README.md

TSLint Rules of Hooks

A TSLint rule that enforces the Rules of Hooks for React hooks.

The rule is based on an ESLint plugin for react hooks.

Features

• detects using React hooks inside potentially-conditional branches:
  • if statements
  • short-circuit conditional expressions (&&, ||)
  • ternary expressions
  • loops (while, for, do ... while)
  • functions that themselves are not custom hooks or components
• detects using React hooks in spite of an early return
• support for detecting hooks from namespaces other than React (e.g. MyHooks.useHook) (optional)

Installation

First, install the rule:

npm install tslint-react-hooks --save-dev

Then, enable the rule by modifying tslint.json:

{
  "extends": [
    // your other plugins...
    "tslint-react-hooks"
  ],
  "rules": {
    // your other rules...
    "react-hooks-nesting": "error"
  }
}

To use report rule violations as warnings intead of errors, set it to "warning".

Options

While the rule works fine out-of-the-box, it can be customized. To specify options, use the following syntax when modifying tslint.json:

{
  "extends": [
    // your other plugins...
    "tslint-react-hooks"
  ],
  "rules": {
    // your other rules...
    "react-hooks-nesting": {
      "severity": "error", // "error", "warning", "default" or "off"
      "options": {
        // options go here
      }
    }
  }
}

Available options

• "detect-hooks-from-non-react-namespace" - when set to true, violations will be also reported hooks accessed from sources other than the React namespace (e.g. MyHooks.useHook will be treated as a hook).

  By default, only direct calls (e.g. useHook) or calls from React namespace (e.g. React.useState) are treated as hooks.

Have an idea for an option? Create a new issue.

Workarounds

For some arrow functions/function expressions, the rule has no way to determine whether those are a component, a hook, both of which could contain hook calls, or a regular function that should not contain hook calls.

const withHoc = <TProps extends object>(Component: ComponentType<TProps>) => (
  props: TProps,
) => {
  const [state] = useState();
  return <Component {...props} />;
};

The workaround in those cases is to use a named function expression:

const withHoc = <TProps extends object>(Component: ComponentType<TProps>) =>
  function WrappedComponent(props: TProps) {
    const [state] = useState();
    return <Component {...props} />;
  };

Naming the function like a component (in PascalCase) unambiguously lets the rule treat the function as a component.

False positives and not-covered cases

There are some cases that seem hard to analyze and may result in false positives or false negatives.

In such cases, disable the rule for a specific line using the following comment:

// tslint:disable:react-hooks-nesting
useEffect(() => {});

Looping over static arrays

The rule may report false positives, for example in:

function MyComponent() {
  const array = [1, 2, 3];

  array.forEach(value => {
    React.useEffect(() => console.log(value));
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [A hook cannot be used inside of another function]
  });
}

The useEffect hook will be called unconditionally and the call-order will be the same between renders.

Using renamed hooks (that do not start with use)

The rule only treats functions that start with use as hooks. Therefore, renaming the hook will result in avoiding the rule:

const renamedUseState = React.useState;

function MyComponent() {
  const [state, setState] = renamedUseState(0);
}

Unconditional nesting

Unconditional nesting, for example:

function MyComponent() {
  if (true) {
    const variableThatCannotBeLeaked = useContext(SomeContext);
    useEffect(() => {
      console.log(variableThatCannotBeLeaked);
    });
  }

  return <div>Text</div>;
}

is treated as conditional nesting. It seems hard to verify if the condition is in fact a constant, therefore such a situation will always result in a rule violation.

In situations where such an if statement was used to create an additional block scope, use the block scope directly:

function MyComponent() {
  {
    const variableThatCannotBeLeaked = useContext(SomeContext);
    useEffect(() => {
      console.log(variableThatCannotBeLeaked);
    });
  }

  return <div>Text</div>;
}

Development

After pulling the repository, make sure to run

npm install

The source code for the rule is located in the src directory.

For more information about the developing custom TSLint rules, take a look at TSLint's documentation.

Testing the rule

Run

npm run test

to compile the rule and run automatic TSLint tests.

They are located in the test directory.

Author

The author of this rule is Grzegorz Rozdzialik.