# TSLint Rules of Hooks

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![Demo](https://i.imgur.com/SGHlOvF.png)

A TSLint rule that enforces the [Rules of Hooks](https://reactjs.org/docs/hooks-rules.html) for
React hooks.

The rule is based on an [ESLint plugin for react hooks](https://github.com/facebook/react/blob/master/packages/eslint-plugin-react-hooks/README.md).

## 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](https://www.npmjs.com/package/tslint-react-hooks):

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

Then, enable the rule by modifying `tslint.json`:

```js
{
  "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`:

```js
{
  "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](https://github.com/Gelio/tslint-react-hooks/issues/new).

## 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.

```ts
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**:

```ts
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:

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

### Looping over static arrays

The rule may report false positives, for example in:

```ts
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:

```ts
const renamedUseState = React.useState;

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

### Unconditional nesting

Unconditional nesting, for example:

```tsx
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:

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

  return <div>Text</div>;
}
```

## Development

After pulling the repository, make sure to run

```sh
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](https://palantir.github.io/tslint/develop/custom-rules/).

### Testing the rule

Run

```sh
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.