ECMAScript proposal and reference implementation for Map.prototype.getOrInsert, Map.prototype.getOrInsertComputed, WeakMap.prototype.getOrInsert, and WeakMap.prototype.getOrInsertComputed.

Authors: Daniel Minor (Mozilla) Lauritz Thoresen Angeltveit (Bergen) Jonas Haukenes (Bergen) Sune Lianes (Bergen) Vetle Larsen (Bergen) Mathias Hop Ness (Bergen)

Champion: Daniel Minor (Mozilla)

Original Author: Brad Farias (GoDaddy)

Former Champion: Erica Pramer (GoDaddy)

Stage: 3

A common problem when using a Map or WeakMap is how to handle doing an update when you're not sure if the key already exists in the map. This can be handled by first checking if the key is present, and then inserting or updating depending upon the result, but this is both inconvenient for the developer, and less than optimal, because it requires multiple lookups in the map that could otherwise be handled in a single call.

We propose the addition of a method that will return the value associated with key if it is already present in the Map or WeakMap, and otherwise insert the key with the provided default value, or the result of calling a provided callback function, and then return that value.

Earlier versions of this proposal had an getOrInsert method that provided two callbacks, one for insert and the other for update, however the current champion thinks that the get / insert if necessary is a sufficiently common usecase that it makes sense to focus on it, rather than trying to create an API with maximum flexibility. It also strongly follows precedent from other languages, in particular Python.

Using getOrInsert simplifies handling default values because it will not overwrite an existing value.

// Currently
let prefs = getUserPrefsMap();
if (!prefs.has("useDarkmode")) {
  prefs.set("useDarkmode", true); // default to true
}

// Using getOrInsert
let prefs = getUserPrefsMap();
prefs.getOrInsert("useDarkmode", true); // default to true

By using getOrInsert, default values can be applied at different times, with the assurance that later defaults will not overwrite an existing value. For example, in a situation where there are user preferences, operating system preferences, and application defaults, we can use getOrInsert to apply the user preferences, and then the operating system preferences, and then the application defaults, without worrying about overwriting the user's preferences.

A typical usecase is grouping data based upon key as new values become available. This is simplified by being able to specify a default value rather than having to check for whether the key is already present in the Map before trying to update.

// Currently
let grouped = new Map();
for (let [key, ...values] of data) {
  if (grouped.has(key)) {
    grouped.get(key).push(...values);
  } else {
    grouped.set(key, values);
  }
}

// Using getOrInsert
let grouped = new Map();
for (let [key, ...values] of data) {
  grouped.getOrInsert(key, []).push(...values);
}

It's true that a common usecase for this pattern is already covered by Map.groupBy. However, that method requires that all data be available prior to building the groups; using getOrInsert would allow the Map to be built and used incrementally. It also provides flexibility to work with data other than objects, such as the array example above.

Another common use case is maintaining a counter associated with a particular key. Using getOrInsert makes this more concise, and is the kind of access and then mutate pattern that is easily optimizable by engines.

// Currently
let counts = new Map();
if (counts.has(key)) {
  counts.set(key, counts.get(key) + 1);
} else {
  counts.set(key, 1);
}

// Using getOrInsert
let counts = new Map();
counts.set(key, counts.getOrInsert(key, 0) + 1);

For some usecases, determining the default value is potentially a costly operation that would be best avoided if it will not be used. In this case, we can use getOrInsertComputed.

// Using getOrInsertComputed
let grouped = new Map();
for (let [key, ...values] of data) {
  grouped.getOrInsertComputed(key, () => []).push(...values);
}

Similar functionality exists in other languages.

Java

• computeIfPresent remaps existing entry
• computeIfAbsent insert if empty. computes the insertion value with a mapping function

Scala

• getOrElseUpdate inserts if missing.

C++

• 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

C#

• GetOrAdd Adds a key/value pair if the key does not already exist and returns the new value, or the existing value if the key already exists.

Rust

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

Python

• setdefault Performs a get and an insert
• defaultdict A subclass of dict that takes a callback function that is used to construct missing values on get.

Elixir

• Map.update/4 Updates the item with given function if key exists, otherwise inserts given initial value

• Ecmarkup source
• HTML version

The proposal is trivially polyfillable:

Map.prototype.getOrInsert = function (key, defaultValue) {
  if (!this.has(key)) {
    this.set(key, defaultValue);
  }
  return this.get(key);
};

Map.prototype.getOrInsertComputed = function (key, callbackFunction) {
  if (!this.has(key)) {
    this.set(key, callbackFunction(key));
  }
  return this.get(key);
};