This style guide is mostly based on the standards that are currently prevalent in JavaScript, although some conventions (i.e async/await or static class fields) may still be included or prohibited on a case-by-case basis. Currently, anything prior to stage 3 is not included nor recommended in this guide.
- However, multiple [Stateless, or Pure, Components](https://facebook.github.io/react/docs/reusable-components.html#stateless-functions) are allowed per file. eslint: [`react/no-multi-comp`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/no-multi-comp.md#ignorestateless).
- [`react/forbid-prop-types`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/forbid-prop-types.md) will allow `arrays` and `objects` only if it is explicitly noted what `array` and `object` contains, using `arrayOf`, `objectOf`, or `shape`.
- If you have internal state and/or refs, prefer `class extends React.Component` over `React.createClass`. eslint: [`react/prefer-es6-class`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/prefer-es6-class.md) [`react/prefer-stateless-function`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/prefer-stateless-function.md)
- [Do not use mixins](https://facebook.github.io/react/blog/2016/07/13/mixins-considered-harmful.html).
> Why? Mixins introduce implicit dependencies, cause name clashes, and cause snowballing complexity. Most use cases for mixins can be accomplished in better ways via components, higher-order components, or utility modules.
- **Extensions**: Use `.jsx` extension for React components. eslint: [`react/jsx-filename-extension`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/jsx-filename-extension.md)
- **Reference Naming**: Use PascalCase for React components and camelCase for their instances. eslint: [`react/jsx-pascal-case`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/jsx-pascal-case.md)
- **Component Naming**: Use the filename as the component name. For example, `ReservationCard.jsx` should have a reference name of `ReservationCard`. However, for root components of a directory, use `index.jsx` as the filename and use the directory name as the component name:
- **Higher-order Component Naming**: Use a composite of the higher-order component’s name and the passed-in component’s name as the `displayName` on the generated component. For example, the higher-order component `withFoo()`, when passed a component `Bar` should produce a component with a `displayName` of `withFoo(Bar)`.
> Why? A component’s `displayName` may be used by developer tools or in error messages, and having a value that clearly expresses this relationship helps people understand what is happening.
> Why? People expect props like `style` and `className` to mean one specific thing. Varying this API for a subset of your app makes the code less readable and less maintainable, and may cause bugs.
- Always use double quotes (`"`) for JSX attributes, but single quotes (`'`) for all other JS. eslint: [`jsx-quotes`](https://eslint.org/docs/rules/jsx-quotes)
- Always include a single space in your self-closing tag. eslint: [`no-multi-spaces`](https://eslint.org/docs/rules/no-multi-spaces), [`react/jsx-tag-spacing`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/jsx-tag-spacing.md)
- Do not pad JSX curly braces with spaces. eslint: [`react/jsx-curly-spacing`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/jsx-curly-spacing.md)
- Omit the value of the prop when it is explicitly `true`. eslint: [`react/jsx-boolean-value`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/jsx-boolean-value.md)
- Always include an `alt` prop on `<img>` tags. If the image is presentational, `alt` can be an empty string or the `<img>` must have `role="presentation"`. eslint: [`jsx-a11y/alt-text`](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/blob/master/docs/rules/alt-text.md)
- Do not use words like "image", "photo", or "picture" in `<img>` `alt` props. eslint: [`jsx-a11y/img-redundant-alt`](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/blob/master/docs/rules/img-redundant-alt.md)
- Use only valid, non-abstract [ARIA roles](https://www.w3.org/TR/wai-aria/#usage_intro). eslint: [`jsx-a11y/aria-role`](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/blob/master/docs/rules/aria-role.md)
- Do not use `accessKey` on elements. eslint: [`jsx-a11y/no-access-key`](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/blob/master/docs/rules/no-access-key.md)
- Avoid using an array index as `key` prop, prefer a stable ID. eslint: [`react/no-array-index-key`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/no-array-index-key.md)
> Why? Not using a stable ID [is an anti-pattern](https://medium.com/@robinpokorny/index-as-a-key-is-an-anti-pattern-e0349aece318) because it can negatively impact performance and cause issues with component state.
We don’t recommend using indexes for keys if the order of items may change.
- Always define explicit defaultProps for all non-required props.
> Why? propTypes are a form of documentation, and providing defaultProps means the reader of your code doesn’t have to assume as much. In addition, it can mean that your code can omit certain type checks.
> Why? Otherwise you’re more likely to pass unnecessary props down to components. And for React v15.6.1 and older, you could [pass invalid HTML attributes to the DOM](https://reactjs.org/blog/2017/09/08/dom-attributes-in-react-16.html).
- Wrap JSX tags in parentheses when they span more than one line. eslint: [`react/jsx-wrap-multilines`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/jsx-wrap-multilines.md)
- Always self-close tags that have no children. eslint: [`react/self-closing-comp`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/self-closing-comp.md)
- If your component has multiline properties, close its tag on a new line. eslint: [`react/jsx-closing-bracket-location`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/jsx-closing-bracket-location.md)
- Use arrow functions to close over local variables. It is handy when you need to pass additional data to an event handler. Although, make sure they [do not massively hurt performance](https://www.bignerdranch.com/blog/choosing-the-best-approach-for-react-event-handlers/), in particular when passed to custom components that might be PureComponents, because they will trigger a possibly needless rerender every time.
- Bind event handlers for the render method in the constructor. eslint: [`react/jsx-no-bind`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/jsx-no-bind.md)
> Why? A bind call in the render path creates a brand new function on every single render. Do not use arrow functions in class fields, because it makes them [challenging to test and debug, and can negatively impact performance](https://medium.com/@charpeni/arrow-functions-in-class-properties-might-not-be-as-great-as-we-think-3b3551c440b1), and because conceptually, class fields are for data, not logic.
> Why? Underscore prefixes are sometimes used as a convention in other languages to denote privacy. But, unlike those languages, there is no native support for privacy in JavaScript, everything is public. Regardless of your intentions, adding underscore prefixes to your properties does not actually make them private, and any property (underscore-prefixed or not) should be treated as being public. See issues [#1024](https://github.com/airbnb/javascript/issues/1024), and [#490](https://github.com/airbnb/javascript/issues/490) for a more in-depth discussion.
- Be sure to return a value in your `render` methods. eslint: [`react/require-render-return`](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/require-render-return.md)
