Finding a unique word in string with Javascript

I am trying to find a unique word in my input string with JS and have written a code. However, my code doesn’t work and I don’t know why. Could anyone help me please?

function findUniqueWord(index){
    var show = []
    var result = [...index]
    for (let i = 0; i < result.length; i++) {
        for (let j = i + 1; j < result.length; j++) {
            if(result[i] === result[j]){
                
            }
            else{
                return show.push(result[i])
            }
            
        } 
    }
    return show
}

console.log(findUniqueWord('A2A'));