Check for repeated characters in a string Javascript

Question

I was wondering if there is a way to check for repeated characters in a string without using double loop. Can this be done with recursion?

An example of the code using double loop (return true or false based on if there are repeated characters in a string):

var charRepeats = function(str) {
    for(var i = 0; i <= str.length; i++) {
        for(var j = i+1; j <= str.length; j++) {
            if(str[j] == str[i]) {
                return false;
            }
        }
    }
    return true;
}

Many thanks in advance!

Answer 1

(A recursive solution can be found at the end, of this answer.)

You could use javascript builtin Array functions some MDN some reference

 var text = "test".split("");
 text.some(function(v,i,a){
   return a.lastIndexOf(v)!=i;
 });

callback parameters:
v current value of the iteration
i current index of the iteration
a current array

.split("") create an array from a string
.some(function(v,i,a){ ... }) goes through an array until the function returns true, and ends than right away. (doesn't loop through the whole array, if it finds an match earlier)

Details to the some function can be found here

Tests, with several strings:

var texts = ["test", "rest", "why", "puss"];


for(var idx in texts){
    var text = texts[idx].split("");
    document.write(text + " -> " + text.some(function(v,i,a){return a.lastIndexOf(v)!=i;}) +"<br/>");
    
  }
  //tested on win7 in chrome 46+

If recursion is needed.

Update for recursion:

//recursive function
function checkString(text,index){
    if((text.length - index)==0 ){ //stop condition
        return false; 
    }else{
        return checkString(text,index + 1) 
        || text.substr(0, index).indexOf(text[index])!=-1;
    }
}

// example Data to test
var texts = ["test", "rest", "why", "puss"];

for(var idx in texts){
    var txt = texts[idx];
    document.write( txt +  " ->" + checkString(txt,0) + "<br/>");
}
//tested on win7 in chrome 46+