类数组转数组

arrayLikeConvert.js

const arrLike = {
    length: 4,
    2: "foo"
};
Array.from(arrLike); // [undefined, undefined, "foo", undefined] , length 是类数组对象必须的属性

// Array.from还可以接受一个映射函数作为第二个参数类似Array.map(..)
var arrLike = {
    length: 4,
    2: "foo"
};

Array.from(arrLike, function mapper(val, idx){
    if (typeof val == "string") {
        return val.toUpperCase();
    }
    else {
        return idx;
    }
} );
// [ 0, 1, "FOO", 3 ]