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Profiler

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:

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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
  • Create React App
  • Webpack 4

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.

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This API was removed in React 17

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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.

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Start with this HTML:

<div>click here</div>
<div style="user-select: none;">read-only (no-select)</div>
<div contenteditable style="user-select: none;">editable (no-select)</div>
<div>release here</div>

Click at the top ("click here") and drag down and release at the bottom ("release here"). The following text will be selected:

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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:

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Infinite lists and reflow

In my experience, infinite lists use two basic layout strategies. The first uses absolute positioning to control where visible items are rendered. The second uses relative positioning (with top/left padding to offset for unrendered items).

In both cases, the list abstraction caches some metadata about the size of items once they have been rendered– so that it knows where to position the items that come after them.

Both of these strategies need to handle reflow. For example, changing the width of a list often affects the height of its itesm. Generally speaking, only the "window" of rendered (visible) items are remeasured in this case (because it would be too slow to rerender and remeasure all of the items before). But once a user scrolls backwards (up/left)– the list needs to account for the reflowed sizes. If it didn't, items would appear to jump up or down (depending on the delta between the previous, cached sizes and the new/reflowed sizes).

How the list deals with new sizes

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The MIT License (MIT)

Copyright © <year> <copyright holders>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell

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#!/usr/bin/env node

const { spawn } = require("child_process");
const util = require("node:util");

const exec = util.promisify(require("node:child_process").exec);

async function main() {
  let { REPLAY_CHROME, RECORD_REPLAY_API_KEY } = process.env;
  if (!REPLAY_CHROME || !RECORD_REPLAY_API_KEY) {

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// 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
  };

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Pre-rendering virtual rows

Example list with ten total rows and a viewport large enough to display two rows.

Legend

█ - Visible row ▓ - Hidden/display-locked row ░ - Empty space (nothing rendered here)