Javascript - insert an array inside another array

You can now do this if using ES2015 or later:

var a1 = [1,2,3,4,5];
var a2 = [21,22];
a1.splice(2, 0, ...a2);
console.log(a1) // => [1,2,21,22,3,4,5]

Refer to this for documenation on the spread (...) operator https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator

You can use splice combined with some apply trickery:

a1 = [1,2,3,4,5];
a2 = [21,22];

a1.splice.apply(a1, [2, 0].concat(a2));

console.log(a1); // [1, 2, 21, 22, 3, 4, 5];

In ES2015+, you could use the spread operator instead to make this a bit nicer

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