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+# eslint-visitor-keys
+
+[![npm version](https://img.shields.io/npm/v/eslint-visitor-keys.svg)](https://www.npmjs.com/package/eslint-visitor-keys)
+[![Downloads/month](https://img.shields.io/npm/dm/eslint-visitor-keys.svg)](http://www.npmtrends.com/eslint-visitor-keys)
+[![Build Status](https://travis-ci.org/eslint/eslint-visitor-keys.svg?branch=master)](https://travis-ci.org/eslint/eslint-visitor-keys)
+[![Dependency Status](https://david-dm.org/eslint/eslint-visitor-keys.svg)](https://david-dm.org/eslint/eslint-visitor-keys)
+
+Constants and utilities about visitor keys to traverse AST.
+
+## 💿 Installation
+
+Use [npm] to install.
+
+```bash
+$ npm install eslint-visitor-keys
+```
+
+### Requirements
+
+- [Node.js] 10.0.0 or later.
+
+## 📖 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) // → ["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)) // → ["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"]
+})) // → { ..., MethodDefinition: ["decorators", "key", "value"], ... }
+```
+
+## 📰 Change log
+
+See [GitHub releases](https://github.com/eslint/eslint-visitor-keys/releases).
+
+## 🍻 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