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判断是否为数组
1. Array.isArray([])
2. var arr=[1,2] arr instanceof Array
-->true arr instanceof String -->false
map ---- 返回数组得出的结果
const filtered = [1, 2, 3].map(x => x*x);
// filtered: [1, 4, 9];
const filtered = [1, 2, 3].map(x => x != 1);
// filtered: [false, true, true];
filter ---- 返回一个新的数组,内容为符合条件的下标
const filtered = [1, 2, 3].filter(element => element > 1);
// filtered: [2, 3];
every ---- 检测数组中的每一个元素是否都通过了callback测试,全部通过返回true,否则返回false。
const filtered = [1, 2, 3].every(x => x != 1);
// filtered: false;
const filtered = [1, 2, 3].every(x => x != 0);
// filtered: true;