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ECMAScript Proposal, specs, and reference implementation for Map.prototype.upsert
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ECMAScript proposal and reference implementation for Map.prototype.upsert.

Author: Brad Farias (GoDaddy)

Champion: Erica Pramer (GoDaddy)

Stage: 1


Adding and updating values of a Map are tasks that developers often perform in conjunction. There are currently no Map prototype methods for either of those two things, let alone a method that does both. The workarounds involve multiple lookups and developer inconvenience.

Solution: upsert

We propose the addition of a method that will add a value to a map if the map does not already have something at key, and will also update an existing value at key. It’s worthwhile having this API for the average case to cut down on lookups. It is also worthwhile for developer convenience and expression of intent.

Examples & Proposed API

The following examples would all be optimized and made simpler by upsert. The proposed API allows a developer to do one lookup and update in place:

upsert(key, old => updated, () => insertionValue)

Normalization of values during insertion

Currently you would need to do 3 lookups:

if (!map.has(key)) {
  map.set(key, value);

With this proposal:

map.upsert(key, o => o, () => value).doThing();
// or
map.upsert(key, undefined, () => value).doThing();

Either update or insert for a specific key

You might get new data and want to calculate some aggregate if the key exists, but just insert if it's the first value at that key.

// two lookups
old = map.get(key);
if (!old) {
  map.set(key, value);
} else {
  map.set(key, old => updated);

With this proposal:

map.upsert(key, () => updated, () => value)

Just insert if missing

You might omit an update if you're handling data that doesn't change, but can still be appended.

// two lookups
if (!map1.has(key)) {
  map1.set(key, value);

With this proposal:

map.upsert(key, o => o, () => value);
// or
map.upsert(key, undefined, () => value);

Just update if present

You might want to omit an insert if you want to perform a function on all existing values in a Map (ex. normalization).

// three lookups
if (map.has(key)) {
  old = map.get(key);
  updated = old.doThing();
  map.set(key, updated);

With this proposal:

map.upsert(key, old => old.doThing());
// or
map.upsert(key, old => old.doThing(), undefined);

Implementations in other languages

Similar functionality exists in other languages.



  • emplace inserts if missing
  • map[] assignment opts inserts if missing at key but also returns a value if it exists at key
  • insert_or_assign inserts if missing. updates existing value by replacing with a specific new one, not by applying a function to the existing value


  • and_modify Provides in-place mutable access to an occupied entry
  • or_insert_with inserts if empty. insertion value comes from a mapping function



  • Is the goal to simplify the API or to optimize it?

    • This proposal seeks to simplify expressing intent for programmers, and should ease optimization without complex analysis. For engines without complex analysis like IOT VMs this should see wins by avoiding multiple entry lookups, at potential call stack cost.
  • Why not use existing JS engines that optimize by coalescing the lookup and mutation?

    • This does not cover all patterns (of which there are many), things such as ordering can cause the optimization to fail.
    if (!x.has(a)) x.set(a, []);
    if (!y.has(b)) y.set(b, []);
  • Why use functions instead of values for the parameters?

    • You may want to apply a factory function when inserting to avoid costs of potentially heavy allocation, or the key may be determined at insertion time.
    • When updating, we will be able to perform a function on the existing value instead of just replacing the value. The action may also cause mutation or side-effects, which would want to be avoided if not updating.
  • Why are we calling this upsert?

    • It is a combination of "update" & "insert" that is already used in other programming situations and many SQL variants use that exact term.
    • updateOrInsert and insertOrUpdate seem too wordy.


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