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fast, tiny `queueMicrotask` shim for modern engines
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README.md

queue-microtask travis npm downloads javascript style guide

fast, tiny queueMicrotask shim for modern engines

  • Use queueMicrotask in all JS engines.
  • No dependencies. Less than 10 lines. No shims or complicated fallbacks.
  • Optimal performance in all modern environments.
    • Use queueMicrotask in modern environments (optimal)
    • Fallback to Promise.resolve().then(fn) in Node.js 10 and earlier, and old browsers (optimal)
    • Fallback to setTimeout in JS environments without Promise (slow)

install

npm install queue-microtask

usage

const queueMicrotask = require('queue-microtask')

queueMicrotask(() => { /* this will run soon */ })

What is queueMicrotask and why would one use it?

The queueMicrotask function is a WHATWG standard. It queues a microtask to be executed prior to control returning to the event loop.

A microtask is a short function which will run after the current task has completed its work and when there is no other code waiting to be run before control of the execution context is returned to the event loop.

The code queueMicrotask(fn) is equivalent to the code Promise.resolve().then(fn). It is also very similar to process.nextTick(fn) in Node.

Using microtasks lets code run without interfering with any other, potentially higher priority, code that is pending, but before the JS engine regains control over the execution context.

See the spec or Node documentation for more information.

Who is this package for?

This package allows you to use queueMicrotask safely in all JS engines. Use it if you prioritize small JS bundle size over optimal performance in old browsers.

Why not use process.nextTick?

In Node, queueMicrotask and process.nextTick are essentially equivalent, though there are subtle differences that don't matter in most situations.

If you just need to support Node 12 and later, use queueMicrotask directly. If you need to support all versions of Node, use this package.

Why not use setTimeout(fn, 0)?

This approach is the most compatible, but it has problems. Modern browsers throttle timers severely, so setTimeout(…, 0) usually takes at least 4ms to run. Furthermore, the throttling gets even worse if the page is backgrounded. If you have many setTimeout calls, then this can severely limit the performance of your program.

Why not use a microtask library like immediate or asap?

These packages are great! However, if you prioritize small JS bundle size over optimal performance in old browsers then you may want to consider this package.

This package (queue-microtask) is four times smaller than immediate, twice as small as asap, and twice as small as using process.nextTick and letting the browser bundler shim it automatically.

Note: This package does not have proper microtask support in old browsers. Instead, old browsers fallback to setTimeout. This will be slower, but it allows us to avoid including a complicated solution.

Since the queueMicrotask API is supported in Chrome, Firefox, Safari, Opera, and Edge (canary), the vast majority of users will get the optimal experience. Any JS environment with Promise, which is almost all of them, also get the optimal experience. If you need optimal performance in old browsers, use one of the alternative packages.

What is a shim?

In computer programming, a shim is a library that transparently intercepts API calls and changes the arguments passed, handles the operation itself or redirects the operation elsewhere. – Wikipedia

This package could also be described as a "ponyfill".

A ponyfill is almost the same as a polyfill, but not quite. Instead of patching functionality for older browsers, a ponyfill provides that functionality as a standalone module you can use. – PonyFoo

API

queueMicrotask(fn)

The queueMicrotask() method queues a microtask.

The fn argument is a function to be executed after all pending tasks have completed but before yielding control to the browser's event loop.

license

MIT. Copyright (c) Feross Aboukhadijeh.

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