# no-restricted-syntax

> Disallow the syntax you describe with selectors

Sometimes you want to ban one piece of the language in your project and there's no rule for it, like `for..in` loops, TypeScript enums, or a call to one function with one kind of argument. This rule takes a list of selectors and reports every syntax tree node that matches one.

A selector names an ESTree node type, such as `ForInStatement`, and can narrow it down the way a CSS selector does: by attribute (`[name="eval"]`), by parent (`A > B`), by ancestor (`A B`), or with `:not()`, `:is()` and `:has()`. TypeScript syntax uses typescript-eslint's node types, such as `TSEnumDeclaration`.

The rule does nothing until you give it a selector.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

Off by default. To turn it on:

```toml bunfig.toml
[lint.rules]
"no-restricted-syntax" = "error" # or "warn"
```

{/* End of the generated section. */}

## Examples

```toml bunfig.toml
[lint.rules]
"no-restricted-syntax" = ["error", "ForInStatement"]
```

Reported:

{/* bad {"options": ["ForInStatement"]} */}
```js
for (const key in settings) {
  apply(key, settings[key]);
}
```

Not reported:

{/* good {"options": ["ForInStatement"]} */}
```js
for (const key of Object.keys(settings)) {
  apply(key, settings[key]);
}
```

## Options

Each option is a selector, or an object with a selector and the message to show.

| Option | What it does |
| --- | --- |
| `"<selector>"` | Report anything that matches, with the message `Using '<selector>' is not allowed.` |
| `{ "selector": "..", "message": ".." }` | Report anything that matches, with your message. |

This selector matches a literal, such as a string, that's the first argument of `setTimeout`:

```toml bunfig.toml
[lint.rules]
"no-restricted-syntax" = ["error", {
  selector = "CallExpression[callee.name='setTimeout'] > Literal:first-child",
  message = "Pass a function to setTimeout.",
}]
```

{/* bad {"options": [{"selector": "CallExpression[callee.name='setTimeout'] > Literal:first-child", "message": "Pass a function to setTimeout."}]} */}
```js
setTimeout("refresh()", 1000);
```

{/* good {"options": [{"selector": "CallExpression[callee.name='setTimeout'] > Literal:first-child", "message": "Pass a function to setTimeout."}]} */}
```js
setTimeout(refresh, 1000);
```

In a TypeScript file:

{/* bad {"options": [{"selector": "TSEnumDeclaration", "message": "Use a union of strings."}]} */}
```ts
enum Color {
  Red,
  Blue,
}
```

{/* good {"options": [{"selector": "TSEnumDeclaration", "message": "Use a union of strings."}]} */}
```ts
type Color = "red" | "blue";
```