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Everything new coming in ES2022

By Alberto Montalesi
Published in Article
September 08, 2020
5 min read
Check out my Github for my free-to-read JavaScript Ebook that covers all the new features from ES6 to 2021. If you want find a great place for interactive tutorials, i recommend Educative where you can find my JavaScript course
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Every year since 2015, JavaScript has been receiving constant yearly updates to its specification with new interesting features added.

In this article we will have a look at what’s to come since many features already reached stage 4 and will be included in the specification.

For those of you who don’t know, there are 4 stages in the proposal process, with the 4th one being the last one which marks the proposal as finished.

As a developer, it’s important to stay updated with the new specs of a language and if you feel like you’ve been left behind by the many updates that JavaScript received in the past years, I can recommend you my book that covers everything from the basics of the language all the way to the latest ES2020 specs, including a little intro to TypeScript. You can read it for free on Github where you will also find links where to buy the ebook or you can check out my course on Educative

Now, let’s get started with the first of the new ES2022 features:

Class Fields

Class public Instance Fields & private Instance Fields

Before ES2022 we would define properties of a class in its constructor like this:

class ButtonToggle extends HTMLElement {
    constructor(){
        super();
        // public field
        this.color = 'green'
        // private field
        this._value = true;
    }

    toggle(){
        this.value = !this.value
    }
}

const button = new ButtonToggle();
console.log(button.color);
// green - public fields are accessible from outside classes

button._value = false;
console.log(button._value);
// false - no error thrown, we can access it from outside the class

Inside of the constructor, we defined two fields. As you can see one of them is marked with an _ in front of the name which is just a JavaScript naming convention to declare the field as private meaning that it can only be accessed from inside of a class method. Of course, that’s just a naming convention and not something that the language itself enforces and that’s why when we tried to access it, it didn’t raise any error.

In ES2022 we have an easier way to declare both public and private fields. Let’s have a look at this updated example:

class ButtonToggle extends HTMLElement {
   
    color = 'green';
    #value = true;

    toggle(){
        this.#value = !this.#value;
    }
}
const button = new ButtonToggle();
console.log(button.color);
// green - public fields are accessible from outside classes

// SyntaxError - cannot be accessed or modified from outside the class
console.log(button.#value); 
button.#value = false;

The first thing to notice is that don’t have to define them inside of the constructor. Secondly, we can also define private fields by pre-pending # to their names.

The main difference with the previous example is that this time an actual error will be thrown if we try to access or modify the field outside of the class.

Private methods and getter/setters for JavaScript classes

Similar to how we did in the previous example, we can also define private methods and getter/setters for our classes.

class ButtonToggle extends HTMLElement {
   
    color = 'green'
    #value = true;

    #toggle(){
        this.#value = !this.#value
    }

    set #setFalseValue(){
        this.#value = false;
    }
}
const button = new ButtonToggle();
// SyntaxError - cannot be accessed or modified from outside the class
button.#toggle();
// SyntaxError - cannot be accessed or modified from outside the class
button.#setFalseValue;

In the example above we replaced toggle() with #toggle() thus making the toggle method private and only accessible from inside of the class.

Static class fields and private static methods

A static field or method is only accessible in the prototype and not in every instance of a class and ES2022 provides us with the means to define static fields and static public/private methods by using the static keyword.

Previously we would have to define them outside of the class body such as:

class ButtonToggle extends HTMLElement {
    // ... class body
}
ButtonToggle.toggle(){
    // static method define outside of the class body
}

Now, instead, we can define them directly inside of the class body with the use of the static keyword:

class ButtonToggle extends HTMLElement {
   
    #value = true;

    static toggle(){
        this.#value = !this.#value
    }
}
// this will work
ButtonToggle.toggle();

// SyntaxError - private static field
const button = new ButtonToggle();
button.toggle();

As you can see in the example above, we can access toggle() directly on our ButtonToggle but we cannot do the same on a new instance of it.

We can use the static keyword in front of fields and methods (both private and public) and by combining it with the # (private) we can create a private static method only accessible from inside of our prototype class.

class ButtonToggle extends HTMLElement {
   
    #value = true;

    static #toggle(){
        this.#value = !this.#value
    }
}
// this will error, it's a private static method
ButtonToggle.#toggle();

Ergonomic brand checks for private Fields

As we saw in the examples above, if we try to access a private field outside of a class it will throw an exception and will not return undefined like it does with public fields.

We could try using a simple try/catch inside of the class to check if the field exists:

class ButtonToggle extends HTMLElement {
   
   // initialised as null
    #value = null;

    get #getValue(){
        if(!this.#value){
            throw new Error('no value');
        } 
        return this.#value
    }

    static isButtonToggle(obj){
        try {
            obj.#getValue;
            return true;
        } catch {
            // could be an error internal to the getter
            return false; 
        }
    }

}

In the example above we added a private getter that will throw an error if there is no value yet. We then created a static method to access that getter and tried to determine if it exists by checking with a try/catch. The problem lies in the fact that we don’t know if the code in the catch is executed because the getter is not present or simply because it threw an error.

ES2022 provides us with an easy way to check if said field belongs to a class by using the operator in. Let’s rework our example code:

class ButtonToggle extends HTMLElement {
   
   // initialised as null
    value = null;

    get #getValue(){
        if(!this.#value){
            throw new Error('no value');
        } 
        return this.#value;
    }

    static isButtonToggle(obj){
       return #value in obj && #getValue in obj
    }

}

Our method isButtonToggle will check if the class contains the private fields ‘value’ and ‘getValue’.

Class Static Block

This is yet another upgrade to the static fields in ES2022 that allows us to have static blocks inside of classes. The issue this is trying to solve arises from the fact that we cannot evaluate statements such as a try/catch during initialization meaning that we would have to put that code outside of the class body:

class ButtonToggle{
    value = false;

    get getValue(){
        if(!this.#value){
            throw new Error('no value');
        } 
        return this.#value
    }
}

// this has to sit outside of the class body
try {
    const val = ButtonToggle.getValue;
    ButtonToggle.value = val
} catch {
    ButtonToggle.value = false
}

As you can see, our try/catch had to be put outside of the class body. Thankfully we can replace that with a static block like the following:

// method defined outside of the class body
let initVal;

class ButtonToggle{
    #value = false;

    get getValue(){
        if(!this.#value){
            throw new Error('no value');
        } 
        return this.#value
    }

    static {
        initVal = () => {
            this.#value = this.getValue;
        }
    }
}

initVal();

We created a static block inside of our class that defines a function that we declared outside of the context of that class. As you can see, the method will have access to ‘#value’ which is a private field or our class. They will have access to private methods and fields, being them instance-private (meaning non static, private fields) or static-private.

RegExp Match Indices

This upgrade will allow us to use the d character to specify that we want to get the indices (starting and ending) of the matches of our RegExp.

We can use Regexp.exec or String.matchAll to find a list of matches, with the main difference between them being that Regexp.exec returns its results one by one whereas String.matchAll returns an iterator. Let’s see them in practice:

const fruits = 'Fruits: mango, mangosteen, orange'
const regex = /(mango)/g;

// .exec
RegExp(regex).exec(fruits);
// [
//   'mango',
//   index: 8,
//   input: 'Fruits: mango, mangosteen, orange',
//   groups: undefined
// ]

// matchAll
const matches = [...fruits.matchAll(regex)];
matches[0];
// [
//   'mango',
//   'mango',
//   index: 8,
//   input: 'Fruits: mango, mangosteen, orange',
//   groups: undefined
// ]

Both return the index of the match, the match itself, and the initial input. What we don’t know are the indices at which the string ends, something that we will now be able to do like this:

const fruits = 'Fruits: mango, mangosteen, orange'
// /gd instead of the previous /g
const regex = /(mango)/gd;

const matches = [...fruits.matchAll(regex)];
matches[0];

// [
// "mango",
// "mango",
// groups: undefined
// index: 8
// indices:[]
//  [8, 13],
//  [8, 13]
// ]
// groups: undefined

As you can see it returned [8,13] as the indices of the first occurrence of ‘mango’ in our string.]

Top-level await

await operator can only be used within an async method” is probably an error you have encountered frequently. In ES2022 we will be able to use it outside of the context of an async method in our modules. For example, we could defer the execution of a module and its parent until something else is imported.

This can be useful in many scenarios, for example when we have a dynamic path for a dependency that depends on a runtime value:

// we need to get the appropriate translation keys based on the language
const translationKeys = await import(`/i18n/${navigator.language}`);

Another use could be to provide a fallback for a dependency:

let jQuery;
try {
  jQuery = await import('https://cdn-a.com/jQuery');
} catch {
  jQuery = await import('https://cdn-b.com/jQuery');
}

.at()

In JavaScript you can do arr[1] to access the value at index 1 of an Array but you cannot do arr[-1] to count backward from the ending of the Array. The reason is that the brackets syntax is used not only for arrays but also for Objects, where obj[-1] would simply refer to the property ‘-1’ of that Object.

With the .at() method we now have an easy way to access any index, positive or negative of arrays and strings:

const arr = [10,20,30,40];

// same -> 10
arr[1];
arr.at(1);

// same -> 40
arr[arr.length -1];
arr.at(-1);

Note that a negative value simply means: ‘Start counting backward from the end of the array’.  

Accessible Object.prototype.hasOwnProperty

In JavaScript we already have an Object.prototype.hasOwnProperty but, as the MDN documentation also suggests, it’s best to not use hasOwnProperty outside the prototype itself as it is not a protected property, meaning that an object could have its property called hasOwnProperty that has nothing to do with Object.prototype.hasOwnProperty.

For example:

const obj = {
    hasOwnProperty:()=> {
        return false
    }
}

obj.hasOwnProperty('prop'); // false

As you can see, we defined our own method hasOwnProperty that has overridden the one on the prototype, an issue that is not present with Object.hasOwn().

Object.hasOwn() takes our Object as the first argument and the property we want to check as the second:

const student = {
    name: 'Mark',
    age: 18
}

Object.hasOwn(student,'age'); // true
Object.hasOwn(student,'grade'); // false

Thank you very much for reading, if you enjoyed this article, please share it with friends and colleagues and if there is a topic you would like me to cover, reach out to me on twitter at @montalesi. Follow me on DevTo or on Twitter for more.

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