It’s a string manipulation problem with JS. I cant figure out why the main test fails

A string manipulation exercise that is failimg in the main test.

I tried to figure out if some character was missing or broken, but I can’t figure out.

function hasNoneLetters(phrase, blacklist) {
    let x = phrase.toLowerCase();
    let y = blacklist.toLowerCase();
    let z = '';
    for (let i = 0; i <= blacklist.length-1; i++) {
        z = blacklist[i];
        if (x.lastIndexOf(z)) {
            return false;
    return true;

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