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# Summary note: This implements the idea discussed in https://github.com/reactwg/react/discussions/389#discussioncomment-14252280 This is a large PR that significantly changes our impurity and ref validation to address multiple issues. The goal is to reduce false positives and make the errors we do report more actionable. ## Validating Against Impure Values In Render Currently we create `Impure` effects for impure functions like `Date.now()` or `Math.random()`, and then throw if the effect is reachable during render. However, impurity is a property of the resulting value: if the value isn't accessed during render then it's okay: maybe you're console-logging the time while debugging (fine), or storing the impure value into a ref and only accessing it in an effect or event handler (totally ok). This PR updates to validate that impure values are not transitively consumed during render, building on the new effects system: rather than look at instruction types, we use effects like `Capture a -> b`, `Impure a`, and `Render b` to determine how impure values are introduced and where they flow through the program. We're intentionally conservative, and do _not_ propagate impurity through MaybeCapture effects, which is used for functions we don't have signatures for. This means that things like `identity(performance.now())` drop the impurity. This feels like a good compromise since it means we have very high confidence in the errors that we report and can always add increased strictness later as our confidence increases. An example error: ``` Error: Cannot access impure value during render Calling an impure function can produce unstable results that update unpredictably when the component happens to re-render. (https://react.dev/reference/rules/components-and-hooks-must-be-pure#components-and-hooks-must-be-idempotent). error.invalid-impure-value-in-render-helper.ts:5:17 3 | const now = () => Date.now(); 4 | const render = () => { > 5 | return <div>{now()}</div>; | ^^^^^ Cannot access impure value during render 6 | }; 7 | return <div>{render()}</div>; 8 | } error.invalid-impure-value-in-render-helper.ts:3:20 1 | // @validateNoImpureFunctionsInRender 2 | function Component() { > 3 | const now = () => Date.now(); | ^^^^^^^^^^ `Date.now` is an impure function. 4 | const render = () => { 5 | return <div>{now()}</div>; 6 | }; ``` Impure values are allowed to flow into refs, meaning that we now allow `useRef(Date.now())` or `useRef(localFunctionThatReturnsMathDotRandom())` which would have errored previously. The next PR reuses this improved impurity tracking to validate ref access in render as well. ## Refs Now Treated As Impure Values in Render Reading a ref now produces an `Impure` effect, and reading refs in render is validated using the above validation against impure values in render. The error category and message is customized for refs, we're just reusing the validation implementation. This means you get consistent results for `performance.now()` as for `ref.current`. A nice consistency win. ## Simplified writing-ref validation Now that _reading_ a ref in render is validated using the impure values infra, I also dramatically simplified ValidateNoRefAccessInRender to focus solely on validation against _writing_ refs during render. It was harder to use the new effects infra for this since we intentionally do not record ref mutations as a `Mutate` effect. So for now, the pass switches on InstructionValue variants. We continue to support the `if (ref.current == null) { ref.current = <init> }` pattern and reasonable variants of it. We're conservative about what we consider to be a write of a ref - `foo(ref)` now assumes you're not mutating rather than the inverse. # Takeaways * Impure-values-in-render logic is more conservative about what it considers an error (ie to users it appears more persmissive). We follow clearly identifiable (and if we wanted traceable/explainable) paths from impure sources through to where they are rendered. We allow many more cases than before, notably `x = foo(ref)` optimistically assumes you don't read the ref. Concretely, this follows from not tracking impure values across MaybeCapture effects. * No-writing-refs-in-render is also more conservative about what it counts as a write, appearing to users as allowing more cases. We look for very direct evidence of writing to refs, ie `ref.current = <value>` or `ref.current.property = <value>`, and allow potential writes through eg `writeToRef(ref, value)`.
React · 
React is a JavaScript library for building user interfaces.
- Declarative: React makes it painless to create interactive UIs. Design simple views for each state in your application, and React will efficiently update and render just the right components when your data changes. Declarative views make your code more predictable, simpler to understand, and easier to debug.
- Component-Based: Build encapsulated components that manage their own state, then compose them to make complex UIs. Since component logic is written in JavaScript instead of templates, you can easily pass rich data through your app and keep the state out of the DOM.
- Learn Once, Write Anywhere: We don't make assumptions about the rest of your technology stack, so you can develop new features in React without rewriting existing code. React can also render on the server using Node and power mobile apps using React Native.
Learn how to use React in your project.
Installation
React has been designed for gradual adoption from the start, and you can use as little or as much React as you need:
- Use Quick Start to get a taste of React.
- Add React to an Existing Project to use as little or as much React as you need.
- Create a New React App if you're looking for a powerful JavaScript toolchain.
Documentation
You can find the React documentation on the website.
Check out the Getting Started page for a quick overview.
The documentation is divided into several sections:
- Quick Start
- Tutorial
- Thinking in React
- Installation
- Describing the UI
- Adding Interactivity
- Managing State
- Advanced Guides
- API Reference
- Where to Get Support
- Contributing Guide
You can improve it by sending pull requests to this repository.
Examples
We have several examples on the website. Here is the first one to get you started:
import { createRoot } from 'react-dom/client';
function HelloMessage({ name }) {
return <div>Hello {name}</div>;
}
const root = createRoot(document.getElementById('container'));
root.render(<HelloMessage name="Taylor" />);
This example will render "Hello Taylor" into a container on the page.
You'll notice that we used an HTML-like syntax; we call it JSX. JSX is not required to use React, but it makes code more readable, and writing it feels like writing HTML.
Contributing
The main purpose of this repository is to continue evolving React core, making it faster and easier to use. Development of React happens in the open on GitHub, and we are grateful to the community for contributing bugfixes and improvements. Read below to learn how you can take part in improving React.
Code of Conduct
Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please read the full text so that you can understand what actions will and will not be tolerated.
Contributing Guide
Read our contributing guide to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to React.
Good First Issues
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.
License
React is MIT licensed.