JSFiddle - React, Tailwind, and code Playground

JavaScript

function testStr (str) {
    let arr = str.split('')
    let unique = [...new Set(arr)]
    return unique.filter(char => {
        let count = 0
        let searchIndex = 0
        while (true) {
            index = str.indexOf(char, searchIndex)
            if (index == -1) break;
            count+=1
            searchIndex = index + 1
        }
        return !!(count % 2)
    })
}
console.log(testStr ('abbadddd'));
console.log(testStr ('abbbaccddd'));