# @babel/plugin-transform-instanceof

> Wraps `instanceof` expressions to allow constructors to customize the logic with a `Symbol.hasInstance` method

## Example

**In**

```javascript
foo instanceof Bar;
```

**Out**

```javascript
function _instanceof(left, right) {
  if (right != null && typeof Symbol !== "undefined" && right[Symbol.hasInstance]) {
    return right[Symbol.hasInstance](left);
  } else {
    return left instanceof right;
  }
}

_instanceof(foo, Bar);
```

## Installation

```sh
npm install --save-dev @babel/plugin-transform-instanceof
```

## Usage

### Via `.babelrc` (Recommended)

**.babelrc**

```json
{
  "plugins": ["@babel/plugin-transform-instanceof"]
}
```

### Via CLI

```sh
babel --plugins @babel/plugin-transform-instanceof script.js
```

### Via Node API

```javascript
require("@babel/core").transform("code", {
  plugins: ["@babel/plugin-transform-instanceof"]
});
```

## References

* [ES6 Spec: InstanceOf Operator Semantics](https://www.ecma-international.org/ecma-262/6.0/#sec-instanceofoperator)
* [MDN: Symbol.hasInstance](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/hasInstance)