Array methods in ECMAScript 6

[mqy1023]

1、Array.prototype.find
快速获取到第一个匹配值

let numbers = [1, 2, 3];
let oddNumber = numbers.find((x) => x % 2 == 1);
console.log(oddNumber); // 1

2、Array.prototype.findIndex
返回第一个匹配值的下标

let people = ['jamie', 'jack', 'isaac'];
let jackIndex = people.findIndex((x) => x === 'jack');
console.log(jackIndex); // 1

3、Array.prototype.entries
返回一个数组迭代器(Array Iterator), that can be used to loop through the array’s keys and values. entries will return an array of arrays, where each child array is an array of [index, value].

let people = ['jamie', 'jack', 'isaac'];
let entries = people.entries();
console.log(entries.next().value); // [0, 'jamie']
console.log(entries.next().value); // [1, 'jack']
console.log(entries.next().value); // [2, 'isaac']

We can also use the spread operator to get back an array of the entries in one go:

let people = ['jamie', 'jack', 'isaac'];
let entries = people.entries();
console.log([...entries]); // [[0, 'jamie'], [1, 'jack'], [2, 'isaac']]

4、Array.from

Array.from('hello'); // ['h', 'e', 'l', 'l', 'o']

Array.from([1, 2, 3]); // [1, 2, 3]

let namesSet = new Set(['jamie', 'jack']);
Array.from(namesSet); // ['jamie', 'jack']

from can also take a second argument, which is a map function to be applied to each element:

Array.from([1, 2, 3], (x) => x * x); // [1, 4, 9]

Because the method can take array like objects, we can use it to generate arrays of values too:

Array.from({ length: 4 }, (val, key) => key); // [0, 1, 2, 3]

Each time the mapping function gets called, the val argument will be undefined, as this object has no actual values, but the key argument will be 0, then 1 and so on. This lets us generate arrays of numbers, but we can also return whatever we’d like from the mapping function:

Array.from({ length: 2 }, () => 'jack'); // ['jack', 'jack']

5、Array.map
返回新的数组

const arr = ['1', '2', '3'].map(() => false);
console.log(arr); //[false, false, false]

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