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Brian Vaughn bvaughn

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bvaughn /
Last active January 9, 2023 15:46
Experimental React DOM test selector API

RFC Test Selectors

Owner: Brian Vaughn

As of facebook/react/pull/22760, the experimental Test Selector API is now available in the experimental release channel.

To test the API, first install the experimental release:

bvaughn /
Last active December 29, 2022 15:15
How to use profiling in production mode for react-dom

React recently introduced an experimental profiler API. This page gives instructions on how to use this API in a production release of your app.

Table of Contents

Profiling in production

React DOM automatically supports profiling in development mode for v16.5+, but since profiling adds some small additional overhead it is opt-in for production mode. This gist explains how to opt-in.

View console-colors.js
const createLogger = (backgroundColor, color) => {
const logger = (message, ...args) => {
if (logger.enabled === false) {
`background-color: ${backgroundColor}; color: ${color}; padding: 2px 4px;`,
bvaughn / example.jsx
Last active December 23, 2022 04:56
Advanced example for manually managing subscriptions in an async-safe way using hooks
View example.jsx
import React, { useMemo } from "react";
import useSubscription from "./useSubscription";
// In this example, "source" is an event dispatcher (e.g. an HTMLInputElement)
// but it could be anything that emits an event and has a readable current value.
function Example({ source }) {
// In order to avoid removing and re-adding subscriptions each time this hook is called,
// the parameters passed to this hook should be memoized.
const subscription = useMemo(
() => ({
bvaughn /
Last active December 17, 2022 00:48
Notes about the in-development React <Profiler> component


React 16.4 will introduce a new Profiler component (initially exported as React.unstable_Profiler) for collecting render timing information in order to measure the "cost" of rendering for both sync and async modes.

Profiler timing metrics are significantly faster than those built around the User Timing API, and as such we plan to provide a production+profiling bundle in the future. (The initial release will only log timing information in DEV mode, although the component will still render its children- without timings- in production mode.)

How is it used?

Profiler can be declared anywhere within a React tree to measure the cost of rendering that portion of the tree. For example, a Navigation component and its descendants:

bvaughn / updating-external-data-when-props-changes-using-promises.js
Last active November 30, 2022 21:22
Example for loading new external data in response to updated props
View updating-external-data-when-props-changes-using-promises.js
// This is an example of how to fetch external data in response to updated props,
// If you are using an async mechanism that does not support cancellation (e.g. a Promise).
class ExampleComponent extends React.Component {
_currentId = null;
state = {
externalData: null
bvaughn / eager-prefetching-async-data-example.js
Last active November 30, 2022 21:16
Advanced example for eagerly prefetching async data in a React component.
View eager-prefetching-async-data-example.js
// This is an advanced example! It is not intended for use in application code.
// Libraries like Relay may make use of this technique to save some time on low-end mobile devices.
// Most components should just initiate async requests in componentDidMount.
class ExampleComponent extends React.Component {
_hasUnmounted = false;
state = {
externalData: null,
bvaughn /
Last active November 28, 2022 08:27
Interaction tracing with React

This API was removed in React 17

Interaction tracing with React

React recently introduced an experimental profiler API. After discussing this API with several teams at Facebook, one common piece of feedback was that the performance information would be more useful if it could be associated with the events that caused the application to render (e.g. button click, XHR response). Tracing these events (or "interactions") would enable more powerful tooling to be built around the timing information, capable of answering questions like "What caused this really slow commit?" or "How long does it typically take for this interaction to update the DOM?".

With version 16.4.3, React added experimental support for this tracing by way of a new NPM package, scheduler. However the public API for this package is not yet finalized and will likely change with upcoming minor releases, so it should be used with caution.

bvaughn /
Last active October 27, 2022 04:42
Profiling a custom Chrome extension

Chrome's profiler ("Performance tab) is very useful for measuring JavaScript performance, but what if you want to measure the performance of a custom extension?

For example, what if I would like to profile the following interaction:

The interaction we want to profile

bvaughn /
Last active October 13, 2022 18:34
React DevTools: Unsupported backend version

Unsupported DevTools backend version

This page contains instructions for updating a local React DevTools application to match a version embedded in a renderer such as React Native. Instructions below cover NPM, Flipper, and React Native Debugger. If you use React DevTools in a different way, please let us know.

If you are viewing this page, you have likely seen one of the dialogs below:

Dialog displaying downgrade instructions for the React DevTools frontend to connect to an older backend version