// 原始赋值
let a = [1, 2, 3]
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// 利用call和apply方法
var a = [1, 2, 3, 4], b = ['1', '2', '3']
a.splice.call(a, 0, a.length, ...b);
console.log(a)
(3) ["1", "2", "3"]
-----------------------------------------
var a = [1, 2, 3, 4], b = ['a', 'b', 'c']
a.splice.apply(a, [0, a.length, ...b]);
console.log(a)
(3) ["a", "b", "c"]
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// es6
let a = [...b, ...c]