ES2019.MD

ES2019 - ES10

See the ES2019 standard for full specification of the ECMAScript 10 language.

ES10 includes the following new feature proposals:

• Array.prototype.{flat,flatMap}
• Object.fromEntries
• String.prototype.{trimStart,trimEnd}
• Symbol.prototype.description
• Optional catch binding
• Array.prototype.sort()

ECMAScript 10 Features

Array.prototype.{flat,flatMap}

The flat() method enables you to easily concatenate all sub-array elements of an array.

Example:

let arr = ['a', 'b', ['c', 'd']];
let flattened = arr.flat();

console.log(flattened);    // => ["a", "b", "c", "d"]

arr = ['a', , , 'b', ['c', 'd']];
flattened = arr.flat();

console.log(flattened);    // => ["a", "b", "c", "d"]

arr = [10, [20, [30]]];

console.log(arr.flat());     // => [10, 20, [30]]
console.log(arr.flat(1));    // => [10, 20, [30]]
console.log(arr.flat(2));    // => [10, 20, 30]

The flatMap() method combines map() and flat() into one method. It first creates a new array with the return value of a provided function and then concatenates all sub-array elements of the array.

Example:

const arr = [4.25, 19.99, 25.5];

console.log(arr.map(value => [Math.round(value)]));    
// => [[4], [20], [26]]

console.log(arr.flatMap(value => [Math.round(value)]));    
// => [4, 20, 26]

Object.fromEntries

This static method allows you to easily transform a list of key-value pairs into an object.

Example:

const myArray = [['one', 1], ['two', 2], ['three', 3]];
const obj = Object.fromEntries(myArray);

console.log(obj);    // => {one: 1, two: 2, three: 3}

String.prototype.{trimStart,trimEnd}

The trimStart() and trimEnd() methods are technically the same as trimLeft() and trimRight(). These methods are currently stage 4 proposals and will be added to the specification for consistency with padStart() and padEnd().

Example:

const str = "   string   ";

// es2019
console.log(str.trimStart());    // => "string   "
console.log(str.trimEnd());      // => "   string"

// the same as
console.log(str.trimLeft());     // => "string   "
console.log(str.trimRight());    // => "   string"

For web compatibility, trimLeft() and trimRight() will remain as aliases for trimStart() and trimEnd().

Symbol.prototype.description

When creating a Symbol, you can add a description to it for debugging purposes. Sometimes, it’s useful to be able to directly access the description in your code.

This ES2019 proposal adds a read-only description property to the Symbol object, which returns a string containing the description of the Symbol.

Example:

let sym = Symbol('foo');
console.log(sym.description);    // => foo

sym = Symbol();
console.log(sym.description);    // => undefined

// create a global symbol
sym = Symbol.for('bar');
console.log(sym.description);    // => bar

Optional catch binding

This makes a small change to the ECMAScript specification that allows you to omit the catch binding and its surrounding parentheses.

Example:

try {
  // use a feature that the browser might not have implemented
} catch {
  // do something that doesn’t care about the value thrown
}

Array.prototype.sort()

The sort() method sorts the elements of an array in place and returns the reference to the same array, now sorted. The default sort order is ascending, built upon converting the elements into strings, then comparing their sequences of UTF-16 code units values.

Example:

const months = ['March', 'Jan', 'Feb', 'Dec'];
months.sort();
console.log(months);
// Expected output: Array ["Dec", "Feb", "Jan", "March"]

const array1 = [1, 30, 4, 21, 100000];
array1.sort();
console.log(array1);
// Expected output: Array [1, 100000, 21, 30, 4]