JS 2019

New Features in JavaScript 2019

Browser Support

ECMAScript 2019 is supported in all modern browsers since January 2020:

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();

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();

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);

Optional catch Binding

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

Example

try {

  // code

} catch (err) {

  // code

}

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();

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, x * 10]);

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 }

];

In the example above, when sorting on price, the result should not have names in a different relative position, such as 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");

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";

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;

}

let text = myFunction.toString();