Javascript JS Remove Anything that is NOT a Letter

Javascript Function to Remove Anything that is NOT a Letter, Meaning Remove Any Numbers, Special Characters etc

function funcNonLettersRemoved(input, ballow_space = false){ if(ballow_space){ out = input.replace(/[^a-z\s]/gi, ''); }else{ out = input.replace(/[^a-z]/gi, ''); } return out; }