In Javascript how do I create a secondary array in a specific index?

How do I create a subarray from an existing array in Javascript?

For example;

Arr = [5,2,1,2]

Then I want to insert 8 in position 1 of Arr, but keep the original value 2. So arr can become:

Arr = [5,[2,8],1,2]

I tried using concat, which sort of did something but duplicates all values.

Bear in mind that this can grow e.g. Arr = [5,[2,8,3,4],1,[2,3]]

Thanks!

>Solution :

You could assign the concatinated values.

const
    addAt = (array, value, index) => array[index] = [].concat(array[index], value),
    array = [5, 2, 1, 2];

addAt(array, 8, 1);
console.log(array)

addAt(array, 3, 1);
console.log(array)

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