If you see the reference for Array.prototype.splice on mdn:

The splice() method changes the contents of an array by removing or replacing existing elements and/or adding new elements in place

Return Value: An array containing the deleted elements.

At the end of your code, when you do .splice(1, 0, '.'), it deletes 0 elements so it will return an empty array.

I'm assuming you want to get the previous array with the '.' element inserted in the first position. For that you'll want to do:

const arr = num.toString().split('').splice(3);
arr.splice(1, 0, '.');
console.log(arr)

And If you want to join it to a string just use arr.join('')

Anyway, for your use case, you could just use the * multiplication operator rather than converting to a string array and attempting to manipulate that.

You can simply do:

let num = 0.0134;
console.log(num * 100);