ECMAScript 2019

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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

String.trimStart()
String.trimEnd()
Object.fromEntries
Optional catch binding
Array.flat()
Array.flatMap()
Revised Array.Sort()
Revised JSON.stringify()
Separator symbols allowed in string litterals
Revised Function.toString()


Warning
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.

Example

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.

Example

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.

Example

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:.

Example
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.

Example

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.

Example

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.

Example

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 an other relelative position like
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 \.

Example

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:

Example

// This is valid in ES2019:
let text = "\u2028";

Try it Yourself »


Note
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.

Example

function myFunction(p1, p2) {
  return p1 * p2;
}

Try it Yourself »













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Reference: https://www.w3schools.com/js/js_2019.asp