What's new in ES2022? 🤔

Soon the new version of ECMA Script will become standard in few months. So let's take a glimpse at the features that will be part of ES2022.

Features of ES2022

1. Method .at() of indexable values.

This function let's us reads an element at the given index. It can accept negative indexes to read elements from the end of the given datatype.

For example

[1,2,3,4,5].at(3)  // returns 4

[1,2,3,4,5].at(-2)   // returns 4

Datatypes supporting this function.

• String
• Array
• All Typed Array classes: Uint8Array etc.

2. RegExp match indices

Adding a flag /d to a regular expression produces match objects that records the start and end of each group capture.

There are different ways to match indices

• Match indices for numbered group

const matchObj = /(a+)(b+)/d.exec('aaaabb');

console.log(matchObj);
/*
Output -
['aaaabb', 'aaaa', 'bb', index: 0, input: 'aaaabb', groups: undefined, indices: Array(3)]
*/

Due to the regular expression flag /d, matchObj also has a property .indices that records for each numbered group where it was captured in the input string.

matchObj.indices[1];
/*
Output - 
[0, 4]
*/

• Match indices for named groups

const matchObj = /(?<as>a+)(?<bs>b+)/d.exec('aaaabb');

console.log(matchObj);
/*
Output - 
['aaaabb', 'aaaa', 'bb', index: 0, input: 'aaaabb', groups: {as: 'aaaa', bs: 'bb'}, indices: Array(3)]
*/

Their indices are stored in matchObj.indices.groups

matchObj.indices.groups;
/*
Output -
{ as: [0,4], bs: [4,6] }
*/

3. Object.hasOwn(obj, propKey)

It is a safe way to check that propKey is the own property of obj object. It is similar to Object.prototype.hasOwnProperty but it supports all object types.

const proto = {
  protoProp: 'protoProp',
};

const obj = {
  __proto__: proto,
  objProp: 'objProp',
};

console.log('protoProp' in obj); // output - true.
console.log(Object.hasOwn(obj, 'protoProp')) // output - false
console.log(Object.hasOwn(proto, 'protoProp')); // output - true.

4. error.cause

Error and it's subclasses now let us specify the reason behind the error. This is useful in deeply nested function where we have chained error blocks to quickly find the error. Read here for more info

function readFiles(filePaths) {
  return filePaths.map(
    (filePath) => {
      try {
        // ···
      } catch (error) {
        throw new Error(
          `While processing ${filePath}`,
          {cause: error}
        );
      }
    });
}

5. Top-level await modules

We can now use await at the top levels of modules and don’t have to enter async functions or methods anymore.

• Loading modules dynamically

const messages = await import(`./messages-${language}.mjs`);

• Using a fallback if module loading fails

let lodash;
try {
  lodash = await import('https://primary.example.com/lodash');
} catch {
  lodash = await import('https://secondary.example.com/lodash');
}

• Using whichever resource loads fastest

const resource = await Promise.any([
  fetch('http://example.com/first.txt')
    .then(response => response.text()),
  fetch('http://example.com/second.txt')
    .then(response => response.text()),
]);

6. New members of classes

• Public properties can be created via
  • Instance public fields

class InstPublicClass {
  // Instance public field
  instancePublicField = 0; // (A)

  constructor(value) {
    // We don’t need to mention .property elsewhere!
    this.property = value; // (B)
  }
}

const inst = new InstPublicClass('constrArg');

• Static public fields

const computedFieldKey = Symbol('computedFieldKey');
class StaticPublicFieldClass {
  static identifierFieldKey = 1;
  static 'quoted field key' = 2;
  static [computedFieldKey] = 3;
}
console.log(StaticPublicFieldClass.identifierFieldKey) //output -> 1
console.log(StaticPublicFieldClass['quoted field key']) //output -> 2
console.log(StaticPublicFieldClass[computedFieldKey]) //output -> 3

• Private slots are new and can be created via
  • Instance private fields

class InstPrivateClass {
  #privateField1 = 'private field 1'; // (A)
  #privateField2; // (B) required!
  constructor(value) {
    this.#privateField2 = value; // (C)
  }
  /**
   * Private fields are not accessible outside the class body.
   */
  checkPrivateValues() {
  console.log(this.#privateField1); // output -> 'private field 1'
  console.log(this.#privateField2); // output -> 'constructor argument'

  }
}

const inst = new InstPrivateClass('constructor argument');
  inst.checkPrivateValues();


console.log("inst", Object.keys(inst).length === 0) //output -> inst, true

• Instance and static private fields

class InstPrivateClass {
  #privateField1 = 'private field 1'; // (A)
  #privateField2; // (B) required!
  static #staticPrivateField = 'hello';
  constructor(value) {
    this.#privateField2 = value; // (C)
  }
  /**
   * Private fields are not accessible outside the class body.
   */
  checkPrivateValues() {
    console.log(this.#privateField1); // output -> 'private field 1'
    console.log(this.#privateField2); // output -> 'constructor argument'

  }

  static #twice() {
    return this.#staticPrivateField + " " + this.#staticPrivateField;
  }

  static getResultTwice() {
    return this.#twice()
  }
}

const inst = new InstPrivateClass('constructor argument');
inst.checkPrivateValues();


console.log("inst", Object.keys(inst).length === 0) //output -> "inst", true
console.log(InstPrivateClass.getResultTwice()); // output -> "hello hello"

• Private methods and accessors

class MyClass {
  #privateMethod() {}
  static check() {
    const inst = new MyClass();

    console.log(#privateMethod in inst) // output-> true

    console.log(#privateMethod in MyClass.prototype) // output-> false

    console.log(#privateMethod in MyClass) // output-> false
  }
}
MyClass.check();

• Static initialisation blocks in classes. For static data we have Static fields and Static Blocks that are executed when the class is created.

class Translator {
  static translations = {
    yes: 'ja',
    no: 'nein',
    maybe: 'vielleicht',
  };
  static englishWords = [];
  static germanWords = [];
  static { // (A)
    for (const [english, german] of Object.entries(this.translations)) {
      this.englishWords.push(english);
      this.germanWords.push(german);
    }
  }
}


console.log(Translator.englishWords, Translator.germanWords)
//Output -> ["yes", "no", "maybe"], ["ja", "nein", "vielleicht"]

• Private slot checks - This functionality helps us to check that the object has the given private slot in it.

class C1 {
  #priv() {}
  static check(obj) {
    return #priv in obj;
  }
}

console.log(C1.check(new C1())) // output true

These amazing features will help us to enhance our projects and improve our coding techniques. I am really excited to try these features out in my project. 💃

Happy Coding! 👩🏻‍💻

Comments

[Baraa Baba]

I will miss arr[arr.Length -2]

[Jasmin Virdi]

True 😅

[Massimo Artizzu]

... or not! 😅

[Anna R Dunster]

Don't worry, you can still use it if you want ;)

[Blessing Hirwa]

I didn't like this one. It could mislead me sometimes 😂/

[Adnane Chaikhi]

hhhhhhhhhhhhhhhhh

[Mohamed ID-ABDELLAH]

so true hahahahah

[🚩 Atul Prajapati 🇮🇳]

Ohh wow

[Jasmin Virdi]

Thanks👍

[Rasmus Schultz]

What is error.cause for?

Normally, the cause of the error is what I would put in the error message. I mean, what else would you put there?

Why do we need another field for that?

I don't know of any other language that has two text messages in the same exception. What for? 🤷‍♂️

[Alex Bea]

The error message might sometimes be used for a public-facing message. You could then put more technical information in the cause as well. That's my guess, at least.

[Rasmus Schultz]

Ah, mystery solved!

2ality.com/2021/06/error-cause.html

It's for chaining errors - like the "inner exception" you have in many other languages.

Because you can throw anything in JS, the property type isn't specified as Error - but the intention is similar.

[Jasmin Virdi]

Thanks for attaching this link for reference. 🙂

The way this would be super helpful is in deeply nested functions with chained errors.
Like if we have mutiple try catch block inside a deeply nested function so we pass the string as the purpose of that block and cause as the argument of catch block. For example:

function deeplyNested() {
  try {
    // outer block logic.

    let appData = [{...}]
    // app data array parsing.
    appData.map(
      (data) => {
        try {
          // ···
        } catch (error) {
          throw new Error(`While processing ${data}`, {
            cause: error
          });
        }
      });
  } catch (error) {
    throw Error('Error while processing outer block', {
      cause: error
    });
  }
}

[Rasmus Schultz]

That sounds unlikely?

At least, I've never heard of anyone using exceptions for display purposes - exceptions generally do contain technical information, they are usually intended for developers, not for users. How would you deal with localization, for instance... That's just not what exceptions are for...

[Juvenal PENGELE]

I may be wrong but I do not think this is correct "For static data we have Static fields and Static Blocks that are executed every time a new instance is created.".

Static blocks are evaluated once during Class definition not every time a new instance is created.
When a new instance is created, it is the code in the construction that is executed.

[Jasmin Virdi]

Yes you are right, static blocks are executed when a class is created not in case of instance. Updated the statement.

Thanks👍

[Brett Zamir]

Informative post, thank you. There is apparently an error in the output listed for matchObj.indices.groups however which should instead by an object keyed by group.

[Jasmin Virdi]

Hey,

Actually, I included the full output which is an array having an object keyed by group.

I should’ve included just the groups output for more clarity.
Thanks for pointing out buddy!😀👍

[Brett Zamir]

This is what I see as the output showing on the page:

matchObj.indices.groups;
/*
Output -
[0,4]
*/

There's no object showing there...

[Jasmin Virdi]

I was referring to the snippet just above this one 😅
I should’ve returned the full object instead of returning just the indices.

Updated the post. Thanks 👍

[Conner Ow]

Top-level await is what I've been waiting for forever!!

Can't wait for these to come out!!

[sanidz]

flatMap operator is alternative but sure its nice to have

[Mike Bybee]

I'm very excited for being able to use the top level await examples you gave (minus the lodash part).

[Jasmin Virdi]

😁

[Jim Aho]

Can you give an example as of why?

/ C# dude here

[Mateusz Skrobiś]

superb post, thanks.

[Jasmin Virdi]

🙂

[SeongKuk Han]

Thank you for sharing the post. :)

[Jasmin Virdi]

🙂

[Sahan]

Thanks for the breakdown. Nicely written too 👏

[Jasmin Virdi]

Thanks👍