How to iterate over two arrays and exclude duplicates from them, taking into account that duplicates can be located asynchronously?

Question

Arrays

Can you please tell me how to iterate over two arrays and exclude duplicates from them, taking into account that duplicates can be located asynchronously?
Example
There are 2 arrays (numbers are arbitrary)
let a = [1, 2, 3,4,5, -1,9]
let b - [3,4, -1,5,6,7]
After iteration, return array a without the elements that are in the array b. The array b does not need to be changed.
t.s. Array a should be = [1,2,9]
Array b unchanged
Thank you!

Answer 1

You can sort both arrays and then walk through them with two pointers in parallel, as in a merge.
Or add a second array to some hash table and then iterate over the first array to see if the current element is in the table.