Why do I have to use ==, not === in Javascript 'for in' loop?

Question

I was solving basic algorithm problems, and I got stuck in this code.

The problem was to get string data and remove duplicated charactors. For instance, input: 'ksekkset', output: 'kset'

And this is my solution.

function solution(s) {
  let answer = "";
  for(let i in s) {
    if(s.indexOf(s[i]) == i) answer+=s[i];
  }
  return answer;
}

My question is, why do I get correct answer only when I put '==' inside if()? When I put '===' why if() only returns false?

If I use for loop, '===' also works..

function solution2(s) {
  let answer = "";
  for(let i=0; i<s.length; i++) {
    if(s.indexOf(s[i])===i) answer+=s[i];
  }
  return answer;
}

I'm so confused!

With `for..in` the `i` would be a string. And a string doesn't strictly equal a number. `5 === "5"` is `false` while with loose equality `5 == "5"` is `true`. — VLAZ, Jul 10 '21 at 07:33
[Why is using “for…in” for array iteration a bad idea?](https://stackoverflow.com/questions/500504/why-is-using-for-in-for-array-iteration-a-bad-idea) — Andreas, Jul 10 '21 at 07:39

score 1 · Accepted Answer · answered Jul 10 '21 at 08:02

As @VLAZ mentioned above.. the == vs === isn't the only difference.

I wrote changed around your program to try all combinations of the for/in change and === change, and you can see that the only one that doesn't work as expected is === with for/in.

Strange that for a string, for/in gives you the indices as strings even though they are the numbers!

const s = "stackoverflow"; // repeated 'o' makes answer interesting

const answers = {
  answerForInTripple: "",
  answerForInDouble: "",
  answerForTripple: "",
  answerForDouble: "",
};

for(let i in s) {
  if(s.indexOf(s[i]) == i) {
    answers.answerForInDouble += s[i];
  }
  if(s.indexOf(s[i]) === i) {
    answers.answerForInTripple += s[i];
  }
}

for(let i=0; i < s.length; i++) {
  if(s.indexOf(s[i]) == i) {
    answers.answerForDouble += s[i];
  }
  if(s.indexOf(s[i]) === i) {
    answers.answerForTripple += s[i];
  }
}

console.log(answers);
console.log('here is the difference between what you are comparing');
for(let i in s) {
  const idx = s.indexOf(s[i]);
  console.log(typeof i, i, typeof idx, idx);
}

console.log('simpler example');

for (let i in 'test') console.log(typeof i, i);

Thanks Alex! Now I can move on to the next problem :) – siennaY Jul 11 '21 at 12:56 — siennaY, Jul 11 '21 at 12:56

Why do I have to use ==, not === in Javascript 'for in' loop?

1 Answers1