JavaScript 实现 —— 将一个数组插入另一个数组中

方法一： unshift + splice

var arr1 = ['a', 'b', 'c'];   
var arr2 = ['1', '2', '3'];   
var index = 2;   
arr2.unshift(index, 0);  
Array.prototype.splice.apply(arr1, arr2);   
console.log(arr1);     // ["a", "b", "1", "2", "3", "c"] 

方法二：splice + concat

var arr1 = ['a', 'b', 'c'];   
var arr2 = ['1', '2', '3'];   
var index = 2;   
arr1.splice.apply(arr1, [index, 0].concat(arr2));
console.log(arr1);     // ["a", "b", "1", "2", "3", "c"] 

方法三： splice + ES6 rest

var arr1 = ['a', 'b', 'c'];   
var arr2 = ['1', '2', '3']; 
arr1.splice(2, 0, ...arr2)
console.log(arr1)   //  ["a", "b", "1", "2", "3", "c"]