return the first non repeating character in a string in javascript

Question

So I tried looking for this in the search but the closest I could come is a similar answer in several different languages, I would like to use Javascript to do it.

T

Answer 1

Here's a Solution using Regex to replace all repeating characters and then returning the first character.

function firstNonRepeat(str) {

   // Sorting str so that all repeating characters will come together & replacing it with empty string and taking first character using substr.

   var rsl = str.split('').sort().join('').replace(/(\w)\1+/g,'').substr(0,1);

   if(rsl) return rsl;

   else return 'All characters are repeated in ' + str;

}

console.log(firstNonRepeat('aaabcccdeeef'));
console.log(firstNonRepeat('aaacbdcee'));
console.log(firstNonRepeat('aabcbd'));

Answer 2

 function FirstNotRepeatedChar(str) {
 var arr = str.split('');
 var result = '';
 var ctr = 0; 
  for (var x = 0; x < arr.length; x++) {
   ctr = 0;
  for (var y = 0; y < arr.length; y++) {
  if (arr[x] === arr[y]) {
    ctr+= 1;
    }
   }

if (ctr < 2) {
  result = arr[x];
  break;
  }
}
return result;
}
console.log(FirstNotRepeatedChar('asif shaik'));

Answer 3

The most satisfactory and easy to understand answer is the following.

function firstNotRepeatingCharacter(s) {
        
    const arr = s.split("");

    for(let i = 0; i < arr.length; i++){
        let chr = arr[i];
        if( arr.indexOf(arr[i]) == arr.lastIndexOf(arr[i])){
            return arr[i]
        }            
    }
    
    return "_"
}

Explanation: It loops through all the characters of a string from forward and backward and then compares the values. If the index of both forward and backward search is true then it returns that character.

Answer 4

You can use the indexOf method to find the non repeating character. If you look for the character in the string, it will be the first one found, and you won't find another after it:

function firstNonRepeatedCharacter(string) {
  for (var i = 0; i < string.length; i++) {
    var c = string.charAt(i);
    if (string.indexOf(c) == i && string.indexOf(c, i + 1) == -1) {
      return c;
    }
  }
  return null;
}

Demo: http://jsfiddle.net/Guffa/Se4dD/

Answer 5

Fill an empty array with zeros, with same length as the string array, and tally up how many times they appear through the loop. Grab the first one in the tallied array with a value of 1.

function firstNotRepeatingCharacter(s) {
    const array = s.split("");
    let scores = new Array(array.length).fill(0);

    for (let char of array) {
        scores[array.indexOf(char)]++;
    }

    const singleChar = array[scores.indexOf(1)];
    return singleChar ? singleChar : "_"
}

Answer 6

var firstNonRepeatedCharacter = function(string) {
  var chars = string.split('');
  for (var i = 0; i < string.length; i++) {
    if (chars.filter(function(j) { 
                        return j == string.charAt(i); 
               }).length == 1) return string.charAt(i);
  }
};

So we create an array of all the characters, by splitting on anything.

Then, we loop through each character, and we filter the array we created, so we'll get an array of only those characters. If the length is ever 1, we know we have a non-repeated character.

Fiddle: http://jsfiddle.net/2FpZF/