@arthurgeron/eslint-plugin-react-usememo

eslint-plugin-react-usememo

Enforce that functions or complex objects that can generate unecessary renders or side-effects are wrapped in useMemo or useCallback, allow for devs to enforce that functional components be wrapped in memo programatically, and that all props and deps are wrapped in useMemo/useCallback; The intended outcome is that component's tree and/or expensive lifecycles (e.g. React Native's FlatLists, useEffect, useMemo, etc) only re-calculate or render again when really necessary, controlling expensive expressions and bringing out the best scalability and performance that your application can get.

Rationale

React Native's own docs state how it's important to use static or memoized as props for complex children (FlatList on that case), that applies even more broadly when we are talking about custom components (the Components you've created), it might not seem necessary at first but you'll be making a bet that the component in question will never grow to use memo or those props in hooks (i.e. useEffect, useMemo, useCallback), you'll only notice once your solution starts freezing and dropping frames, that's why using the require-usememo rule is recommended.

Installation

yarn add @arthurgeron/eslint-plugin-react-usememo --dev

or

npm install @arthurgeron/eslint-plugin-react-usememo --save-dev

Usage

To enable the plugin add the following to the plugin property your eslintrc file:

"plugins": ["@arthurgeron/react-usememo"],

Then enable any rules as you like, example:

"rules": {
    "@arthurgeron/react-usememo/require-usememo": [2],
},

Rules

require-usememo Recommended

Requires complex values (objects, arrays, functions, and JSX) that get passed props or referenced as a hook dependency to be wrapped in React.useMemo() or React.useCallback().

Options:

• {strict: true}: Fails even in cases where it is difficult to determine if the value in question is a primitive (string or number) or a complex value (object, array, etc.);

• {checkHookReturnObject: true}: Will require Object Expressions passed in return statements (e.g. return {someFunc}) to also be memoised (e.g. return useMemo(() => ({someFunc}), [someFunc])); Disabled by default;

• {checkHookCalls: true}: Will require objects/data passed to a non-native/Custom hook to be memoized (e.g. in this code const data = useParse(unparsedData) it will check unparsedData for memo status), this check can be ignored by a hook/name basis, check next item for details; Enabled by default;

• {ignoredHookCallsNames: Record<string, boolean>}: You can add specific hooks names here, individually disabling or enabling them to be checked when used, e.g. if you have a custom hook named useX and it should be called with unmemoized parameters you can set { ignoredHookCallsNames: { useX: false } } and then have somethning like const data = {}; const x = useX(data); not generate any errors.

Function Components

Incorrect

  function Component() {

    const [data, setData] = useState([]);
    
    
    // This will be redeclared each render
    function renderItem({ item }) {
      return (<Text>item.name</Text>);
    }

    // Data isn't redeclared each ender but `[]` is
    return (<FlatList renderItem={renderItem} data={data ?? []} />);
  }

Correct

  // Has no dynamic dependencies therefore should be static, will be declared only once.
  function renderItem({ item }) {
    return <Text>item.name</Text>;
  }

  const EMPTY_ARRAY = [];

  function Component() {

    const [data, setData] = useState(EMPTY_ARRAY);
    
    // Will only render again if data changes
    return (<FlatList renderItem={renderItem} data={data ?? EMPTY_ARRAY} />);
  }

Class Components

Incorrect

  class Component() {

    constructor(props) {
      super(props);
      this.state = {
        data: undefined,
        propDrivenData: props.,
      };
    }
    
    
    // This will NOT be redeclared each render
    getItemName(item) {
      return item.name;
    }

    render() {
      // This function will be redeclared each render
      function renderItem({ item }) {
        return (<Text>{this.getItemName(item)}</Text>);
      }

      // Data isn't redeclared each ender but [] is
      // Extradata has a exponential complexity (will iterate the entire array for each render, could render once or several times in a second)
      // Outcome will be that any new render on this component will cause the entire FlatList to render again, including children components, even if the data hasn't changed.
      return (<FlatList
        renderItem={renderItem}
        data={data ?? []} 
        extraData={dataArray.filter(id => !!id)}
      />);
    }
  }

In the previous example there are two issues, a function and a object that will be dynamically redeclared each time the component renders, which will cause FlatList to keep re-rendering even when the input data hasn't changed.

Correct

  // Static therefore is only declared once
  const EMPTY_ARRAY = [];

  class Component() {
    constructor(props) {
      super(props);
      this.state = {
        data: undefined,
        propDrivenData: props.dataArray.filter(id => !!id),
      };
    }
    
    // Properly regenerate state driven data only when props change instead of during each render
    static getDerivedStateFromProps(props) {
      if (props.propDrivenData !== this.props.propDrivenData) {
        return {
          propDrivenData: props.dataArray.filter(id => !!id),
        };
      }
      return null;
    }

    // Will be declared only once.
    getItemName({item}) {
      const { data } = this.state;
      const dataLength = data ? data.length : 0;
      return (<Text>{item.name} {dataLength}</Text>);
    }

    render() {
      const { data } = this.state;
      // Will only cause a new render if data changes
      return (<FlatList renderItem={this.renderItem} data={data ?? EMPTY_ARRAY} />);
    }
  }

Correct

  const EMPTY_ARRAY = [];

  function Component() {
    const [data, setData] = useState(EMPTY_ARRAY);
    const [isEditing, setIsEditing] = useState(false);

    // Has dynamic dependencies but will only be re-declared when isEditing or the input data changes
    const renderItem = useCallback(({ item }) => {
      return (<Text>{isEditing ? 'item.name' : 'Editing'}</Text>);
    }, [isEditing]);

    return (<FlatList renderItem={renderItem} data={data ?? EMPTY_ARRAY} />);
  }

Hooks

Hooks return statements follow the same motto, they can and usually are used inside other hooks.

Incorrect

  function useData() {

    let otherData = {};
    function getData() {}
    
    return { getData, otherData};
  }

Incorrect (checkHookReturnObject: true)

  function getData() {}

  function useData() {
    
    return { getData };
  }

Incorrect Calling a custom hook

  function doSomething() {}

  function useStateManagement(someFlag) {
    useEffect(() => {
      doSomething(someFlag);
    }, [someFlag]);
  }

  function useData() {
    const someFlag = {};
    // Some flag is memoized, so not unwanted side effects are generated in the other hook
    const data = useStateManagement(someFlag);
  }

Correct

  const otherData = {}; // Or declare inside hook with useMemo

  function useData() {
    const getData = useCallback(() => { // Or declare statically
    }, []);
    return {getData, otherData};
  }

Correct (checkHookReturnObject: true)

  function getData() {
    // Doing something
  }
  function useData() {
    
    return useMemo(() => ({ getData }), []);
  }

Correct Passing a dependency to a hook

  function getData() {}

  function useData() {
    
    return useMemo(() => ({ getData }), []);
  }

Correct Calling a custom hook

  function doSomething() {}

  function useStateManagement(someFlag) {
    useEffect(() => {
      doSomething(someFlag);
    }, [someFlag]);
  }

  function useData() {
    const someFlag = useMemo(() => ({}), []);
    // Some flag is memoized, so not unwanted side effects are generated in the other hook
    const data = useStateManagement(someFlag);
  }

"Correct" Calling a custom hook (checkHookCalls: true) or (ignoredHookCallsNames: { useStateManagement: false })

  function doSomething() {
  }
  function useStateManagement(someFlag) {
    useEffect(() => {
      doSomething(someFlag);
    }, [someFlag]);
  }

  function useData() {
    const someFlag = useMemo(() => ({}), []);
    // Some flag is regenerated each render, hence useEffect in "useStateManagement" will execute a new cycle each render
    // Either way this will not generate a error/warning because of the options passed
    const data = useStateManagement(someFlag);
  }

require-memo

Requires all function components to be wrapped in React.memo().
May be useful when used with overrides in your eslint config, I do not recommend enabling this globally, while there's great advantaje in memoing a complex tree of components some smaller/basic components with no children might not need to be memoized.

Incorrect

  export default function Component() {
    return (<Text>This is a component</Text>);
  }

Correct

  export default memo(function Component() {
    return (<Text>This is a component</Text>);
  });

require-usememo-children Advanced

Requires complex values (objects, arrays, functions, and JSX) that get passed as children to be wrapped in React.useMemo() or React.useCallback().

Options:

• {strict: true}: Fails even in cases where it is difficult to determine if the value in question is a primitive (string or number) or a complex value (object, array, etc.).

Incorrect

  function Component() {

    return (<View>
      <>
      <OtherComponent />
      </>
    </View>);
  }

Correct

  function Component() {
    const children = useMemo(() => (<OtherComponent />), []);
    
    return (<View>
      {children}
    </View>);
  }