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.
这 trimStart() method works like trim(), but removes whitespace only from the start of a string.
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.
这 trimEnd() method works like trim(), but removes whitespace only from the end of a string.
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.
这 fromEntries() method creates an object from iterable key / value pairs.
例子
const fruits = [
["apples", 300],
["pears", 900],
["bananas", 500]
];
const myObj = Object.fromEntries(fruits);
亲自尝试 »
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:
例子
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.
这 flat() method creates a new array by flattening a nested array.
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.
这 flatMap() method first maps all elements of an array and then creates a new array by flattening the array.
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.
例子
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 }
];
亲自尝试 »
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 \.
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.
这 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.
