concat

concat 用来合并两个或者多个数组。concat 不影响原始数组，会返回一个新数组

const letters = ['a', 'b', 'c'];
const numbers = [1, 2, 3];

letters.concat(numbers);
// result in ['a', 'b', 'c', 1, 2, 3]

concat 可以传入数值

const letters = ['a', 'b', 'c'];

const alphaNumeric = letters.concat(1, [2, 3]);

console.log(alphaNumeric); 
// results in ['a', 'b', 'c', 1, 2, 3]

concat 内部是对原对象的引用，原对象改变，concat的返回对象也改变

const num1 = [[1]];
const num2 = [2, [3]];

const numbers = num1.concat(num2);

console.log(numbers);
// results in [[1], 2, [3]]

// modify the first element of num1
num1[0].push(4);

console.log(numbers);
// results in [[1, 4], 2, [3]]