first unique character in a string javascript code example

Example: first non repeating character javascript

function firstNotRepeatingCharacter(s) {
    for (let i = 0; i < s.length; i++) {
        if(s.indexOf(s.charAt(i)) == s.lastIndexOf(s.charAt(i))) {
            return s.charAt(i)
        }
    }
    return '_'    
}