How to insert into an array at a specific index in JavaScript?

You want the splice function on the native array object.

arr.splice(index, 0, item) will insert item into arr at the specified index (deleting 0 items first and then insert item at index).

In this example we will create an array and add an element to it into index 2:

let arr = [];
arr[0] = "Jani";
arr[1] = "Hege";
arr[2] = "Stale";
arr[3] = "Kai Jim";
arr[4] = "Borge";

console.log(arr.join()); // Jani, Hege, Stale, Kai Jim,Borge
arr.splice(2, 0, "Lene");
console.log(arr.join()); // Jani, Hege, Lene, Stale, Kai Jim, Borge

Why is it called splice and not insert or remove?

Splice means to join or connect, also to change. You have an established array that you are now "changing" which would involve adding or removing elements.