function mostFrequentChar(str) {
  const count = {};
  let best = null;
  for (const ch of str) {
    count[ch] = (count[ch] || 0) + 1;
    if (best === null || count[ch] > count[best]) best = ch;
  }
  return best;
}
mostFrequentChar("javascript"); // "a"

Updating best inside the loop keeps it single-pass. On a tie the character that reached the count first wins - state that convention out loud in an interview.

sandbox.js

function mostFrequentChar(str) {
  // TODO
}

Output

Hit “Run tests” to check your solution.

All problems

Next question

JavaScriptMedium14/15

Write mostFrequentChar(str) returning the most frequent character (null for an empty string)

By FrontendPro Editorial Team Updated 8/14/2026

Explore the JavaScript interview guide

Examples

Write mostFrequentChar so that the following cases hold.

InputmostFrequentChar("javascript")
Output"a"
InputmostFrequentChar("aabbb")
Output"b"
InputmostFrequentChar("x")
Output"x"

1 more hidden test cases - 4 in total will be graded.

Answer

function mostFrequentChar(str) {
  const count = {};
  let best = null;
  for (const ch of str) {
    count[ch] = (count[ch] || 0) + 1;
    if (best === null || count[ch] > count[best]) best = ch;
  }
  return best;
}
mostFrequentChar("javascript"); // "a"