数组的map()方法用于遍历数组，每遍历一个元素就调用回调方法一次，并将回调函数的返回结果作为新数组的元素，被遍历的数组不会被改变。
语法：let newAarray = arr.map(function callback(currentValue, index, array) { // Return element for newArray }
示例：
let numbers = [1, 5, 10, 15];
let doubles = numbers.map((x) => {
return x * 2;
}); // doubles is now [2, 10, 20, 30]
// numbers is still [1, 5, 10, 15] let numbers = [1, 4, 9];
let roots = numbers.map(Math.sqrt); // roots is now [1, 2, 3]
// numbers is still [1, 4, 9]
参考博文：javascript map()方法解析