ето с някои поправки но може да го рефакторираш както ти е удобно на теб:

function merlahShake(input) {
 
    let text = input.shift();
    let pattern = input.shift();
 
    while (pattern.length > 0) {
        let firstMatch = text.indexOf(pattern);
        let lastMatch = text.lastIndexOf(pattern);
        if ((firstMatch > -1 && lastMatch > -1) && firstMatch !== lastMatch) {
        let firstRemove = text.substr(0, firstMatch);
        let lastRemove = text.substr(firstMatch + pattern.length);
        text = firstRemove + lastRemove;
        lastMatch = text.lastIndexOf(pattern);
        firstRemove = text.substring(0, lastMatch);
        lastRemove = text.substring(lastMatch + pattern.length);
        text = firstRemove + lastRemove;
        let pat1 = pattern.substring(0, pattern.length / 2);
        let pat2 = pattern.substring(pattern.length / 2 + 1);
        pattern = pat1 + pat2;
        console.log('Shaked it.');
        } else {
            break;
        }
    }
console.log('No shake.');
console.log(text);
}