9.7 JS 2019

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 1: JS Strings - The trimStart() Method

let text1 = "     Hello World!     ";
let text2 = text1.trimStart();

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 2: JS Strings - The trimEnd() Method

let text1 = "     Hello World!     ";
let text2 = text1.trimEnd();

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 3: JS Objects - The fromEntries() Method

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

const myObj = Object.fromEntries(fruits);

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

JavaScript Array flat()

• ES2019 added the Array flat() method to JavaScript.
• The flat() method creates a new array by flattening a nested array.

Example 4: JS Arrays - The flat() Method

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

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 5: JS Arrays - The flatMap() Method

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

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 6: JS Stable Sort

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 is not allowed to come out with the names in an other relative 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 \.

Example 7: JS JSON - Revised stringify()

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 8: JS Strings - Separator Symbols

// 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 9: JS Functions - The toString() Method

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