WeakMap.prototype.getOrInsertComputed()

Try it

const alan = { name: "Alan" };
const map = new WeakMap([[alan, 35]]);
const defaultCreator = (obj) => `${obj.name}'s age is unknown`;

console.log(map.getOrInsertComputed(alan, defaultCreator));
// Expected output: 35

const brett = { name: "Brett" };
console.log(map.getOrInsertComputed(brett, defaultCreator));
// Expected output: "Brett's age is unknown"

Syntax

getOrInsertComputed(key, callback)

Parameters

key

The key of the element to return from the Map object. Must be either an object or a non-registered symbol. Object keys are compared by reference, not by value.

callback

A function that returns the value to insert and return if the key is not already present in the Map object. The function is called with the following argument:

key

The same key that was passed to getOrInsertComputed().

Return value

The value associated with the specified key in the WeakMap object. If the key can't be found, the result of callback(key) is inserted and returned.

Exceptions

TypeError

Thrown in one of the following cases:

• key is not an object or a non-registered symbol.
• callback isn't callable.

Examples

Caching

Caching involves building a mapping from the parameter of an expensive function to its return value, so that future calls with the same parameter can return the cached value instead of recomputing it. When no cached value exists, it needs to be computed and inserted into the cache.

We use a WeakMap here instead of a Map, so that the cached argument values do not prevent them from being garbage-collected if they are no longer needed elsewhere in the program. If your cached function takes non-object arguments, you can use a Map instead.

// Any expensive function you want to cache
function computeExpensiveValue(requestOptions) {
  // Imagine expensive computation, like fetching data or complex calculations
  console.log(`Fetching from ${requestOptions.url}`);
  return new Response("Fake response");
}

// A higher-order function that adds caching to any function
function withCache(fn) {
  const cache = new WeakMap();
  return (param) => cache.getOrInsertComputed(param, fn);
}

const computeWithCache = withCache(computeExpensiveValue);
const options = { url: "https://example.com/a", method: "GET" };
const value1 = computeWithCache(options); // Logs "Fetching from https://example.com/a"
const value2 = computeWithCache(options); // No log

Specifications

Browser compatibility

See also

• Polyfill of WeakMap.prototype.getOrInsertComputed in core-js
• es-shims polyfill of WeakMap.prototype.getOrInsertComputed
• WeakMap
• WeakMap.prototype.get()
• WeakMap.prototype.set()
• WeakMap.prototype.has()
• WeakMap.prototype.getOrInsert()