Ihab Khattab

Map.getOrInsert: one less if between you and the job offer

Also on X

Half of LeetCode is the same ritual: check if the key exists, add it if it doesn’t, then use it. Map.prototype.getOrInsert() performs the ritual for you, so you can retire the if you’ve typed 4,000 times.

Take Group Anagrams: put words made of the same letters in one group. Sort a word’s letters and you get its group’s key: eat and tea both become aet. Each key holds a list of its words.

groupAnagrams.js
  function groupAnagrams(words) {
    const groups = new Map();
    for (const word of words) {
      const key = [...word].sort().join("");
-     if (!groups.has(key)) {
-       groups.set(key, []);
-     }
-     groups.get(key).push(word);
+     groups.getOrInsert(key, []).push(word);
    }
    return [...groups.values()];
  }

Same answer, with time left to panic about the follow-up question.

Yes, Map.groupBy solves this one too. Let me have this.

getOrInsertComputed() takes a function instead of a value and only calls it when the key is missing. With getOrInsert(), the default gets evaluated first, every time, hit or miss. So:

  • Cheap default like [] or new Set(): getOrInsert().
  • Expensive default, or one with side effects: getOrInsertComputed().

Pick wrong and you get a memo that remembers everything and saves nothing. Here’s Fibonacci, with each version’s own Map:

// 2,692,537 calls for n = 30. Both recursive calls run before the map is even asked.
const slow = (n) =>
  n < 2 ? n : slowMemo.getOrInsert(n, slow(n - 1) + slow(n - 2));

// 59 calls. The callback only runs on a miss.
const fast = (n) =>
  n < 2 ? n : fastMemo.getOrInsertComputed(n, () => fast(n - 1) + fast(n - 2));

Both have been Baseline since February 2026: Chrome and Edge 145, Firefox 144, Safari 26.2. Node 26 has them, Node 24 doesn’t. For anything older, use a polyfill like core-js. WeakMap gets both too.

The catch: LeetCode runs Node 22.14, where this throws a TypeError. The if keeps its job a little longer.

Outside LeetCode, Sébastien Lorber uses it to lazily fill a cache:

return cache.getOrInsertComputed(options, () =>
  createProcessors({ options }),
);

More: MDN: getOrInsert() · MDN: getOrInsertComputed()

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