JSFiddle - React, Tailwind, and code Playground

JavaScript

const removeLetters = lettersArr => pred => 
        lettersArr.split('')
                .filter(l => !pred(l, lettersArr))
                .join('');

const predicate = (letter, string) => 
        string.match(new RegExp(letter, 'gi')).length % 2 === 0;

console.log(removeLetters('ffsafafasd')(predicate));