js 数组方法

1 Array.from

Array.from 用于将类数组转换成数组。只要有length属性

Array.from({length:3})  // [undefined, undefined, undefined]

第二个参数回调可以对数组进行map

Array.from({length:3}, val => 'kimi')  // ['kimi', 'kimi', 'kimi']

2 Array.of

Array.of用于新建数组，来弥补之前构造函数Array()创建的不足。

Array() // []
Array(3) // [, , ]   一个参数代表长度
Array(1,2,3) // [1,2,3]

Array.of() // []
Array.of(3) // [3]
Array.of(1,2,3) // [1,2,3] 

3 Array.keys()、 Array.values()、 Array.entries()

这三个api都用于遍历数组，返回的都是一个遍历器对象，用for of遍历。

const a = [1,2,3].keys(); // Iterator
const b = [1,2,3].values(); // Iterator
const c = [1,2,3].entries(); // Iterator
for (let [v, k] of c) {
    console.log(v, k)  // 0 1,   1 2,   2 3
}
console.log(a.next().value) // 0 

4 Array.map

map polyfill

Array.prototype._map = function(fn, context) {
    const results = [];
    for (let i = 0; i < this.length; i++) {
        results.push(fn.call(context, this[i], i, this));
    }
    return results;
}
const a = [1,2,3,45]
console.log(a._map(v => v*2));

5 Array.filter

filter polyfill

Array.prototype._filter = function(fn, context) {
    const results = [];
    for(let i = 0; i < this.length; i++) {
        if (fn.call(context, this[i], i, this)) results.push(this[i])
    }
    return results;
}
const a = [1,2,3,4,5,7];
console.log(a._filter(val => val > 4));

6 Array.some()、 Array.every()

polyfill

Array.prototype._every = function(fn, context) {
    for(let i = 0; i < this.length; i++) {
        if (!fn.call(context, this[i], i, this)) return false;
    }
    return true;
}
const b = ['a', 'v', 'abc'];
const c = {a:'a'}
console.log(b._every(function(val) {
    return val.indexOf(this.a) > -1
}, c))

Array.prototype._some = function(fn, context) {
    for (let i = 0; i < this.length; i++) {
        if (fn.call(context, this[i], i, this)) return true;
    }
    return false;
}
const b = ['e', 'av', 'bc'];
const c = {a:'a'}
console.log(b._some(function(val) {
    return val.indexOf(this.a) > -1
}, c))

7 Array.reduce()

reduce polyfill

Array.prototype._reduce = function(fn, initVal) {
    let res = initVal || Array.prototype.shift.call(this),
        startI = initVal ? 0 : 1;
    
    for (let i = startI; i < this.length; i++) {
        res = fn.call(this, res, this[i], i, this);
    }
    return res;
}

let a = [1,2,3,4].reduce(function(memo, val, index, arr) {
    return memo += val;
})