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Upsert--getOrInsert()

A very common Map pattern is 'get the value for this key, or compute and store a default if it is not there yet' — previously requiring an awkward .has()-then-.get()-or-.set() dance (and two lookups instead of one). ECMAScript 2026 adds an 'upsert' (update-or-insert) pair of methods, shared by Map and WeakMap, that do this atomically in a single operation.

Map.prototype.getOrInsert(key, value) returns the existing value for key if present; otherwise it inserts value under key and returns that value.

Map.prototype.getOrInsertComputed(key, callbackFn) is the same, except the default is computed lazily by calling callbackFn(key) — and only when key is actually missing — instead of always being evaluated up front like the plain value passed to getOrInsert().

getOrInsertComputed() is preferable whenever building the default value is expensive (or has side effects), since it is skipped entirely when the key already exists.
const cache = new Map();

function getGroup(name) {
  return cache.getOrInsertComputed(name, () => {
    console.log('creating group:', name);   // only logged the first time per name
    return [];
  });
}

getGroup('admins').push('alice');
getGroup('admins').push('bob');
console.log(cache.get('admins'));

const counts = new Map();
counts.getOrInsert('x', 0);
counts.set('x', counts.get('x') + 1);
console.log(counts.get('x'));

creating group: admins ['alice', 'bob'] 1

WeakMap.prototype.getOrInsert() and WeakMap.prototype.getOrInsertComputed() (15) work identically, for the same key-must-already-exist-or-get-created situation with object keys.