JS Tutorial

JS HOME JS Introduction JS Where To JS Output JS Statements JS Syntax JS Comments JS Variables JS Let JS Const JS Operators JS Arithmetic JS Assignment JS Data Types JS Functions JS Objects JS Events JS Strings JS String Methods JS String Search JS String Templates JS Numbers JS Number Methods JS Arrays JS Array Methods JS Array Sort JS Array Iteration JS Array Const JS Dates JS Date Formats JS Date Get Methods JS Date Set Methods JS Math JS Random JS Booleans JS Comparisons JS If Else JS Switch JS Loop For JS Loop For In JS Loop For Of JS Loop While JS Break JS Iterables JS Sets JS Maps JS Typeof JS Type Conversion JS Bitwise JS RegExp JS Errors JS Scope JS Hoisting JS Strict Mode JS this Keyword JS Arrow Function JS Classes JS Modules JS JSON JS Debugging JS Style Guide JS Best Practices JS Mistakes JS Performance JS Reserved Words

JS Versions

JS Versions JS 2009 (ES5) JS 2015 (ES6) JS 2016 JS 2017 JS 2018 JS 2019 JS 2020 JS 2021/2022 JS IE / Edge JS History

JS Objects

Object Definitions Object Properties Object Methods Object Display Object Accessors Object Constructors Object Prototypes Object Iterables Object Sets Object Maps Object Reference

JS Functions

Function Definitions Function Parameters Function Invocation Function Call Function Apply Function Bind Function Closures

JS Classes

Class Intro Class Inheritance Class Static

JS Async

JS Callbacks JS Asynchronous JS Promises JS Async/Await


DOM Intro DOM Methods DOM Document DOM Elements DOM HTML DOM Forms DOM CSS DOM Animations DOM Events DOM Event Listener DOM Navigation DOM Nodes DOM Collections DOM Node Lists

JS Browser BOM

JS Window JS Screen JS Location JS History JS Navigator JS Popup Alert JS Timing JS Cookies


Web API Intro Web Forms API Web History API Web Storage API Web Worker API Web Fetch API Web Geolocation API


AJAX Intro AJAX XMLHttp AJAX Request AJAX Response AJAX XML File AJAX PHP AJAX ASP AJAX Database AJAX Applications AJAX Examples



JS vs jQuery

jQuery Selectors jQuery HTML jQuery CSS jQuery DOM

JS Graphics

JS Graphics JS Canvas JS Plotly JS Chart.js JS Google Chart JS D3.js

JS Examples

JS Examples JS HTML DOM JS HTML Input JS HTML Objects JS HTML Events JS Browser JS Editor JS Exercises JS Quiz JS Bootcamp JS Certificate

JS References

JavaScript Objects HTML DOM Objects

JavaScript. W3Schools in English. Lessons for beginners


ECMAScript 2019

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 ES2019


These features are relatively new.

Older browsers may need an alternative code (Polyfill)

JavaScript String trimStart()

ES2019 added the String method trimStart() to JavaScript.

The trimStart() method works like trim(), but removes whitespace only from the start of a string.


let text1 = "     Hello World!     ";
let text2 = text1.trimStart();
Try it Yourself »

JavaScript String trimStart() is supported in all modern browsers since January 2020:

Chrome 66 Edge 79 Firefox 61 Safari 12 Opera 50
Apr 2018 Jan 2020 Jun 2018 Sep 2018 May 2018

JavaScript String trimEnd()

ES2019 added the String method trimEnd() to JavaScript.

The trimEnd() method works like trim(), but removes whitespace only from the end of a string.


let text1 = "     Hello World!     ";
let text2 = text1.trimEnd();
Try it Yourself »

JavaScript String trimEnd() is supported in all modern browsers since January 2020:

Chrome 66 Edge 79 Firefox 61 Safari 12 Opera 50
Apr 2018 Jan 2020 Jun 2018 Sep 2018 May 2018

JavaScript Object fromEntries()

ES2019 added the Object method fromEntries() to JavaScript.

The fromEntries() method creates an object from iterable key / value pairs.


const fruits = [
["apples", 300],
["pears", 900],
["bananas", 500]

const myObj = Object.fromEntries(fruits);
Try it Yourself »

JavaScript Object fromEntries() is supported in all modern browsers since January 2020:

Chrome 73 Edge 79 Firefox 63 Safari 12.1 Opera 60
Mar 2019 Jan 2020 Oct 2018 Mar 2019 Apr 2019

Optional catch Binding

From ES2019 you can omit the catch parameter if you don't need it:


Before 2019:

try {
// code
} catch (err) {
// code

After 2019:

try {
// code
} catch {
// code

Optional catch binding is supported in all modern browsers since January 2020:

Chrome 66 Edge 79 Firefox 58 Safari 11.1 Opera 53
Apr 2018 Jan 2020 Jan 2018 Mar 2018 May 2018

JavaScript Array flat()

ES2019 added the Array flat() method to JavaScript.

The flat() method creates a new array by flattening a nested array.


const myArr = [[1,2],[3,4],[5,6]];
const newArr = myArr.flat();
Try it Yourself »

JavaScript Array flat() is supported in all modern browsers since January 2020:

Chrome 69 Edge 79 Firefox 62 Safari 12 Opera 56
Sep 2018 Jan 2020 Sep 2018 Sep 2018 Sep 2018

JavaScript Array flatMap()

ES2019 added the Array flatMap() method to JavaScript.

The flatMap() method first maps all elements of an array and then creates a new array by flattening the array.


const myArr = [1, 2, 3, 4, 5, 6];
const newArr = myArr.flatMap((x) => x * 2);
Try it Yourself »

Stable Array sort()

ES2019 revised the Array sort() method.

Before 2019, the specification allowed unstable sorting algorithms such as QuickSort.

After ES2019, browsers must use a stable sorting algorithm:

When sorting elements on a value, the elements must keep their relative position to other elements with the same value.


const myArr = [
  {name:"X00",price:100 },
  {name:"X01",price:100 },
  {name:"X02",price:100 },
  {name:"X03",price:100 },
  {name:"X04",price:110 },
  {name:"X05",price:110 },
  {name:"X06",price:110 },
  {name:"X07",price:110 }
Try it Yourself »

In the example above, when sorting on price, the result is not allowed to come out with the names in another relelative position like this:

X01 100
X03 100
X00 100
X03 100
X05 110
X04 110
X06 110
X07 110

Revised JSON.stringify()

ES2019 revised the JSON stringify() method.

Before 2019, JSON could not stringify character encoded with \.


let text = JSON.stringify("\u26D4");
Try it Yourself »

Before ES2019, using JSON.stringify()JSON on UTF-8 code points (U+D800 to U+DFFF) returned broken Unicode characters like ���.

After this revision, strings with UTF-8 code points convert safely with JSON.stringify(), and back to the original using JSON.parse().

Separator Symbols

Line separators and paragraph separator symbols (\u2028 and \u2029) are now allowed in string literals.

Before 2019, these were treated as line terminators and resulted in error exceptions:


// This is valid in ES2019:
let text = "\u2028";
Try it Yourself »


Now, JavaScript and JSON have equal rules.

Before ES2019:

text = JSON.parse('"\u2028"') would parse to ''.

text = '"\u2028"' would give syntax error.

Revised Function toString()

ES2019 revised the Function toString() method.

The toString() method returns a string representing the source code of a function.

From 2019, toString() must return the source code of the function including comments, spaces, and syntax details.

Before 2019, different browsers returned different variants of the function (like without comments and spaces). From 2019 the function should be returned exactly as it is written.


function myFunction(p1, p2) {
  return p1 * p2;
Try it Yourself »