Insert an item into array at a specific index

Let's assume that we have the following array and we want to complete it:

javascript
let numbers = [1, 2, 3, 4, 6, 7];

Since the number 5 is missing, we want to insert it between 4 and 6. We can slice the array in two parts and then use the spread operator ... to put everything together again:

javascript
// We are slicing the array in two parts: 0 - 4 and 4 - End
let first  = numbers.slice(0, 4);
let second = numbers.slice(4);

// We insert number 5 between those two parts
let completed = [...first, 5, ...second];
// [1, 2, 3, 4, 5, 6, 7]

In one line of code:

javascript
let completed = [...numbers.slice(0, 4), 5, ...numbers.slice(4)];
// [1, 2, 3, 4, 5, 6, 7]