How to search with JavaScript in an array without distinguishing uppercase/lowercase?

I have a function that correctly searches within an array and returns the match, but it is case sensitive. I need the comparison to be done at both ends in lowercase so it doesn't distinguish.

How can I do it?

This is my current code:

function buscar(mi_array, valor, pos){
    var results = mi_array.filter(function (mi_array) { return mi_array[pos].startsWith(valor); });
    var firstObj = (results.length > 0) ? results[0] : null;
    return results;
}