javascriptNull / UndefinedEasy

Word counter shows NaN

Time: 0:00

BugFind the bug and fix the code

What it should do

countWords(words) counts how many times each word appears. Example: countWords(["a", "b", "a"]) → { a: 2, b: 1 }.

What happens instead

countWords(["a", "b", "a"]) returns { a: NaN, b: NaN }.

function countWords(words) {
  const counts = {};
  for (const word of words) {
    counts[word] += 1;
  }
  return counts;
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

⏱️0:00

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This bug belongs to the null and undefined errors pattern

Cannot read properties of undefined, NoneType has no attribute, NaN appearing from nowhere.

Learn the null and undefined errors pattern Step through your own code

Spoiler: why this bug happens(try it first; the hints get warmer one at a time)

What went wrong: the first time a word appears, counts[word] is undefined. undefined + 1 is NaN.

The fix: counts[word] = (counts[word] ?? 0) + 1;

Remember: give a new key a start value of 0 before adding to it.

The pattern behind it: null and undefined errors.

More null and undefined errors to practise

Seen this error in your own code?

Unexpected NaN in a JavaScript calculation

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