ECMAScript 2019
New Features in JavaScript 2019
| Feature | Description | |
|---|---|---|
| String trimStart() | Removes whitespace from the start of a string | |
| String trimEnd() | Removes whitespace from the end of a string | |
| Object.fromEntries | Creates an object from key/value pairs | |
| Optional catch binding | Allows to omit the catch parameter if it is not needed | |
| Array flat() | Creates a new array by flattening a nested array | |
| Array flatMap() | Maps all array elements and creates a new array by flattening the array | |
| Revised Array sort() | Browsers must now use a stable sorting algorithm | |
| Revised JSON.stringify() | Strings with UTF-8 code points now convert safely | |
| Separator symbols in strings | Line and paragraph separators (\u2028 and \u2029) are now allowed in strings | |
| Revised Function.toString() | Now returns source code including comments and spaces and syntax details |
Browser Support
ECMAScript 2019 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 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.
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.
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);
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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
}
JavaScript Array flat()
ES2019 added the Array flat() method to JavaScript.
The flat() method creates a new array by flattening a nested array.
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]);
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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 }
];
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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 \.
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:
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();
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