add element with index to array javascript code example

Example 1: javascript add array to array at index

a1.splice(2, 0, ...a2);

Example 2: add new element by index js

const arr = [ 1, 2, 3, 4 ];
arr[5] = 'Hello, world!';

console.log(arr); // [ 1, 2, 3, 4, <1 empty item>, 'Hello, world!' ]
console.log(arr.length); // 6