javascript declare array class

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An array is a special variable, which can hold more than one value:

Why Use Arrays?

If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:

However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?

The solution is an array!

An array can hold many values under a single name, and you can access the values by referring to an index number.

Creating an Array

Using an array literal is the easiest way to create a JavaScript Array.

It is a common practice to declare arrays with the const keyword.

Learn more about const with arrays in the chapter: JS Array Const .

Spaces and line breaks are not important. A declaration can span multiple lines:

You can also create an array, and then provide the elements:

Using the JavaScript Keyword new

The following example also creates an Array, and assigns values to it:

The two examples above do exactly the same.

There is no need to use new Array() .

For simplicity, readability and execution speed, use the array literal method.

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Accessing Array Elements

You access an array element by referring to the index number :

Note: Array indexes start with 0.

[0] is the first element. [1] is the second element.

Changing an Array Element

This statement changes the value of the first element in cars :

Converting an Array to a String

The JavaScript method toString() converts an array to a string of (comma separated) array values.

Access the Full Array

With JavaScript, the full array can be accessed by referring to the array name:

Arrays are Objects

Arrays are a special type of objects. The typeof operator in JavaScript returns "object" for arrays.

But, JavaScript arrays are best described as arrays.

Arrays use numbers to access its "elements". In this example, person[0] returns John:

Objects use names to access its "members". In this example, person.firstName returns John:

Array Elements Can Be Objects

JavaScript variables can be objects. Arrays are special kinds of objects.

Because of this, you can have variables of different types in the same Array.

You can have objects in an Array. You can have functions in an Array. You can have arrays in an Array:

Array Properties and Methods

The real strength of JavaScript arrays are the built-in array properties and methods:

Array methods are covered in the next chapters.

The length Property

The length property of an array returns the length of an array (the number of array elements).

The length property is always one more than the highest array index.

Accessing the First Array Element

Accessing the last array element, looping array elements.

One way to loop through an array, is using a for loop:

You can also use the Array.forEach() function:

Adding Array Elements

The easiest way to add a new element to an array is using the push() method:

New element can also be added to an array using the length property:

Adding elements with high indexes can create undefined "holes" in an array:

Associative Arrays

Many programming languages support arrays with named indexes.

Arrays with named indexes are called associative arrays (or hashes).

JavaScript does not support arrays with named indexes.

In JavaScript, arrays always use numbered indexes .  

WARNING !! If you use named indexes, JavaScript will redefine the array to an object.

After that, some array methods and properties will produce incorrect results .

 Example:

The difference between arrays and objects.

In JavaScript, arrays use numbered indexes .  

In JavaScript, objects use named indexes .

Arrays are a special kind of objects, with numbered indexes.

When to Use Arrays. When to use Objects.

• JavaScript does not support associative arrays.
• You should use objects when you want the element names to be strings (text) .
• You should use arrays when you want the element names to be numbers .

JavaScript new Array()

JavaScript has a built-in array constructor new Array() .

But you can safely use [] instead.

These two different statements both create a new empty array named points:

These two different statements both create a new array containing 6 numbers:

The new keyword can produce some unexpected results:

A Common Error

is not the same as:

How to Recognize an Array

A common question is: How do I know if a variable is an array?

The problem is that the JavaScript operator typeof returns " object ":

The typeof operator returns object because a JavaScript array is an object.

Solution 1:

To solve this problem ECMAScript 5 (JavaScript 2009) defined a new method Array.isArray() :

Solution 2:

The instanceof operator returns true if an object is created by a given constructor:

Complete Array Reference

For a complete Array reference, go to our:

Complete JavaScript Array Reference .

The reference contains descriptions and examples of all Array properties and methods.

Test Yourself With Exercises

Get the value " Volvo " from the cars array.

Start the Exercise

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Data Structures 101: a tutorial on arrays in JavaScript

The image above shows an array of the elements A, B, C, and D, and their corresponding indexes. The array length would be four since there are four elements. The numbers beneath the elements refer to the places that the letters exist in the computer’s memory. Now, let’s look at an example of a small array of numbers using JavaScript code.

Here, you can see our elements between the brackets [] . Quiz yourself: what would the indexes of each element be?

Advantages and disadvantages of arrays

Every data structure has its advantages and disadvantages due to its basic organization.

Some data structures are better for certain uses than others, so it’s important to know what arrays do well and what they are not best suited to perform.

• Store similar data of the same type.
• Store data when you already know the number of all the elements
• Implement other data structures like trees, linked lists , graphs , stacks , etc.
• Store elements in different dimensions of arrays: two-dimensional or multidimensional.
• An array allocates memory in contiguous memory locations for every element. This avoids memory overflow.
• Iteration in arrays runs faster compared to other data structures.
• An array, once declared, can be reused multiple times. This improves code readability.
• If you want to use an array, you have to know the number of elements you want to store beforehand. It is impossible to add new/extra values once you declare it.
• The memory that is allocated to an array at declaration cannot be increased or decreased.
• Allocating more memory than you need can waste memory space.
• The cost of deleting and inserting elements is high because the elements of an array are stored in contiguous memory locations.
• Errors show up at runtime instead of compile-time because an array does not verify the indexes when compiling.

Uses of arrays

Now we know what arrays look like and their advantages, but what can you actually do with an array? Let’s take a look. An array can:

• Copy and clone elements
• Insert and delete elements
• Search and sort elements

Arrays allow you to maintain multiple variable names and large data with a single name. This removes the confusion of using multiple variables and improves code readability. You can also perform different matrix operations with the use of two-dimensional and multidimensional arrays.

Arrays can be used for sorting data elements. Different sorting techniques like bubble sort, insertion sort, and selection sort use arrays to store and sort elements easily.

An array can also be used for CPU scheduling. It allows one process in your program to use the CPU while the execution of another process is on hold.

Get started with arrays

Writing or declaring an array differs from language to language, but one thing all these different variations have in common is that the array always holds a collection of data. In this section, we will look at how to declare an array in JavaScript, Java, C++, and Python.

In each example, we will use the test scores of a high school student as a use case. Let’s say our student Tonia scored 78, 90, 62, 88, 93, and 50 in six subjects respectively.

To use an array in a program, you must declare a variable to reference the array, and then, create the elements of that array.

Now, let’s build this array in different languages!

Declaring an array in JavaScript

In JavaScript, the syntax to create an array is:

If we want to create an array that holds Tonia’s test scores, we can name our array, test_scores and put all the elements inside the square brackets like so:

Declaring an array in Java

In Java, the syntax is a little different. First, you have to declare the variable type and then insert the values you want to store with an array literal. If we wanted to create an array of Tonia’s subjects, it would look like this:

An array of integers, the corresponding scores for these subjects, would look like this:

Declaring an array in C++

Arrays in C++ are more similar to Java than JavaScript. In C++, you can create an array by creating the array variable and initializing the elements like so:

Here, the compiler will create an array of size 6, that houses all of Tonia’s scores.

Declaring an array in Python

Python does not have built-in support for arrays, so we use lists to create them instead. First, we create the array variable name then, we initialize all the elements we need like this:

Using array literal and array constructor

It’s important to note that there is another common way to create an array in popular languages. You can either use an array literal , as we showed above with an array declaration in JavaScript, but we can an array constructor .

The Array() constructor is used to create Array objects. A constructor is a function that creates an instance of a class called an object . With an array constructor, we can have multiple parameters.

To use an array constructor, you declare the array with the help of a built-in class and initialize your elements by passing them into the brackets.

In JavaScript, it looks like this:

No matter the way you declare an array using the methods above, it works. If you console.log both pieces of code separately, you will get the same array.

To learn more about constructors in JavaScript, check out our EdPresso shot What is a constructor in JavaScript?

Keep the learning going.

Learn data structures for JavaScript coding interviews without scrubbing through videos or documentation. Educative’s text-based courses are easy to skim and feature live coding environments - making learning quick and efficient.

Data Structures for Coding Interviews in JavaScript

Using and modifying an array

Now that we know how to create an array, we need to learn how to apply operations and functions to make them useful for our program. We have the stored data, but what do we do next?

Well, let’s take a look at some of the common uses and modifications to arrays that are applied to most computer programs.

Accessing a value in an array

If you want to access an array, you first need to know the location of that array. This is why knowing how indexes work is important.

For instance, if we need to access Tonia’s score for Computer Science (93, the highest score) and store it in a variable highestScore , we first need to know its location. If we count from the left, starting at zero, the index of 93 will be 4. Our code for this would look like this:

We console.log the highestScore and indeed get 93, which is at index 4. Accessing elements in arrays comes down to knowing the index.

Modify an array

So, what if we wanted to change the value of a score? Let’s assume that the last score was miscalculated, and Tonia actually got a score of 71.

To modify the last element, we find the index of the element we want to modify and set it equal to the new value we want to exchange it with.

In JavaScript, this would look like this:

Here, we found that the index of the element we need to change is 5. We console that to be sure and then set it to be equivalent to the new value. Now, if you console.log the last line, you will get the correct test score 71.

Inserting and deleting values

Say Tonia took an extra Accounting class, and we wanted to add that score to the testScores array. To insert an element, there are a couple of methods you can use.

If you want to add a new element to the end of your array, you can use the push() method. Let’s add the score for her Accounting class at the end of the array.

You can do it in JavaScript like so:

As we can see, this adds the new score to the end of the array and increments the index by 1.

You can also add a new element to the beginning of an array using the unshift() method. Say Tonia has another two classes with scores of 74 and 58, and we want them at the front of our array. Let’s see how we’d do that below.

Say we want to add an element in the middle of our aray. To insert a new element anywhere in your array except for the beginning or end, you can use the splice() method. In JavaScript, the syntax is like this:

Here, the index refers to the location you want to add or remove elements. howmany refers to the number of items you need removed, and it is optional. The last part, item1, ..., itemX , defines the new items to be added to the array. They are also optional.

Deleting elements, on the other hand, has a number of methods you can use. The first one is the pop() method. This method lets you delete the last element in an array. If you wanted to remove the last test score, you can use it like so with JavaScript:

This method works by appending pop(); to the array variable. If you console.log this, you’ll notice that the last element is gone. In contrast to the unshift() method, you can use the shift() method to remove the first element of an array.

In our example below, we will remove the first element, 78, from the array of test scores and shift all the remaining elements to a lower index. Here’s how it’s done in JavaScript.

Passing arrays to a function

A function is like a command or action that manipulates our data. Say we want to apply commands to Tonia’s score.

If you want to pass an array to a function in your program, you can use the spread operator. This is a new JavaScript feature you can use instead of apply() . The spread operator allows you to access the elements of an iterable object.

Since arrays are a type of object that can be traversed in a sequential fashion, it is a good choice here. If we wanted to write a function that takes Tonia’s scores and apply an action to them, we have to pass the testScores array to the function as a parameter.

Our syntax would look like this:

Here we have written a placeholder function that will take Tonia’s six scores and do something with them. We create a new variable (an array) to hold the scores and in the last line, and we pass the array to our function.

Print an array

To print an array in JavaScript, you can use the built-in array.values() function. This prints all the elements in a given array.

If you wanted to print out all the elements in the testScores array we have been working with, it would look like so:

Now we know how to create an array, modify it, delete values, and apply functions to our data. Well done!

Questions about data structures are very common for coding interviews. Since arrays are so popular in computer programs, you will likely encounter coding problems about arrays in any coding interview.

Here are common JavaScript interview questions for arrays that demonstrate your mastery of arrays.

• Find the length of an array
• Reverse the array in place
• Find the smallest and largest numbers
• Remove all the duplicates of an array in place
• Find the length of the longest consecutive elements’ sequence
• Remove the first two elements using array destructuring
• Write a function that determines if an object is an array
• Given an array of coins , write a function to compute the number of ways you can make that amount using those coins

Well done! You’ve now completed your Data Structures 101 tour of arrays in JavaScript. You’re well on your way to mastering array. There is still a lot to learn about arrays, so keep reading, practicing, and taking courses to aid your learning. One of the best ways to learn about data structures is through hands-on practice.

By working with real-world coding questions or projects, you’ll quickly learn how arrays work and how they integrate with other data structures.

Educative’s course Data Structures for Coding Interviews in JavaScript is an ideal place to start. This course contains a detailed review of all the common data structures and provides implementation level details in JavaScript.

You’ll become well equipped with all the different data structures they can leverage to write better code!

Class basic syntax

In object-oriented programming, a class is an extensible program-code-template for creating objects, providing initial values for state (member variables) and implementations of behavior (member functions or methods). Wikipedia

In practice, we often need to create many objects of the same kind, like users, or goods or whatever.

As we already know from the chapter Constructor, operator "new" , new function can help with that.

But in the modern JavaScript, there’s a more advanced “class” construct, that introduces great new features which are useful for object-oriented programming.

The “class” syntax

The basic syntax is:

Then use new MyClass() to create a new object with all the listed methods.

The constructor() method is called automatically by new , so we can initialize the object there.

For example:

When new User("John") is called:

• A new object is created.
• The constructor runs with the given argument and assigns it to this.name .

…Then we can call object methods, such as user.sayHi() .

A common pitfall for novice developers is to put a comma between class methods, which would result in a syntax error.

The notation here is not to be confused with object literals. Within the class, no commas are required.

What is a class?

So, what exactly is a class ? That’s not an entirely new language-level entity, as one might think.

Let’s unveil any magic and see what a class really is. That’ll help in understanding many complex aspects.

In JavaScript, a class is a kind of function.

Here, take a look:

What class User {...} construct really does is:

• Creates a function named User , that becomes the result of the class declaration. The function code is taken from the constructor method (assumed empty if we don’t write such method).
• Stores class methods, such as sayHi , in User.prototype .

After new User object is created, when we call its method, it’s taken from the prototype, just as described in the chapter F.prototype . So the object has access to class methods.

We can illustrate the result of class User declaration as:

Here’s the code to introspect it:

Not just a syntactic sugar

Sometimes people say that class is a “syntactic sugar” (syntax that is designed to make things easier to read, but doesn’t introduce anything new), because we could actually declare the same thing without using the class keyword at all:

The result of this definition is about the same. So, there are indeed reasons why class can be considered a syntactic sugar to define a constructor together with its prototype methods.

Still, there are important differences.

First, a function created by class is labelled by a special internal property [[IsClassConstructor]]: true . So it’s not entirely the same as creating it manually.

The language checks for that property in a variety of places. For example, unlike a regular function, it must be called with new :

Also, a string representation of a class constructor in most JavaScript engines starts with the “class…”

There are other differences, we’ll see them soon.

Class methods are non-enumerable. A class definition sets enumerable flag to false for all methods in the "prototype" .

That’s good, because if we for..in over an object, we usually don’t want its class methods.

Classes always use strict . All code inside the class construct is automatically in strict mode.

Besides, class syntax brings many other features that we’ll explore later.

Class Expression

Just like functions, classes can be defined inside another expression, passed around, returned, assigned, etc.

Here’s an example of a class expression:

Similar to Named Function Expressions, class expressions may have a name.

If a class expression has a name, it’s visible inside the class only:

We can even make classes dynamically “on-demand”, like this:

Getters/setters

Just like literal objects, classes may include getters/setters, computed properties etc.

Here’s an example for user.name implemented using get/set :

Technically, such class declaration works by creating getters and setters in User.prototype .

Computed names […]

Here’s an example with a computed method name using brackets [...] :

Such features are easy to remember, as they resemble that of literal objects.

Class fields

Class fields are a recent addition to the language.

Previously, our classes only had methods.

“Class fields” is a syntax that allows to add any properties.

For instance, let’s add name property to class User :

= " in the declaration, and that’s it.

The important difference of class fields is that they are set on individual objects, not User.prototype :

We can also assign values using more complex expressions and function calls:

Making bound methods with class fields

As demonstrated in the chapter Function binding functions in JavaScript have a dynamic this . It depends on the context of the call.

So if an object method is passed around and called in another context, this won’t be a reference to its object any more.

For instance, this code will show undefined :

The problem is called "losing this ".

There are two approaches to fixing it, as discussed in the chapter Function binding :

• Pass a wrapper-function, such as setTimeout(() => button.click(), 1000) .
• Bind the method to object, e.g. in the constructor.

Class fields provide another, quite elegant syntax:

The class field click = () => {...} is created on a per-object basis, there’s a separate function for each Button object, with this inside it referencing that object. We can pass button.click around anywhere, and the value of this will always be correct.

That’s especially useful in browser environment, for event listeners.

The basic class syntax looks like this:

MyClass is technically a function (the one that we provide as constructor ), while methods, getters and setters are written to MyClass.prototype .

In the next chapters we’ll learn more about classes, including inheritance and other features.

Rewrite to class

The Clock class (see the sandbox) is written in functional style. Rewrite it in the “class” syntax.

P.S. The clock ticks in the console, open it to see.

Open a sandbox for the task.

Open the solution in a sandbox.

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• To insert few words of code, use the <code> tag, for several lines – wrap them in <pre> tag, for more than 10 lines – use a sandbox ( plnkr , jsbin , codepen …)

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JavaScript Array of Objects Tutorial – How to Create, Update, and Loop Through Objects Using JS Array Methods

On average I work with JSON data 18 times a week. And I still need to google for specific ways to manipulate them almost every time. What if there was an ultimate guide that could always give you the answer?

In this article, I'll show you the basics of working with object arrays in JavaScript.

If you ever worked with a JSON structure, you've worked with JavaScript objects. Quite literally. JSON stands for JavaScript Object Notation.

Creating an object is as simple as this:

This object represents a car. There can be many types and colors of cars, each object then represents a specific car.

Now, most of the time you get data like this from an external service. But sometimes you need to create objects and their arrays manually. Like I did when I was creating this e-shop:

Considering each category list item looks like this in HTML:

I didn't want to have this code repeated 12 times, which would make it unmaintainable.

Creating an array of objects

But let's get back to cars. Let's take a look at this set of cars:

We can represent it as an array this way:

Arrays of objects don't stay the same all the time. We almost always need to manipulate them. So let's take a look at how we can add objects to an already existing array.

Add a new object at the start - Array.unshift

To add an object at the first position, use Array.unshift .

Add a new object at the end - Array.push

To add an object at the last position, use Array.push .

Add a new object in the middle - Array.splice

To add an object in the middle, use Array.splice . This function is very handy as it can also remove items. Watch out for its parameters:

So if we want to add the red Volkswagen Cabrio on the fifth position, we'd use:

Looping through an array of objects

Let me ask you a question here: Why do you want to loop through an array of objects? The reason I'm asking is that the looping is almost never the primary cause of what we want to achieve.

JavaScript provides many functions that can solve your problem without actually implementing the logic in a general cycle. Let's take a look.

Find an object in an array by its values - Array.find

Let's say we want to find a car that is red. We can use the function Array.find .

This function returns the first matching element:

It's also possible to search for multiple values:

let car = cars.find(car => car.color === "red" && car.type === "cabrio");

In that case we'll get the last car in the list.

Get multiple items from an array that match a condition - Array.filter

The Array.find function returns only one object. If we want to get all red cars, we need to use Array.filter .

Transform objects of an array - Array.map

This is something we need very often. Transform an array of objects into an array of different objects. That's a job for Array.map . Let's say we want to classify our cars into three groups based on their size.

It's also possible to create a new object if we need more values:

Add a property to every object of an array - Array.forEach

But what if we want the car size too? In that case we can enhance the object for a new property size . This is a good use-case for the Array.forEach function.

Sort an array by a property - Array.sort

When we're done with transforming the objects, we usually need to sort them one way or another.

Typically, the sorting is based on a value of a property every object has. We can use the Array.sort function, but we need to provide a function that defines the sorting mechanism.

Let's say we want to sort the cars based on their capacity in descending order.

The Array.sort compares two objects and puts the first object in the second place if the result of the sorting function is positive. So you can look at the sorting function as if it was a question: Should the first object be placed in second place?

Make sure to always add the case for zero when the compared value of both objects is the same to avoid unnecessary swaps.

Checking if objects in array fulfill a condition - Array.every, Array.includes

Array.every and Array.some come handy when we just need to check each object for a specific condition.

Do we have a red cabrio in the list of cars? Are all cars capable of transporting at least 4 people? Or more web-centric: Is there a specific product in the shopping cart?

You may remember the function Array.includes which is similar to Array.some , but works only for primitive types.

In this article, we went through the basic functions that help you create, manipulate, transform, and loop through arrays of objects. They should cover most cases you will stumble upon.

If you have a use-case that requires more advanced functionality, take a look at this detailed guide to arrays or visit the W3 schools reference .

Or get in touch with me and I will prepare another article :-)

Jamstack dev, YouTube video maker & streamer, dev evangelist @Kontent, 3D printing enthusiast, bad AoE2 player, German Shepherd lover

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How to Declare an Array in JavaScript ?

• How to clone an array in JavaScript ?
• How to create a array in JavaScript?
• How to deep flatten an array in JavaScript?
• How to Filter an Array in JavaScript ?
• How to empty an array in JavaScript ?
• How to Declare a Variable in JavaScript ?
• How to check object is an array in JavaScript ?
• How to truncate an array in JavaScript ?
• How to get the size of an array in JavaScript ?
• How Dynamic Arrays Work in JavaScript ?
• How to append an element in an array in JavaScript ?
• How to initialize an array in JavaScript ?
• How to declare namespace in JavaScript ?
• How to compare Arrays of Objects in JavaScript ?
• How to convert Set to Array in JavaScript ?
• How to Convert an Array to a String in JavaScript ?
• How to initialize a boolean array in JavaScript ?
• How to Convert Array to Set in JavaScript?
• How to create a JavaScript class in ES6 ?

In Javascript , Arrays can contain any type of data, numbers, strings, booleans, objects, etc. But typically all elements are of the same type.

Arrays can be single-dimensional with one level of elements, or multi-dimensional with nested arrays.

Following are the ways to declare an Array in JavaScript:

Table of Content

• Using Array Literal
• Using Array constructor
• Using new keyword

Approach 1: Using Array Literal

Array literals use square brackets to enclose comma-separated elements for initializing arrays in JavaScript .

Example: This example shows the different types of array declaration.

Approach 2: Using Array constructor

The Array constructor allows dynamically creating arrays and setting their initial state. It allows passing in elements as arguments to populate the array. If no arguments are passed, an empty array is created.

Example: This example shows the declaration of an array using the array constructor.

Approach 3: Using new keyword

The new Array() constructor initializes the array and returns a reference to the array object. It can be used to create arrays of different data types like numbers, strings, nested arrays etc.

Example: This example shows that we can create an array using new keyword also.

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ECMAScript 2024 features you can use now

Ecmascript 2024 is expected to be finalized in june, but four new javascript features are already supported in browsers and node.js. here's how to start using them today..

Contributor, InfoWorld |

Promise.withResolvers

Object.groupby & map.groupby, atomics.waitasync, string.iswellformed & string.towellformed .

The ECMAScript specification is like a portrait of the JavaScript language that is repainted every year. As is typical of modern JavaScript, the spec and real-world practice move in tandem. The newest version of the spec, ECMAScript 2024 , includes seven new JavaScript features and is expected to be finalized in June. This article introduces four of the new features that are already available in browsers and server-side environments, and ready for you to use today:

• Promise.withResolvers  is a powerful mechanism for managing asynchronous operations when external control over resolution and rejection is necessary.
• Object.groupBy and Map.groupBy  let you organize collections based on key properties.
• Atomics.waitAsync  facilitates safe communication and synchronization between worker threads.
• String.isWellFormed and String.toWellFormed  add valuable tools for handling user input and network data.

Let’s start with the new static method on Promise , called withResolvers() . JavaScript promises give us various ways to deal with asynchronous operations. The withResolvers() method is used to create the three parts of a Promise : the Promise itself and the resolve() and reject() functions. 

The benefit of withResolvers() is that it creates all three as externally exposed references. In cases where you want to create a promise, and also have access to the resolution and rejection of the promise from external code, this is the method to use.

The spec itself is characteristically spartan in its description . The Mozilla documentation provides more detail . The key takeaway is that withResolvers() is equivalent to:

In the above snippet, we declare the resolve and reject references in the enclosing scope, then use them inside the body of the Promise callback to refer to the resolve and reject arguments. In this way, we're getting a handle on the promise callback from outside the callback itself.

The Promise.withResolvers() syntax is more compact and we don't have to declare the resolve and reject separately. With this method, we could write the exact same functionality as above like so:

The essence of this capability is that you use it when you need outside access to resolve() and reject() . This isn’t a very common scenario but it happens. Let’s consider a simple example:

Here, we have two operations, fetchData1() and fetchData2() , that return promises. They run timeouts, and fetchData2() is always fastest at 500 milliseconds. We use withResolvers() inside the race() function to expose the resolve and reject functions. These functions are then used by a new promise, called racePromise .

We then use the resolve and reject functions to respond to the two fetchData operations. This is interesting because you can see we don’t even provide a callback to racePromise . Instead, we control the promise externally. We use that control to bind the outcome of the other two promises to racePromise .

This is a contrived example, and somewhat unrealistic because the two racing promises do not begin at the same time. The point is to show the essence of how and when to use withResolvers() .

The handy groupBy method is a quick way to organize collections based on a string value. It's a static method on Object and Map , which works on an array-like collection. The groupBy() method takes two arguments: the collection and a callback that operates on it. You get back a new object instance that has string label values as keys, and the corresponding array elements as the value.

So, when you have an array, and you need to divvy up the elements into string-labeled buckets according to some internal criteria, this is the method to use.

This is a fairly common thing that comes up in day-to-day coding. Looking at an example should make it clear. Say we have a collection of dog breeds and their size:

Now, say we want to organize this collection by size. We want to end up with a collection of objects where the keys are the breed size and the values are the dog breed. Normally, we’d write a loop to do this but it's a bit finicky; it seems like there should be a better way. Now, with groupBy() , there is:

This gives you a simple, functional way to group collections of objects based on some property of the objects themselves. 

The groupBy() method takes whatever is returned by the callback and automatically collects all the elements that are equal according to String equality . If the return value is not a string, it’ll be coerced into a string. If it can’t be coerced, it’ll error out.

The new Atomics.waitAsync() method is designed for sharing data across worker threads safely. It does not block the main thread like Atomics.wait() does. Because it is used between threads, it relies on the SharedArrayBuffer class. 

This class is disabled by default in modern browsers unless security requirements are met . In Node.js, however, the class is enabled by default. 

Here’s a simple example for usage in Node. Note that the imports are built into Node, so no NPM installs are required (but note that the import statement is):

To run this program, just enter: $ node asyncWait.js

The program declares a SharedArrayBuffer (wrapped around an int32Array ) and then checks if we are on the main thread. If it is the main thread, we spawn a worker thread. (If you are new to worker threads, here's a good intro .)

The main thread waits for an update from the worker via the Atomics.waitAsync() call. The three args to waitAsync(1, 2, 3) are:

• Shared array ( int32Array ): The shared memory space.
• Element to watch ( 0 ): The index of the array to wait upon.
• Initial value ( initialValue = 0 ): Only notify the main thread if this value is different (i.e., if the worker sets the value to the initial value of 0, a notification will not occur).

User input, bad data, and network glitches are all sources of malformed strings.  String.isWellFormed is a sanity check. It determines if a string is UTF-16 valid. UTF-16 is the encoding JavaScript itself uses, so String.isWellFormed() ensures a given string is something JavaScript can handle:

You can learn more about what constitutes valid strings in JavaScript in this section of the Mozilla reference . The bad guys in UTF-16 encoding are known as “lone surrogates.”

The active partner to String.isWellFormed , String.toWellFormed transforms a given string into something valid. Any lone surrogates found will be replaced by U+FFFD, the black diamond question mark character: �.

We’ve got a nice collection of new features in ECMAScript 2024.  Promise.withResolvers() and Atomics.waitAsync() are more advanced use cases, while groupBy is a convenient addition that often comes in handy, and the new string methods are perfect for certain situations. All of these features are supported for JavaScript in browsers and server-side environments, so you can start using them today.

Next read this:

• Why companies are leaving the cloud
• 5 easy ways to run an LLM locally
• Coding with AI: Tips and best practices from developers
• Meet Zig: The modern alternative to C
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Matthew Tyson is a founder of Dark Horse Group, Inc. He believes in people-first technology. When not playing guitar, Matt explores the backcountry and the philosophical hinterlands. He has written for JavaWorld and InfoWorld since 2007.

Copyright © 2024 IDG Communications, Inc.

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Public class fields

Public fields are writable, enumerable, and configurable properties. As such, unlike their private counterparts, they participate in prototype inheritance.

There are some additional syntax restrictions:

• The name of a static property (field or method) cannot be prototype .
• The name of a class field (static or instance) cannot be constructor .

Description

This page introduces public instance fields in detail.

• For public static fields, see static .
• For private fields, see private properties .
• For public methods, see method definitions .
• For public accessors, see getter and setter .

Public instance fields exist on every created instance of a class. By declaring a public field, you can ensure the field is always present, and the class definition is more self-documenting.

Public instance fields are added to the instance either at construction time in the base class (before the constructor body runs), or just after super() returns in a subclass. Fields without initializers are initialized to undefined . Like properties, field names may be computed.

Computed field names are only evaluated once, at class definition time . This means that each class always has a fixed set of field names, and two instances cannot have different field names via computed names. The this value in the computed expression is the this surrounding the class definition, and referring to the class's name is a ReferenceError because the class is not initialized yet. await and yield work as expected in this expression.

In the field initializer, this refers to the class instance under construction, and super refers to the prototype property of the base class, which contains the base class's instance methods, but not its instance fields.

The field initializer expression is evaluated each time a new instance is created. (Because the this value is different for each instance, the initializer expression can access instance-specific properties.)

The expression is evaluated synchronously. You cannot use await or yield in the initializer expression. (Think of the initializer expression as being implicitly wrapped in a function.)

Because instance fields of a class are added before the respective constructor runs, you can access the fields' values within the constructor. However, because instance fields of a derived class are defined after super() returns, the base class's constructor does not have access to the derived class's fields.

Fields are added one-by-one. Field initializers can refer to field values above it, but not below it. All instance and static methods are added beforehand and can be accessed, although calling them may not behave as expected if they refer to fields below the one being initialized.

Note: This is more important with private fields , because accessing a non-initialized private field throws a TypeError , even if the private field is declared below. (If the private field is not declared, it would be an early SyntaxError .)

Because class fields are added using the [[DefineOwnProperty]] semantic (which is essentially Object.defineProperty() ), field declarations in derived classes do not invoke setters in the base class. This behavior differs from using this.field = … in the constructor.

Note: Before the class fields specification was finalized with the [[DefineOwnProperty]] semantic, most transpilers, including Babel and tsc , transformed class fields to the DerivedWithConstructor form, which has caused subtle bugs after class fields were standardized.

Using class fields

Class fields cannot depend on arguments of the constructor, so field initializers usually evaluate to the same value for each instance (unless the same expression can evaluate to different values each time, such as Date.now() or object initializers).

However, even declaring an empty class field is beneficial, because it indicates the existence of the field, which allows type checkers as well as human readers to statically analyze the shape of the class.

The code above seems repetitive, but consider the case where this is dynamically mutated: the explicit field declaration makes it clear which fields will definitely be present on the instance.

Because initializers are evaluated after the base class has executed, you can access properties created by the base class constructor.

Specifications

Browser compatibility.

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

• Using classes guide
• Private properties
• The semantics of all JS class elements by Shu-yu Guo (2018)
• Public and private class fields on v8.dev (2018)