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JavaScript. W3Schools in English. Lessons for beginners

Ua

ECMAScript 2021

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JavaScript Version Numbers

Old JS versions are named by numbers: ES5 (2009) and ES6 (2015).

From 2016, versions are named by year: ECMAScript 2016, 2017, 2018, 2019, ...

New Features in ES2021

New Features in ES2022

  • Array at()
  • String at()
  • RegExp /d
  • Object.hasOwn()
  • error.cause
  • await import
  • Private methods and fields
  • Class field declarations

Warning

These features are relatively new.

Older browsers may need an alternative code (Polyfill)

JavaScript String ReplaceAll()

ES2021 introduced the string method replaceAll():

Example

text = text.replaceAll("Cats","Dogs");
text = text.replaceAll("cats","dogs");
Try it Yourself »

The replaceAll() method allows you to specify a regular expression instead of a string to be replaced.

If the parameter is a regular expression, the global flag (g) must be set; otherwise a TypeError is thrown.

Example

text = text.replaceAll(/Cats/g,"Dogs");
text = text.replaceAll(/cats/g,"dogs");
Try it Yourself »

Note

ES2020 introduced the string method matchAll().

JavaScript Numeric Separator (_)

ES2021 intoduced the numeric separator (_) to make numbers more readable:

Example

const num = 1_000_000_000;
Try it Yourself »

The numeric separator is only for visual use.

Example

const num1 = 1_000_000_000;
const num2 = 1000000000;
(num1 === num2);
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The numeric separator can be placed anywhere in a number:

Example

const num1 = 1_2_3_4_5;
Try it Yourself »

Note

The numeric separator is not allowed at the beginning or at the end of a number.

In JavaScript only variables can start with _.

The numeric separator is supported in all modern browsers since January 2020:

Chrome 75 Edge 79 Firefox 74 Safari 13.1 Opera 67
Jun 2019 Jan 2020 Oct 2019 Sep 2019 Jun 2019

JavaScript Array at()

ES2022 intoduced the array method at():

Examples

Get the third element of fruits:

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fruit = fruits.at(2);
Try it Yourself »

Get the third element of fruits:

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fruit = fruits[2];
Try it Yourself »

The at() method returns an indexed element from an array.

The at() method returns the same as [].

The at() method is supported in all modern browsers since March 2022:

Chrome 92 Edge 92 Firefox 90 Safari 15.4 Opera 78
Apr 2021 Jul 2021 Jul 2021 Mar 2022 Aug 2021

Note

Many languages allows negative bracket indexing like [-1] to access elements from the end of an object / array / string.

This is not possible in JavaScript, because [] is used for accessing both arrays and objects. obj[-1] refers to the value of key -1, not to the last property of the object.

The at() method was introduced in ES2022 to solve this problem.

JavaScript String at()

ES2022 intoduced the string method at():

Examples

Get the third letter of name:

const name = "W3Schools";
let letter = name.at(2);
Try it Yourself »

Get the third letter of name:

const name = "W3Schools";
let letter = name[2];
Try it Yourself »

The at() method returns an indexed element from an string.

The at() method returns the same as [].

The at() method is supported in all modern browsers since March 2022:

Chrome 92 Edge 92 Firefox 90 Safari 15.4 Opera 78
Apr 2021 Jul 2021 Jul 2021 Mar 2022 Aug 2021
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