firstNotRepeatingCharacter.js

firstNotRepeatingCharacter = s => {
    r = {}
    for (e of s)
        // ~n = -(n+1)
        // -~n = n+1
        // ~ undefined = -1
        // -~undefined = 1
        // if character is in hash increment else set to 1
        r[e] = -~r[e]
    for (e in r)
        // if character occured once return it
        if (r[e] === 1)
            return e
    return '_'
}

firstNotRepeatingCharacter.min.js

firstNotRepeatingCharacter=s=>{r={}for(e of s){r[e]=-~r[e]};for(e in r){if(r[e]===1){return e}};return'_'}