I'd like someone to help me understand a few lines of code. Can someone help me explain them? Thanks

Question

I'm doing an algorithm on Codesignal.

For s = "abacabad" , the output should be firstNotRepeatingCharacter(a) = 'c' .

There are 2 non-repeating characters in the string: 'c' and 'd'. Return c since it appears in the string first.

For s = "abacabaabacaba" , the output should be firstNotRepeatingCharacter(s) = '_' .

There are no characters in this string that do not repeat.

Code below:

function firstNotRepeatingCharacter(a) {
  let b = {};
  let len = a.length;
  for (let i = 0; i < len; i++) {
    let tmp = a.charAt(i);
    if (b[tmp]) {
      b[tmp] += 1; //explain here
    }
    else {
      b[tmp] = 1;
    }
  }

  for (var prop in b) {
    if (b[prop] == 1) { //explain here
      return prop;
    }
  }
  return '_';
}

Answer 1

Actually in the above b is an object whose keys are different letters of the string and the values of those keys are count of respective lettter in the string.

key => letter
value => Count of that letter

tmp will be character through you are iterating.
if (b[tmp]) checks whether the letter is already added to the object.

• If its already there so increase the count.
• Otherwise set it to one.

In the second loop if (b[prop] == 1) checks whether the count of certain letter is 1 . means that it only occurred once in the string so return the letter.