//find if string has unique chars...
Object.assign(String.prototype, {
    HasUniqueChars(){
        for(let i =0; i < this.length; i++){
            for(let j =i+1; j < this.length; j++){
                if(this[i] == this[j])
                    return false;
            }
         }
         return true;
    }
});


console.log("abcd".HasUniqueChars()); // true

console.log("abccd".HasUniqueChars()); // false

//Big(O) = n*n