You can test React components similar to testing other JavaScript code.

There are a few ways to test React components. Broadly, they divide into two categories:

• Rendering component trees in a simplified test environment and asserting on their output.


• Running a complete app in a realistic browser environment (also known as “end-to-end” tests).

This documentation section focuses on testing strategies for the first case. While full end-to-end tests can be very useful to prevent regressions to important workflows, such tests are not concerned with React components in particular, and are out of the scope of this section.

Tradeoffs

When choosing testing tools, it is worth considering a few tradeoffs:

• Iteration speed vs Realistic environment: Some tools offer a very quick feedback loop between making a change and seeing the result, but don’t model the browser behavior precisely. Other tools might use a real browser environment, but reduce the iteration speed and are flakier on a continuous integration server.


• How much to mock: With components, the distinction between a “unit” and “integration” test can be blurry. If you’re testing a form, should its test also test the buttons inside of it? Or should a button component have its own test suite? Should refactoring a button ever break the form test?

Different answers may work for different teams and products.

Recommended Tools

Jest is a JavaScript test runner that lets you access the DOM via jsdom . While jsdom is only an approximation of how the browser works, it is often good enough for testing React components. Jest provides a great iteration speed combined with powerful features like mocking modules and timers so you can have more control over how the code executes.

React Testing Library is a set of helpers that let you test React components without relying on their implementation details. This approach makes refactoring a breeze and also nudges you towards best practices for accessibility. Although it doesn’t provide a way to “shallowly” render a component without its children, a test runner like Jest lets you do this by mocking.

Learn More

This section is divided in two pages:

• Recipes: Common patterns when writing tests for React components.


• Environments: What to consider when setting up a testing environment for React components.

Common testing patterns for React components.

Note:

This page assumes you’re using Jest as a test runner. If you use a different test runner, you may need to adjust the API, but the overall shape of the solution will likely be the same. Read more details on setting up a testing environment on the Testing Environments page.

On this page, we will primarily use function components. However, these testing strategies don’t depend on implementation details, and work just as well for class components too.

• Setup/Teardown


• act()


• Rendering


• Data Fetching


• Mocking Modules


• Events


• Timers


• Snapshot Testing


• Multiple Renderers


• Something Missing?

Setup/Teardown

For each test, we usually want to render our React tree to a DOM element that’s attached to document . This is important so that it can receive DOM events. When the test ends, we want to “clean up” and unmount the tree from the document .

A common way to do it is to use a pair of beforeEach and afterEach blocks so that they’ll always run and isolate the effects of a test to itself:

import { unmountComponentAtNode } from "react-dom";

let container = null;
beforeEach(() => {
  // setup a DOM element as a render target
  container = document.createElement("div");
  document.body.appendChild(container);
});

afterEach(() => {
  // cleanup on exiting
  unmountComponentAtNode(container);
  container.remove();
  container = null;
});

You may use a different pattern, but keep in mind that we want to execute the cleanup even if a test fails . Otherwise, tests can become “leaky”, and one test can change the behavior of another test. That makes them difficult to debug.

act()

When writing UI tests, tasks like rendering, user events, or data fetching can be considered as “units” of interaction with a user interface. react-dom/test-utils provides a helper called act() that makes sure all updates related to these “units” have been processed and applied to the DOM before you make any assertions:

act(() => {
  // render components
});
// make assertions

This helps make your tests run closer to what real users would experience when using your application. The rest of these examples use act() to make these guarantees.

You might find using act() directly a bit too verbose. To avoid some of the boilerplate, you could use a library like React Testing Library, whose helpers are wrapped with act() .

Note:

The name act comes from the Arrange-Act-Assert pattern.

Rendering

Commonly, you might want to test whether a component renders correctly for given props. Consider a simple component that renders a message based on a prop:

// hello.js

import React from "react";

export default function Hello(props) {
  if (props.name) {
    return <h1>Hello, {props.name}!</h1>;
  } else {
    return <span>Hey, stranger</span>;
  }
}

We can write a test for this component:

// hello.test.js

import React from "react";
import { render, unmountComponentAtNode } from "react-dom";
import { act } from "react-dom/test-utils";

import Hello from "./hello";

let container = null;
beforeEach(() => {
  // setup a DOM element as a render target
  container = document.createElement("div");
  document.body.appendChild(container);
});

afterEach(() => {
  // cleanup on exiting
  unmountComponentAtNode(container);
  container.remove();
  container = null;
});

it("renders with or without a name", () => {
  act(() => {    render(<Hello />, container);  });  expect(container.textContent).toBe("Hey, stranger");
  act(() => {
    render(<Hello name="Jenny" />, container);
  });
  expect(container.textContent).toBe("Hello, Jenny!");

  act(() => {
    render(<Hello name="Margaret" />, container);
  });
  expect(container.textContent).toBe("Hello, Margaret!");
});

Data Fetching

Instead of calling real APIs in all your tests, you can mock requests with dummy data. Mocking data fetching with “fake” data prevents flaky tests due to an unavailable backend, and makes them run faster. Note: you may still want to run a subset of tests using an “end-to-end” framework that tells whether the whole app is working together.

// user.js

import React, { useState, useEffect } from "react";

export default function User(props) {
  const [user, setUser] = useState(null);

  async function fetchUserData(id) {
    const response = await fetch("/" + id);
    setUser(await response.json());
  }

  useEffect(() => {
    fetchUserData(props.id);
  }, [props.id]);

  if (!user) {
    return "loading...";
  }

  return (
    <details>
      <summary>{user.name}</summary>
      <strong>{user.age}</strong> years old
      <br />
      lives in {user.address}
    </details>
  );
}

We can write tests for it:

// user.test.js

import React from "react";
import { render, unmountComponentAtNode } from "react-dom";
import { act } from "react-dom/test-utils";
import User from "./user";

let container = null;
beforeEach(() => {
  // setup a DOM element as a render target
  container = document.createElement("div");
  document.body.appendChild(container);
});

afterEach(() => {
  // cleanup on exiting
  unmountComponentAtNode(container);
  container.remove();
  container = null;
});

it("renders user data", async () => {
  const fakeUser = {    name: "Joni Baez",    age: "32",    address: "123, Charming Avenue"  };  jest.spyOn(global, "fetch").mockImplementation(() =>    Promise.resolve({      json: () => Promise.resolve(fakeUser)    })  );
  // Use the asynchronous version of act to apply resolved promises
  await act(async () => {
    render(<User id="123" />, container);
  });

  expect(container.querySelector("summary").textContent).toBe(fakeUser.name);
  expect(container.querySelector("strong").textContent).toBe(fakeUser.age);
  expect(container.textContent).toContain(fakeUser.address);

  // remove the mock to ensure tests are completely isolated  global.fetch.mockRestore();});

Mocking Modules

Some modules might not work well inside a testing environment, or may not be as essential to the test itself. Mocking out these modules with dummy replacements can make it easier to write tests for your own code.

Consider a Contact component that embeds a third-party GoogleMap component:

// map.js

import React from "react";

import { LoadScript, GoogleMap } from "react-google-maps";
export default function Map(props) {
  return (
    <LoadScript id="script-loader" googleMapsApiKey="YOUR_API_KEY">
      <GoogleMap id="example-map" center={props.center} />
    </LoadScript>
  );
}

// contact.js

import React from "react";
import Map from "./map";

export default function Contact(props) {
  return (
    <div>
      <address>
        Contact {props.name} via{" "}
        <a data-testid="email" href={"mailto:" + props.email}>
          email
        </a>
        or on their <a data-testid="site" href={props.site}>
          website
        </a>.
      </address>
      <Map center={props.center} />
    </div>
  );
}

If we don’t want to load this component in our tests, we can mock out the dependency itself to a dummy component, and run our tests:

// contact.test.js

import React from "react";
import { render, unmountComponentAtNode } from "react-dom";
import { act } from "react-dom/test-utils";

import Contact from "./contact";
import MockedMap from "./map";

jest.mock("./map", () => {  return function DummyMap(props) {    return (      <div data-testid="map">        {props.center.lat}:{props.center.long}      </div>    );  };});
let container = null;
beforeEach(() => {
  // setup a DOM element as a render target
  container = document.createElement("div");
  document.body.appendChild(container);
});

afterEach(() => {
  // cleanup on exiting
  unmountComponentAtNode(container);
  container.remove();
  container = null;
});

it("should render contact information", () => {
  const center = { lat: 0, long: 0 };
  act(() => {
    render(
      <Contact
        name="Joni Baez"
        email="test@example.com"
        site="http://test.com"
        center={center}
      />,
      container
    );
  });

  expect(
    container.querySelector("[data-testid='email']").getAttribute("href")
  ).toEqual("mailto:test@example.com");

  expect(
    container.querySelector('[data-testid="site"]').getAttribute("href")
  ).toEqual("http://test.com");

  expect(container.querySelector('[data-testid="map"]').textContent).toEqual(
    "0:0"
  );
});

Events

We recommend dispatching real DOM events on DOM elements, and then asserting on the result. Consider a Toggle component:

// toggle.js

import React, { useState } from "react";

export default function Toggle(props) {
  const [state, setState] = useState(false);
  return (
    <button
      onClick={() => {
        setState(previousState => !previousState);
        props.onChange(!state);
      }}
      data-testid="toggle"
    >
      {state === true ? "Turn off" : "Turn on"}
    </button>
  );
}

We could write tests for it:

// toggle.test.js

import React from "react";
import { render, unmountComponentAtNode } from "react-dom";
import { act } from "react-dom/test-utils";

import Toggle from "./toggle";

let container = null;
beforeEach(() => {
  // setup a DOM element as a render target
  container = document.createElement("div");
  document.body.appendChild(container);});
afterEach(() => {
  // cleanup on exiting
  unmountComponentAtNode(container);
  container.remove();
  container = null;
});

it("changes value when clicked", () => {
  const onChange = jest.fn();
  act(() => {
    render(<Toggle onChange={onChange} />, container);
  });

  // get a hold of the button element, and trigger some clicks on it
  const button = document.querySelector("[data-testid=toggle]");
  expect(button.innerHTML).toBe("Turn on");

  act(() => {
    button.dispatchEvent(new MouseEvent("click", { bubbles: true }));
  });
  expect(onChange).toHaveBeenCalledTimes(1);
  expect(button.innerHTML).toBe("Turn off");

  act(() => {
    for (let i = 0; i < 5; i++) {
      button.dispatchEvent(new MouseEvent("click", { bubbles: true }));
    }  });

  expect(onChange).toHaveBeenCalledTimes(6);
  expect(button.innerHTML).toBe("Turn on");
});

Different DOM events and their properties are described in MDN. Note that you need to pass { bubbles: true } in each event you create for it to reach the React listener because React automatically delegates events to the root.

Note:

React Testing Library offers a more concise helper for firing events.

Timers

Your code might use timer-based functions like setTimeout to schedule more work in the future. In this example, a multiple choice panel waits for a selection and advances, timing out if a selection isn’t made in 5 seconds:

// card.js

import React, { useEffect } from "react";

export default function Card(props) {
  useEffect(() => {
    const timeoutID = setTimeout(() => {
      props.onSelect(null);
    }, 5000);
    return () => {
      clearTimeout(timeoutID);
    };
  }, [props.onSelect]);

  return [1, 2, 3, 4].map(choice => (
    <button
      key={choice}
      data-testid={choice}
      onClick={() => props.onSelect(choice)}
    >
      {choice}
    </button>
  ));
}

We can write tests for this component by leveraging Jest’s timer mocks, and testing the different states it can be in.

// card.test.js

import React from "react";
import { render, unmountComponentAtNode } from "react-dom";
import { act } from "react-dom/test-utils";

import Card from "./card";
let container = null;
beforeEach(() => {
  // setup a DOM element as a render target
  container = document.createElement("div");
  document.body.appendChild(container);
  jest.useFakeTimers();
});

afterEach(() => {
  // cleanup on exiting
  unmountComponentAtNode(container);
  container.remove();
  container = null;
  jest.useRealTimers();
});

it("should select null after timing out", () => {
  const onSelect = jest.fn();
  act(() => {
    render(<Card onSelect={onSelect} />, container);
  });

  // move ahead in time by 100ms  act(() => {
    jest.advanceTimersByTime(100);
  });
  expect(onSelect).not.toHaveBeenCalled();

  // and then move ahead by 5 seconds  act(() => {
    jest.advanceTimersByTime(5000);
  });
  expect(onSelect).toHaveBeenCalledWith(null);
});

it("should cleanup on being removed", () => {
  const onSelect = jest.fn();
  act(() => {
    render(<Card onSelect={onSelect} />, container);
  });
  act(() => {
    jest.advanceTimersByTime(100);
  });
  expect(onSelect).not.toHaveBeenCalled();

  // unmount the app
  act(() => {
    render(null, container);
  });
  act(() => {
    jest.advanceTimersByTime(5000);
  });
  expect(onSelect).not.toHaveBeenCalled();
});

it("should accept selections", () => {
  const onSelect = jest.fn();
  act(() => {
    render(<Card onSelect={onSelect} />, container);
  });

  act(() => {
    container
      .querySelector("[data-testid='2']")
      .dispatchEvent(new MouseEvent("click", { bubbles: true }));
  });

  expect(onSelect).toHaveBeenCalledWith(2);
});

You can use fake timers only in some tests. Above, we enabled them by calling jest.useFakeTimers() . The main advantage they provide is that your test doesn’t actually have to wait five seconds to execute, and you also didn’t need to make the component code more convoluted just for testing.

Snapshot Testing

Frameworks like Jest also let you save “snapshots” of data with toMatchSnapshot / toMatchInlineSnapshot . With these, we can “save” the rendered component output and ensure that a change to it has to be explicitly committed as a change to the snapshot.

In this example, we render a component and format the rendered HTML with the pretty package, before saving it as an inline snapshot:

// hello.test.js, again

import React from "react";
import { render, unmountComponentAtNode } from "react-dom";
import { act } from "react-dom/test-utils";
import pretty from "pretty";

import Hello from "./hello";

let container = null;
beforeEach(() => {
  // setup a DOM element as a render target
  container = document.createElement("div");
  document.body.appendChild(container);
});

afterEach(() => {
  // cleanup on exiting
  unmountComponentAtNode(container);
  container.remove();
  container = null;
});

it("should render a greeting", () => {
  act(() => {
    render(<Hello />, container);
  });

  expect(    pretty(container.innerHTML)  ).toMatchInlineSnapshot(); /* ... gets filled automatically by jest ... */
  act(() => {
    render(<Hello name="Jenny" />, container);
  });

  expect(
    pretty(container.innerHTML)
  ).toMatchInlineSnapshot(); /* ... gets filled automatically by jest ... */

  act(() => {
    render(<Hello name="Margaret" />, container);
  });

  expect(
    pretty(container.innerHTML)
  ).toMatchInlineSnapshot(); /* ... gets filled automatically by jest ... */
});

It’s typically better to make more specific assertions than to use snapshots. These kinds of tests include implementation details so they break easily, and teams can get desensitized to snapshot breakages. Selectively mocking some child components can help reduce the size of snapshots and keep them readable for the code review.

Multiple Renderers

In rare cases, you may be running a test on a component that uses multiple renderers. For example, you may be running snapshot tests on a component with react-test-renderer , that internally uses render from react-dom inside a child component to render some content. In this scenario, you can wrap updates with act() s corresponding to their renderers.

import { act as domAct } from "react-dom/test-utils";
import { act as testAct, create } from "react-test-renderer";
// ...
let root;
domAct(() => {
  testAct(() => {
    root = create(<App />);
  });
});
expect(root).toMatchSnapshot();

Something Missing?

If some common scenario is not covered, please let us know on the issue tracker for the documentation website.

This document goes through the factors that can affect your environment and recommendations for some scenarios.

Test runners

Test runners like Jest, mocha, ava let you write test suites as regular JavaScript, and run them as part of your development process. Additionally, test suites are run as part of continuous integration.

• Jest is widely compatible with React projects, supporting features like mocked modules and timers, and jsdom support. If you use Create React App, Jest is already included out of the box with useful defaults.


• Libraries like mocha work well in real browser environments, and could help for tests that explicitly need it.


• End-to-end tests are used for testing longer flows across multiple pages, and require a different setup.

Mocking a rendering surface

Tests often run in an environment without access to a real rendering surface like a browser. For these environments, we recommend simulating a browser with jsdom , a lightweight browser implementation that runs inside Node.js.

In most cases, jsdom behaves like a regular browser would, but doesn’t have features like layout and navigation. This is still useful for most web-based component tests, since it runs quicker than having to start up a browser for each test. It also runs in the same process as your tests, so you can write code to examine and assert on the rendered DOM.

Just like in a real browser, jsdom lets us model user interactions; tests can dispatch events on DOM nodes, and then observe and assert on the side effects of these actions (example) .

A large portion of UI tests can be written with the above setup: using Jest as a test runner, rendered to jsdom, with user interactions specified as sequences of browser events, powered by the act() helper (example) . For example, a lot of React’s own tests are written with this combination.

If you’re writing a library that tests mostly browser-specific behavior, and requires native browser behavior like layout or real inputs, you could use a framework like mocha.

In an environment where you can’t simulate a DOM (e.g. testing React Native components on Node.js), you could use event simulation helpers to simulate interactions with elements. Alternately, you could use the fireEvent helper from @testing-library/react-native .

Frameworks like Cypress, puppeteer and webdriver are useful for running end-to-end tests.

Mocking functions

When writing tests, we’d like to mock out the parts of our code that don’t have equivalents inside our testing environment (e.g. checking navigator.onLine status inside Node.js). Tests could also spy on some functions, and observe how other parts of the test interact with them. It is then useful to be able to selectively mock these functions with test-friendly versions.

This is especially useful for data fetching. It is usually preferable to use “fake” data for tests to avoid the slowness and flakiness due to fetching from real API endpoints (example) . This helps make the tests predictable. Libraries like Jest and sinon, among others, support mocked functions. For end-to-end tests, mocking network can be more difficult, but you might also want to test the real API endpoints in them anyway.

Mocking modules

Some components have dependencies for modules that may not work well in test environments, or aren’t essential to our tests. It can be useful to selectively mock these modules out with suitable replacements (example) .

On Node.js, runners like Jest support mocking modules. You could also use libraries like mock-require .

Mocking timers

Components might be using time-based functions like setTimeout , setInterval , or Date.now . In testing environments, it can be helpful to mock these functions out with replacements that let you manually “advance” time. This is great for making sure your tests run fast! Tests that are dependent on timers would still resolve in order, but quicker (example) . Most frameworks, including Jest, sinon and lolex, let you mock timers in your tests.

Sometimes, you may not want to mock timers. For example, maybe you’re testing an animation, or interacting with an endpoint that’s sensitive to timing (like an API rate limiter). Libraries with timer mocks let you enable and disable them on a per test/suite basis, so you can explicitly choose how these tests would run.

End-to-end tests

End-to-end tests are useful for testing longer workflows, especially when they’re critical to your business (such as payments or signups). For these tests, you’d probably want to test how a real browser renders the whole app, fetches data from the real API endpoints, uses sessions and cookies, navigates between different links. You might also likely want to make assertions not just on the DOM state, but on the backing data as well (e.g. to verify whether the updates have been persisted to the database).

In this scenario, you would use a framework like Cypress, Playwright or a library like Puppeteer so you can navigate between multiple routes and assert on side effects not just in the browser, but potentially on the backend as well.

React is one of Facebook’s first open source projects that is both under very active development and is also being used to ship code to everybody on facebook.com. We’re still working out the kinks to make contributing to this project as easy and transparent as possible, but we’re not quite there yet. Hopefully this document makes the process for contributing clear and answers some questions that you may have.

Code of Conduct

Facebook has adopted the Contributor Covenant as its Code of Conduct, and we expect project participants to adhere to it. Please read the full text so that you can understand what actions will and will not be tolerated.

Open Development

All work on React happens directly on GitHub. Both core team members and external contributors send pull requests which go through the same review process.

Semantic Versioning

React follows semantic versioning. We release patch versions for critical bugfixes, minor versions for new features or non-essential changes, and major versions for any breaking changes. When we make breaking changes, we also introduce deprecation warnings in a minor version so that our users learn about the upcoming changes and migrate their code in advance. Learn more about our commitment to stability and incremental migration in our versioning policy.

Every significant change is documented in the changelog file.

Branch Organization

Submit all changes directly to the main branch . We don’t use separate branches for development or for upcoming releases. We do our best to keep main in good shape, with all tests passing.

Code that lands in main must be compatible with the latest stable release. It may contain additional features, but no breaking changes. We should be able to release a new minor version from the tip of main at any time.

Feature Flags

To keep the main branch in a releasable state, breaking changes and experimental features must be gated behind a feature flag.

Feature flags are defined in packages/shared/ReactFeatureFlags.js . Some builds of React may enable different sets of feature flags; for example, the React Native build may be configured differently than React DOM. These flags are found in packages/shared/forks . Feature flags are statically typed by Flow, so you can run yarn flow to confirm that you’ve updated all the necessary files.

React’s build system will strip out disabled feature branches before publishing. A continuous integration job runs on every commit to check for changes in bundle size. You can use the change in size as a signal that a feature was gated correctly.

Bugs

Where to Find Known Issues

We are using GitHub Issues for our public bugs. We keep a close eye on this and try to make it clear when we have an internal fix in progress. Before filing a new task, try to make sure your problem doesn’t already exist.

Reporting New Issues

The best way to get your bug fixed is to provide a reduced test case. This JSFiddle template is a great starting point.

Security Bugs

Facebook has a bounty program for the safe disclosure of security bugs. With that in mind, please do not file public issues; go through the process outlined on that page.

How to Get in Touch

• IRC: #reactjs on freenode


• Discussion forums

There is also an active community of React users on the Discord chat platform in case you need help with React.

Proposing a Change

If you intend to change the public API, or make any non-trivial changes to the implementation, we recommend filing an issue. This lets us reach an agreement on your proposal before you put significant effort into it.

If you’re only fixing a bug, it’s fine to submit a pull request right away but we still recommend to file an issue detailing what you’re fixing. This is helpful in case we don’t accept that specific fix but want to keep track of the issue.

Your First Pull Request

Working on your first Pull Request? You can learn how from this free video series:

How to Contribute to an Open Source Project on GitHub

To help you get your feet wet and get you familiar with our contribution process, we have a list of good first issues that contain bugs that have a relatively limited scope. This is a great place to get started.

If you decide to fix an issue, please be sure to check the comment thread in case somebody is already working on a fix. If nobody is working on it at the moment, please leave a comment stating that you intend to work on it so other people don’t accidentally duplicate your effort.

If somebody claims an issue but doesn’t follow up for more than two weeks, it’s fine to take it over but you should still leave a comment.

Sending a Pull Request

The core team is monitoring for pull requests. We will review your pull request and either merge it, request changes to it, or close it with an explanation. For API changes we may need to fix our internal uses at Facebook.com, which could cause some delay. We’ll do our best to provide updates and feedback throughout the process.

Before submitting a pull request, please make sure the following is done:

1. Fork the repository and create your branch from main .


2. Run yarn in the repository root.


3. If you’ve fixed a bug or added code that should be tested, add tests!


4. Ensure the test suite passes ( yarn test ). Tip: yarn test --watch TestName is helpful in development.


5. Run yarn test --prod to test in the production environment.


6. If you need a debugger, run yarn debug-test --watch TestName , open chrome://inspect , and press “Inspect”.


7. Format your code with prettier ( yarn prettier ).


8. Make sure your code lints ( yarn lint ). Tip: yarn linc to only check changed files.


9. Run the Flow typechecks ( yarn flow ).


10. If you haven’t already, complete the CLA.

Contributor License Agreement (CLA)

In order to accept your pull request, we need you to submit a CLA. You only need to do this once, so if you’ve done this for another Facebook open source project, you’re good to go. If you are submitting a pull request for the first time, just let us know that you have completed the CLA and we can cross-check with your GitHub username.

Complete your CLA here.

Contribution Prerequisites

• You have Node installed at LTS and Yarn at v1.2.0+.


• You have JDK installed.


• You have gcc installed or are comfortable installing a compiler if needed. Some of our dependencies may require a compilation step. On OS X, the Xcode Command Line Tools will cover this. On Ubuntu, apt-get install build-essential will install the required packages. Similar commands should work on other Linux distros. Windows will require some additional steps, see the node-gyp installation instructions for details.


• You are familiar with Git.

Development Workflow

After cloning React, run yarn to fetch its dependencies.
Then, you can run several commands:

• yarn lint checks the code style.


• yarn linc is like yarn lint but faster because it only checks files that differ in your branch.


• yarn test runs the complete test suite.


• yarn test --watch runs an interactive test watcher.


• yarn test --prod runs tests in the production environment.


• yarn test <pattern> runs tests with matching filenames.


• yarn debug-test is just like yarn test but with a debugger. Open chrome://inspect and press “Inspect”.


• yarn flow runs the Flow typechecks.


• yarn build creates a build folder with all the packages.


• yarn build react/index,react-dom/index --type=UMD creates UMD builds of just React and ReactDOM.

We recommend running yarn test (or its variations above) to make sure you don’t introduce any regressions as you work on your change. However, it can be handy to try your build of React in a real project.

First, run yarn build . This will produce pre-built bundles in build folder, as well as prepare npm packages inside build/packages .

The easiest way to try your changes is to run yarn build react/index,react-dom/index --type=UMD and then open fixtures/packaging/babel-standalone/dev.html . This file already uses react.development.js from the build folder so it will pick up your changes.

If you want to try your changes in your existing React project, you may copy build/node_modules/react/umd/react.development.js , build/node_modules/react-dom/umd/react-dom.development.js , or any other build products into your app and use them instead of the stable version.

If your project uses React from npm, you may delete react and react-dom in its dependencies and use yarn link to point them to your local build folder. Note that instead of --type=UMD you’ll want to pass --type=NODE when building . You’ll also need to build the scheduler package:

cd ~/path_to_your_react_clone/
yarn build react/index,react/jsx,react-dom/index,scheduler --type=NODE

cd build/node_modules/react
yarn link
cd build/node_modules/react-dom
yarn link

cd ~/path/to/your/project
yarn link react react-dom

Every time you run yarn build in the React folder, the updated versions will appear in your project’s node_modules . You can then rebuild your project to try your changes.

If some package is still missing (e.g. maybe you use react-dom/server in your project), you can always do a full build with yarn build . Note that running yarn build without options takes a long time.

We still require that your pull request contains unit tests for any new functionality. This way we can ensure that we don’t break your code in the future.

Style Guide

We use an automatic code formatter called Prettier.
Run yarn prettier after making any changes to the code.

Then, our linter will catch most issues that may exist in your code.
You can check the status of your code styling by simply running yarn linc .

However, there are still some styles that the linter cannot pick up. If you are unsure about something, looking at Airbnb’s Style Guide will guide you in the right direction.

Request for Comments (RFC)

Many changes, including bug fixes and documentation improvements can be implemented and reviewed via the normal GitHub pull request workflow.

Some changes though are “substantial”, and we ask that these be put through a bit of a design process and produce a consensus among the React core team.

The “RFC” (request for comments) process is intended to provide a consistent and controlled path for new features to enter the project. You can contribute by visiting the rfcs repository.

License

By contributing to React, you agree that your contributions will be licensed under its MIT license.

What Next?

Read the next section to learn how the codebase is organized.

This section will give you an overview of the React codebase organization, its conventions, and the implementation.

If you want to contribute to React we hope that this guide will help you feel more comfortable making changes.

We don’t necessarily recommend any of these conventions in React apps. Many of them exist for historical reasons and might change with time.

Top-Level Folders

After cloning the React repository, you will see a few top-level folders in it:

• packages contains metadata (such as package.json ) and the source code ( src subdirectory) for all packages in the React repository. If your change is related to the code, the src subdirectory of each package is where you’ll spend most of your time.


• fixtures contains a few small React test applications for contributors.


• build is the build output of React. It is not in the repository but it will appear in your React clone after you build it for the first time.

The documentation is hosted in a separate repository from React.

There are a few other top-level folders but they are mostly used for the tooling and you likely won’t ever encounter them when contributing.

Colocated Tests

We don’t have a top-level directory for unit tests. Instead, we put them into a directory called __tests__ relative to the files that they test.

For example, a test for setInnerHTML.js is located in __tests__/setInnerHTML-test.js right next to it.

Warnings and Invariants

The React codebase uses console.error to display warnings:

if (__DEV__) {
  console.error('Something is wrong.');
}

Warnings are only enabled in development. In production, they are completely stripped out. If you need to forbid some code path from executing, use invariant module instead:

var invariant = require('invariant');

invariant(
  2 + 2 === 4,
  'You shall not pass!'
);

The invariant is thrown when the invariant condition is false .

“Invariant” is just a way of saying “this condition always holds true”. You can think about it as making an assertion.

It is important to keep development and production behavior similar, so invariant throws both in development and in production. The error messages are automatically replaced with error codes in production to avoid negatively affecting the byte size.

Development and Production

You can use __DEV__ pseudo-global variable in the codebase to guard development-only blocks of code.

It is inlined during the compile step, and turns into process.env.NODE_ENV !== 'production' checks in the CommonJS builds.

For standalone builds, it becomes true in the unminified build, and gets completely stripped out with the if blocks it guards in the minified build.

if (__DEV__) {
  // This code will only run in development.
}

Flow

We recently started introducing Flow checks to the codebase. Files marked with the @flow annotation in the license header comment are being typechecked.

We accept pull requests adding Flow annotations to existing code. Flow annotations look like this:

ReactRef.detachRefs = function(
  instance: ReactInstance,
  element: ReactElement | string | number | null | false,
): void {
  // ...
}

When possible, new code should use Flow annotations.
You can run yarn flow locally to check your code with Flow.

Multiple Packages

React is a monorepo. Its repository contains multiple separate packages so that their changes can be coordinated together, and issues live in one place.

React Core

The “core” of React includes all the top-level React APIs, for example:

• React.createElement()


• React.Component


• React.Children

React core only includes the APIs necessary to define components. It does not include the reconciliation algorithm or any platform-specific code. It is used both by React DOM and React Native components.

The code for React core is located in packages/react in the source tree. It is available on npm as the react package. The corresponding standalone browser build is called react.js , and it exports a global called React .

Renderers

React was originally created for the DOM but it was later adapted to also support native platforms with React Native. This introduced the concept of “renderers” to React internals.

Renderers manage how a React tree turns into the underlying platform calls.

Renderers are also located in packages/ :

• React DOM Renderer renders React components to the DOM. It implements top-level ReactDOM APIs and is available as react-dom npm package. It can also be used as standalone browser bundle called react-dom.js that exports a ReactDOM global.


• React Native Renderer renders React components to native views. It is used internally by React Native.


• React Test Renderer renders React components to JSON trees. It is used by the Snapshot Testing feature of Jest and is available as react-test-renderer npm package.

The only other officially supported renderer is react-art . It used to be in a separate GitHub repository but we moved it into the main source tree for now.

Note:

Technically the react-native-renderer is a very thin layer that teaches React to interact with React Native implementation. The real platform-specific code managing the native views lives in the React Native repository together with its components.

Reconcilers

Even vastly different renderers like React DOM and React Native need to share a lot of logic. In particular, the reconciliation algorithm should be as similar as possible so that declarative rendering, custom components, state, lifecycle methods, and refs work consistently across platforms.

To solve this, different renderers share some code between them. We call this part of React a “reconciler”. When an update such as setState() is scheduled, the reconciler calls render() on components in the tree and mounts, updates, or unmounts them.

Reconcilers are not packaged separately because they currently have no public API. Instead, they are exclusively used by renderers such as React DOM and React Native.

Stack Reconciler

The “stack” reconciler is the implementation powering React 15 and earlier. We have since stopped using it, but it is documented in detail in the next section.

Fiber Reconciler

The “fiber” reconciler is a new effort aiming to resolve the problems inherent in the stack reconciler and fix a few long-standing issues. It has been the default reconciler since React 16.

Its main goals are:

• Ability to split interruptible work in chunks.


• Ability to prioritize, rebase and reuse work in progress.


• Ability to yield back and forth between parents and children to support layout in React.


• Ability to return multiple elements from render() .


• Better support for error boundaries.

You can read more about React Fiber Architecture here and here. While it has shipped with React 16, the async features are not enabled by default yet.

Its source code is located in packages/react-reconciler .

Event System

React implements a layer over native events to smooth out cross-browser differences. Its source code is located in packages/react-dom/src/events .

What Next?

Read the next section to learn about the pre-React 16 implementation of reconciler in more detail. We haven’t documented the internals of the new reconciler yet.