bubble sort implementation in typescript

bubbleSort.ts

export function bubbleSort(array: number[]): number[] {
    array = array.slice(); // creates a copy of the array

    for(let i = 0; i < array.length; i++) {
        for(let j = 0; j < array.length - 1; j++) {

            if(array[j] > array[j + 1]) {
                let swap = array[j];
                array[j] = array[j + 1];
                array[j + 1] = swap;
            }
        }
    }
    return array;
}

What is "array = array.slice();"
Why am I using this?
If I do not use this, what happens?

What is "array = array.slice();"
Why am I using this?
If I do not use this, what happens?

slice() without parameters returns the array itself, an easy way to copy the data. slice() accepts two parameters: the value that starts the slice you want and the value that finishes the slice.

It can be easily done with spread operator (ES6).
array = […array]

You can also add - i to the second loop so that the sorting algorithm doesn't consider sorted elements, because in each iteration of the i loop we are guaranteed to have i of sorted elements.
It will look like this:
for(let j = 0; j < array.length - i- 1; j++)

Line 2 is not necessary.