第一种：数组的 concat方法

concat() 方法用于合并两个或多个数组。此方法不会更改现有数组，而是返回一个新数组。

   let arr1 = [1, 2, 3];
   let arr2 = [4, 5, 6, 7];
   let arr3 = arr1.concat(arr2);
   
   console.log(arr1);  // [ 1, 2, 3 ]
   console.log(arr2);  // [ 4, 5, 6, 7 ]
   console.log(arr3);  // [ 1, 2, 3, 4, 5, 6, 7 ]

第二种：ES6的...运算符

  let arr1 = [1, 2, 3];
  let arr2 = [4, 5, 6, 7];
  let arr3 = [...arr1,...arr2];

  console.log(arr1);  // [ 1, 2, 3 ]
  console.log(arr2);  // [ 4, 5, 6, 7 ]
  console.log(arr3);  // [ 1, 2, 3, 4, 5, 6, 7 ]

第三种：使用apply

 let arr1 = [1, 2, 3];
 let arr2 = [4, 5, 6, 7];
 Array.prototype.push.apply(arr1, arr2);

 console.log(arr1);  // [ 1, 2, 3, 4, 5, 6, 7 ]
 console.log(arr2);  // [ 4, 5, 6, 7 ]

第四种：for循环遍历

   let arr1 = [1, 2, 3];
   let arr2 = [4, 5, 6, 7];
   arr2.forEach(el => {
      arr1.push(el);
   });
   console.log(arr1);  // [ 1, 2, 3, 4, 5, 6, 7 ]
   console.log(arr2);  // [ 4, 5, 6, 7 ]