended action steps in sequence.  

Show the Developer tab

In Word, be sure you have the Developer tab displayed in the ribbon.  (See how here:  Show the developer tab .)

Open a template or a blank document on which to base the form

You can start with a template or just start from scratch with a blank document.

Start with a form template

Go to File > New .

In the  Search for online templates  field, type  Forms or the kind of form you want. Then press Enter .

In the displayed results, right-click any item, then select  Create. 

Start with a blank document 

Select Blank document .

Add content to the form

Go to the  Developer  tab Controls section where you can choose controls to add to your document or form. Hover over any icon therein to see what control type it represents. The various control types are described below. You can set properties on a control once it has been inserted.

To delete a content control, right-click it, then select Remove content control  in the pop-up menu. 

Note:  You can print a form that was created via content controls. However, the boxes around the content controls will not print.

Insert a text control

The rich text content control enables users to format text (e.g., bold, italic) and type multiple paragraphs. To limit these capabilities, use the plain text content control . 

Click or tap where you want to insert the control.

Rich text control button

To learn about setting specific properties on these controls, see Set or change properties for content controls .

Insert a picture control

A picture control is most often used for templates, but you can also add a picture control to a form.

Picture control button

Insert a building block control

Use a building block control  when you want users to choose a specific block of text. These are helpful when you need to add different boilerplate text depending on the document's specific purpose. You can create rich text content controls for each version of the boilerplate text, and then use a building block control as the container for the rich text content controls.

building block gallery control

Select Developer and content controls for the building block.

Developer tab showing content controls

Insert a combo box or a drop-down list

In a combo box, users can select from a list of choices that you provide or they can type in their own information. In a drop-down list, users can only select from the list of choices.

combo box button

Select the content control, and then select Properties .

To create a list of choices, select Add under Drop-Down List Properties .

Type a choice in Display Name , such as Yes , No , or Maybe .

Repeat this step until all of the choices are in the drop-down list.

Fill in any other properties that you want.

Note:  If you select the Contents cannot be edited check box, users won’t be able to click a choice.

Insert a date picker

Click or tap where you want to insert the date picker control.

Date picker button

Insert a check box

Click or tap where you want to insert the check box control.

Check box button

Use the legacy form controls

Legacy form controls are for compatibility with older versions of Word and consist of legacy form and Active X controls.

Click or tap where you want to insert a legacy control.

Legacy control button

Select the Legacy Form control or Active X Control that you want to include.

Set or change properties for content controls

Each content control has properties that you can set or change. For example, the Date Picker control offers options for the format you want to use to display the date.

Select the content control that you want to change.

Go to Developer > Properties .

Controls Properties  button

Change the properties that you want.

Add protection to a form

If you want to limit how much others can edit or format a form, use the Restrict Editing command:

Open the form that you want to lock or protect.

Select Developer > Restrict Editing .

Restrict editing button

After selecting restrictions, select Yes, Start Enforcing Protection .

Restrict editing panel

Advanced Tip:

If you want to protect only parts of the document, separate the document into sections and only protect the sections you want.

To do this, choose Select Sections in the Restrict Editing panel. For more info on sections, see Insert a section break .

Sections selector on Resrict sections panel

If the developer tab isn't displayed in the ribbon, see Show the Developer tab .

Open a template or use a blank document

To create a form in Word that others can fill out, start with a template or document and add content controls. Content controls include things like check boxes, text boxes, and drop-down lists. If you’re familiar with databases, these content controls can even be linked to data.

Go to File > New from Template .

New from template option

In Search, type form .

Double-click the template you want to use.

Select File > Save As , and pick a location to save the form.

In Save As , type a file name and then select Save .

Start with a blank document

Go to File > New Document .

New document option

Go to File > Save As .

Go to Developer , and then choose the controls that you want to add to the document or form. To remove a content control, select the control and press Delete. You can set Options on controls once inserted. From Options, you can add entry and exit macros to run when users interact with the controls, as well as list items for combo boxes, .

Adding content controls to your form

In the document, click or tap where you want to add a content control.

On Developer , select Text Box , Check Box , or Combo Box .

Developer tab with content controls

To set specific properties for the control, select Options , and set .

Repeat steps 1 through 3 for each control that you want to add.

Set options

Options let you set common settings, as well as control specific settings. Select a control and then select Options to set up or make changes.

Set common properties.

Select Macro to Run on lets you choose a recorded or custom macro to run on Entry or Exit from the field.

Bookmark Set a unique name or bookmark for each control.

Calculate on exit This forces Word to run or refresh any calculations, such as total price when the user exits the field.

Add Help Text Give hints or instructions for each field.

OK Saves settings and exits the panel.

Cancel Forgets changes and exits the panel.

Set specific properties for a Text box

Type Select form Regular text, Number, Date, Current Date, Current Time, or Calculation.

Default text sets optional instructional text that's displayed in the text box before the user types in the field. Set Text box enabled to allow the user to enter text into the field.

Maximum length sets the length of text that a user can enter. The default is Unlimited .

Text format can set whether text automatically formats to Uppercase , Lowercase , First capital, or Title case .

Text box enabled Lets the user enter text into a field. If there is default text, user text replaces it.

Set specific properties for a Check box .

Default Value Choose between Not checked or checked as default.

Checkbox size Set a size Exactly or Auto to change size as needed.

Check box enabled Lets the user check or clear the text box.

Set specific properties for a Combo box

Drop-down item Type in strings for the list box items. Press + or Enter to add an item to the list.

Items in drop-down list Shows your current list. Select an item and use the up or down arrows to change the order, Press - to remove a selected item.

Drop-down enabled Lets the user open the combo box and make selections.

Protect the form

Go to Developer > Protect Form .

Protect form button on the Developer tab

Note:  To unprotect the form and continue editing, select Protect Form again.

Save and close the form.

Test the form (optional)

If you want, you can test the form before you distribute it.

Protect the form.

Reopen the form, fill it out as the user would, and then save a copy.

Creating fillable forms isn’t available in Word for the web.

You can create the form with the desktop version of Word with the instructions in Create a fillable form .

When you save the document and reopen it in Word for the web, you’ll see the changes you made.

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COMMENTS

  1. 5 ways to Add a Property to object in JavaScript

    1. "object.property_name" syntax The dot notation is the simplest way to access/modify the properties of a JavaScript object. A new property can be initialized by the following syntax: object.new_property = new_value In the below example, we are creating an "id" property with the value 130 for the employee object.

  2. Working with objects

    const obj = { property1: value1, // property name may be an identifier 2: value2, // or a number "property n": value3, // or a string }; Each property name before colons is an identifier (either a name, a number, or a string literal), and each valueN is an expression whose value is assigned to the property name.

  3. Is it possible to add dynamically named properties to JavaScript object

    22 Answers Sorted by: 1505 Yes. var data = { 'PropertyA': 1, 'PropertyB': 2, 'PropertyC': 3 }; data ["PropertyD"] = 4; // dialog box with 4 in it alert (data.PropertyD); alert (data ["PropertyD"]); Share Improve this answer Follow edited Jan 8, 2017 at 16:50 H. Pauwelyn

  4. JavaScript Object Properties

    Properties are the most important part of any JavaScript object. JavaScript Properties Properties are the values associated with a JavaScript object. A JavaScript object is a collection of unordered properties. Properties can usually be changed, added, and deleted, but some are read only. Accessing JavaScript Properties

  5. How to add and remove properties from objects in JavaScript

    For adding any property, one could either use object_name.property_name = value (or) object_name ["property_name"] = value. For deleting any property, one could easily use delete object_name.property_name (or) delete object_name ["property_name"]. here are several methods that can be used to add and remove properties from objects.

  6. Add Property to an Object in Javascript (6 ways)

    Add Property to an Object in Javascript (6 ways) November 29, 2021 / Javascript / By Ritika This article demonstrates easy ways to add a property in the javascript object using different methods and various example illustrations. We will also see how to add new properties dynamically to javascript objects after their creation. Table of Contents:

  7. Object.assign()

    Syntax js Object.assign(target) Object.assign(target, source1) Object.assign(target, source1, source2) Object.assign(target, source1, source2, /* …, */ sourceN) Parameters target The target object — what to apply the sources' properties to, which is returned after it is modified. source1, …, sourceN

  8. How to add a property to an object in JavaScript

    To add a new property to a JavaScript object: You either use the dot (.) notation or the square bracket ( [] ). In dot donation, you use the object name followed by the dot, the name of the new property, an equal sign, and the value for the new property.

  9. Object.defineProperty()

    Object.defineProperty () allows a precise addition to or modification of a property on an object. Normal property addition through assignment creates properties which show up during property enumeration ( for...in, Object.keys (), etc.), whose values may be changed and which may be deleted.

  10. JavaScript: 5 ways to add new properties to an existing object

    Using dot notation is the simplest way to add new properties to an object. You just write the object name, followed by a dot (.), followed by the property name. Example: const obj = { name: 'Sling Academy', address: 'Hidden Forest' }; obj.newProperty = 'New Property'; console.log(obj); Output:

  11. Add a property to an object in JavaScript

    There are many ways to add key-value pairs to an Object in JavaScript. We can add a key/value pair to an Object in JavaScript by the following ways: Using dot (.) notation. Use the bracket [ ] notation. Using the Object.assign () method. Using Spread operator. Using third-party library.

  12. Conditionally Add Property to Object in JavaScript

    The simplest way to conditionally add a property to an object is by using an if statement. This is a simple approach that even beginners in JavaScript should be familiar with. Let's say we have an object representing a book, and we want to add a publisher property, but only if the book is not self-published. let book = { title: "The Great ...

  13. Javascript add property to object

    Method 3: Spread operator. Spread operator allows us to destructure objects and utilizing that we can create a new object with the additional property we want to create. The advantage of this method is that it creates a shallow copy of the original object for us and also allows dynamic keys for the object. let game = {.

  14. How to Add Property to an object in JavaScript?

    Some of the ways to add property to object JavaScript include: using dot notation, using bracket [ ] notation, using defineProperty () method, using spread operator, and using Object.assign () method. Introduction To start with, let us revise and go through the concept of objects and their properties in brief.

  15. Add a property to an object in JavaScript

    This post will discuss how to add a property to an object in JavaScript. 1. Using dot notation. A simple approach is to use the dot notation with an assignment operator to add a property to an existing object. The syntax is: object.property = value. 2.

  16. How to Add Property to an object in JavaScript

    There're a few ways to add properties to an object in JavaScript. One way is to add a property using the dot notation: obj.foo = 1; We added the foo property to the obj object above with value 1. We can also add a property by using the bracket notation: obj ['foo'] = 1;

  17. javascript

    6 Answers Sorted by: 51 In ES6/ ES2015 you can use the Object.assign method let obj = {key1: true}; console.log ('old obj: ', obj); let newObj = {key2: false, key3: false}; Object.assign (obj, newObj); console.log ('modified obj: ', obj); Share Improve this answer Follow edited Jul 16, 2019 at 13:59 Ado 717 1 6 18 answered Apr 25, 2017 at 8:36

  18. How to Add Property to an Object in JavaScript [5 Ways]

    His proficiency in the Python language stands as a testament to his versatility and commitment to the craft. Here are five ways to add a property to a JavaScript Object:Using the dot (.) notation, Using the square brackets, Using the Object.defineProperty () function,Using the Object.assign () function and Using the "Spread operator".

  19. Object

    Unlike normal objects, in which toString() is on the object's prototype, the toString() method here is an own property of nullProtoObj.This is because nullProtoObj has no (null) prototype.. You can also revert a null-prototype object back to an ordinary object using Object.setPrototypeOf(nullProtoObj, Object.prototype).. In practice, objects with null prototype are usually used as a cheap ...

  20. Create a form in Word that users can complete or print

    Add content to the form. Go to Developer, and then choose the controls that you want to add to the document or form. To remove a content control, select the control and press Delete. You can set Options on controls once inserted. From Options, you can add entry and exit macros to run when users interact with the controls, as well as list items ...

  21. javascript

    4,087 3 20 34 7 possible duplicate of Add new element to existing object in JavaScript / jQuery - Foreever Oct 22, 2014 at 6:30 Object.assign (target, source); can be used to copy all the properties from a source object to a target object. - David Spector Aug 25, 2019 at 12:11

  22. javascript

    8 Answers Sorted by: 459 Use Array.prototype.map () Results.map (obj => ( { ...obj, Active: 'false' })) Read the documentation for more information. Share Improve this answer Follow edited Apr 7, 2022 at 12:39 Ivar 6,364 12 51 61 answered Jun 7, 2017 at 8:55 sidonaldson