Flash cards
Review the key moves
What is the main idea behind ECMAScript 2019?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ text1 = " Hello World! ";Put the learning moves in the order that makes the concept easiest to apply.
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()
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()
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()
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:
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.
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 110Revised 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 ���.
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";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();