limit-once

Cache the first successful result of a function call.

Features

• Synchronous variant (tiny 150B)
• Asynchronous variant for promises (tiny 360B)
• Only include the code for the variant(s) you want
• Both variants support cache clearing (avoid memory leaks)
• Both variants respect this control
• Full TypeScript support

Installation

# yarn
yarn add limit-once

# npm
npm install limit-once

# bun
bun add limit-once

Synchronous variant

Create a new function that only allows an existing function to be called once.

import { once } from 'limit-once';

function getGreeting(name: string): string {
  return `Hello ${name}`;
}
const getGreetingOnce = once(getGreeting);

getGreetingOnce('Alex');
// getGreeting called and "Hello Alex" is returned
// "Hello Alex" is put into the cache.
// returns "Hello Alex"

getGreetingOnce('Sam');
// getGreeting is not called
// "Hello Alex" is returned from the cache.

getGreetingOnce('Greg');
// getGreeting is not called
// "Hello Alex" is returned from the cache.

[!NOTE] If you want the wrapped function to cache the result, but recompute the result when the arguments change: use memoize-one

Only successful calls are cached

If the function being wrapped throws an error, then that throw is not cached, and the wrapped function is allowed to be called again

import { once } from 'limit-once';

let callCount = 0;
function maybeThrow({ shouldThrow }: { shouldThrow: boolean }): string {
  callCount++;

  if (shouldThrow) {
    throw new Error(`Call count: ${callCount}`);
  }

  return `Call count: ${callCount}`;
}
const maybeThrowOnce = once(maybeThrow);

expect(() => maybeThrowOnce({ shouldThrow: true })).toThrowError('Call count: 1');

// failure result was not cached, underlying `maybeThrow` called again
expect(() => maybeThrowOnce({ shouldThrow: true })).toThrowError('Call count: 2');

// our first successful result will be cached
expect(maybeThrowOnce({ shouldThrow: false })).toBe('Call count: 3');
expect(maybeThrowOnce({ shouldThrow: false })).toBe('Call count: 3');

Cache clearing (once(fn).clear())

You can clear the cache of a onced function by using the .clear() function property.

import { once } from 'limit-once';

function getGreeting(name: string): string {
  return `Hello ${name}`;
}
const getGreetingOnce = once(getGreeting);

getGreetingOnce('Alex');
// getGreeting called and "Hello Alex" is returned.
// "Hello Alex" is put into the cache
// getGreetingOnce function returns "Hello Alex"

getGreetingOnce('Sam');
// getGreeting is not called
// "Hello Alex" is returned from cache

getGreetingOnce.clear();

getGreetingOnce('Greg');
// getGreeting is called and "Hello Greg" is returned.
// "Hello Greg" is put into the cache
// "Hello Greg" is returned.

Asynchronous variant

Allows you to have a once functionality for a function that returns a Promise.

import { onceAsync } from 'limit-once';

async function getPermissions(): Promise<Record<string, boolean>> {
  // Note: could use "zod" to validate response shape
  const response = await fetch('/permissions');
  return await response.json();
}

// We don't want every call to `getPermissions()` to call `fetch` again.
// Ideally we would store the result of the first successful call and return that!

const getPermissionsOnce = onceAsync(getPermissions);

const user1 = await getPermissionsOnce();

// subsequent calls won't call fetch, and will return the previously fulfilled promise value.
const user2 = await getPermissionsOnce();

Only "fulfilled" Promises are cached

If the wrapped function has it's Promise "rejected", then the "rejected" Promise will not be cached, and the underlying function can be called again.

import { onceAsync } from 'limit-once';

let callCount = 0;
async function maybeThrow({ shouldThrow }: { shouldThrow: boolean }): Promise<string> {
  callCount++;

  if (shouldThrow) {
    throw new Error(`Call count: ${callCount}`);
  }

  return `Call count: ${callCount}`;
}
const maybeThrowOnce = onceAsync(maybeThrow);

expect(async () => await maybeThrowOnce({ shouldThrow: true })).toThrowError('Call count: 1');

// failure result was not cached, underlying `maybeThrow` called again
expect(async () => await maybeThrowOnce({ shouldThrow: true })).toThrowError('Call count: 2');

// our first successful result will be cached
expect(await maybeThrowOnce({ shouldThrow: false })).toBe('Call count: 3');
expect(await maybeThrowOnce({ shouldThrow: false })).toBe('Call count: 3');

Calls while a Promise is "pending"

If multiple calls are made to a onceAsync(fn) function while the original Promise is still "pending", then the original Promise is re-used.

✨ This prevents multiple calls to the underlying function ✨

import { onceAsync } from 'limit-once';

async function getPermissions(): Promise<Record<string, boolean>> {
  // Note: could use "zod" to validate response shape
  const response = await fetch('/permissions');
  return await response.json();
}

export const getPermissionsOnce = onceAsync(getPermissions);

const promise1 = getPermissionsOnce();

// This second call to `getPermissionsOnce()` while the `getPermissions()` promise
// is still "pending" will return the same promise that the first call created.
// `fetch` is only called once (by the first call)
const promise2 = getPermissionsOnce();

console.log(promise1 === promise2); // "true"

Cache clearing (onceAsync(fn).clear())

You can clear the cache of a onceAsync function by using the .clear() function property.

import { onceAsync } from 'limit-once';

let callCount = 0;
async function getCallCount(): Promise<string> {
  return `Call count: ${callCount}`;
}

const onced = onceAsync(getCallCount);

expect(await onced({ shouldThrow: false })).toBe('Call count: 1');
expect(await onced({ shouldThrow: false })).toBe('Call count: 1');

onced.clear();

expect(await onced({ shouldThrow: false })).toBe('Call count: 2');
expect(await onced({ shouldThrow: false })).toBe('Call count: 2');

If onced async function is "pending" when .clear() is called, then the promise(s) that the onced function has returned will be rejected.

import { onceAsync } from 'limit-once';

async function getName(): Promise<string> {
  return 'Alex';
}

const getNameOnce = onceAsync(getName);

const promise1 = getNameOnce().catch(() => {
  console.log('rejected promise 1');
});

const promise2 = getNameOnce().catch(() => {
  console.log('rejected promise 2');
});

// cached cleared while promise was pending
// will cause `promise1` to be rejected
getNameOnce.clear();

// console.log → "rejected promise 1"
// console.log → "rejected promise 2"

Outputs

• limit-once is a dual package that supports ECMAScript modules (esm) and CommonJS (cjs)
• Distributed files are compiled to "es6" which has wide support
• Internet Explorer 11 not supported (it does not support "es6")