Promise Merge

mediumcodingJavaScriptPromisesCaching~25 min

Asked at Google, Meta, Amazon

Overview

Implement promiseMerge(fn) that wraps an async function so concurrent calls with the same key share a single in-flight promise.

Examples

const fetch = promiseMerge(async (id) => api.get(id));
// Two concurrent calls for same id share one request
const [a, b] = await Promise.all([fetch('1'), fetch('1')]); // only 1 call made

Solution

Reveal solution

function promiseMerge(fn) {
  const pending = new Map();
  return function(key) {
    if (pending.has(key)) return pending.get(key);
    const promise = fn(key).finally(() => pending.delete(key));
    pending.set(key, promise);
    return promise;
  };
}

promise-merge.js

Promise Merge

mediumcodingJavaScriptPromises

Overview

Implement promiseMerge(fn) that wraps an async function so concurrent calls with the same key share a single in-flight promise.

Examples

const fetch = promiseMerge(async (id) => api.get(id));
// Two concurrent calls for same id share one request
const [a, b] = await Promise.all([fetch('1'), fetch('1')]); // only 1 call made

Solution

Reveal solution

function promiseMerge(fn) {
  const pending = new Map();
  return function(key) {
    if (pending.has(key)) return pending.get(key);
    const promise = fn(key).finally(() => pending.delete(key));
    pending.set(key, promise);
    return promise;
  };
}

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