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# eslint-visitor-keys
[](https://www.npmjs.com/package/eslint-visitor-keys)
[](http://www.npmtrends.com/eslint-visitor-keys)
[](https://travis-ci.org/eslint/eslint-visitor-keys)
[](https://david-dm.org/eslint/eslint-visitor-keys)
Constants and utilities about visitor keys to traverse AST.
## πΏ Installation
Use [npm] to install.
```bash
$ npm install eslint-visitor-keys
```
### Requirements
- [Node.js] 4.0.0 or later.
## π Usage
```js
const evk = require("eslint-visitor-keys")
```
### evk.KEYS
> type: `{ [type: string]: string[] | undefined }`
Visitor keys. This keys are frozen.
This is an object. Keys are the type of [ESTree] nodes. Their values are an array of property names which have child nodes.
For example:
```
console.log(evk.KEYS.AssignmentExpression) // β ["left", "right"]
```
### evk.getKeys(node)
> type: `(node: object) => string[]`
Get the visitor keys of a given AST node.
This is similar to `Object.keys(node)` of ES Standard, but some keys are excluded: `parent`, `leadingComments`, `trailingComments`, and names which start with `_`.
This will be used to traverse unknown nodes.
For example:
```
const node = {
type: "AssignmentExpression",
left: { type: "Identifier", name: "foo" },
right: { type: "Literal", value: 0 }
}
console.log(evk.getKeys(node)) // β ["type", "left", "right"]
```
### evk.unionWith(additionalKeys)
> type: `(additionalKeys: object) => { [type: string]: string[] | undefined }`
Make the union set with `evk.KEYS` and the given keys.
- The order of keys is, `additionalKeys` is at first, then `evk.KEYS` is concatenated after that.
- It removes duplicated keys as keeping the first one.
For example:
```
console.log(evk.unionWith({
MethodDefinition: ["decorators"]
})) // β { ..., MethodDefinition: ["decorators", "key", "value"], ... }
```
## π° Change log
See [GitHub releases](https://github.com/eslint/eslint-visitor-keys/releases).
## π» Contributing
Welcome. See [ESLint contribution guidelines](https://eslint.org/docs/developer-guide/contributing/).
### Development commands
- `npm test` runs tests and measures code coverage.
- `npm run lint` checks source codes with ESLint.
- `npm run coverage` opens the code coverage report of the previous test with your default browser.
- `npm run release` publishes this package to [npm] registory.
[npm]: https://www.npmjs.com/
[Node.js]: https://nodejs.org/en/
[ESTree]: https://github.com/estree/estree
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