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Enhances a JavaScript class by adding an is<Class> property to compare types between realms.

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# class-is

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[npm-url]:https://npmjs.org/package/class-is
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Enhances a JavaScript class by adding an `is<Class>` property to compare types between realms.

## Motivation

Checking if a value is an instance of a class in JavaScript is not an easy task.

You can use `instanceof`, but that doesn't work between different realms or different versions. Comparing with `constructor.name` could be a solution but if you need to Uglify the module it doesn't work, as it creates different names for the same module.

[Symbols](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol) to the rescue!

## Installation

`$ npm install class-is`

If you want to use this module in the browser you have to compile it yourself to your desired target.

## Usage

### ES6 classes:

```js
// Package X
const withIs = require('class-is');

class Person {
    constructor(name, city) {
        this.name = name;
        this.city = city;
    }
}

module.exports = withIs(Person, {
    className: 'Person',
    symbolName: '@org/package-x/Person',
});
```

```js
// Package Y
const withIs = require('class-is');

class Animal {
    constructor(species) {
        this.species = species;
    }
}

module.exports = withIs(Animal, {
    className: 'Animal',
    symbolName: '@org/package-y/Animal',
});
```

```js
const Person = require('package-x');
const Animal = require('package-y');

const diogo = new Person('Diogo', 'Porto');
const wolf = new Animal('Gray Wolf');

console.log(Person.isPerson(diogo));
console.log(Person.isPerson(wolf));
```

Running the example above will print:

```
true
false
```

### ES5 and below classes:

In ES5 it's not unusual to see constructors like the one below, so you can call it without using the `new` keyword.

```js
function Circle(radius) {
    if (!(this instanceof Circle)) {
        return new Circle(radius);
    }

this.radius = radius;
}
```

In such cases you can use the `withIs.proto` method:

```js
const withIs = require('class-is');

const Circle = withIs.proto(function (radius) {
    if (!(this instanceof Circle)) {
        return new Circle(radius);
    }

this.radius = radius;
}, {
    className: 'Circle',
    symbolName: '@org/package/Circle',
});
```

...or even better:

```js
const withIs = require('class-is');

function Circle(radius) {
    this.radius = radius;
}

module.exports = withIs.proto(Circle, {
    className: 'Circle',
    symbolName: '@org/package/Circle',
    withoutNew: true,
});
```

## API

### withIs(Class, { className, symbolName })

###### class

Type: `class`

The class to be enhanced.

###### className

Type: `String`

The name of the class your passing.

###### symbolName

Type: `String`

Unique *id* for the class. This should be namespaced so different classes from different modules do not collide and give false positives.

Example: `@organization/package/Class`

### withIs.proto(Class, { className, symbolName, withoutNew })

The `className` and `symbolName` parameters are the same as above.

###### withoutNew

Type: `Boolean`   
Default: `false`

Allow creating an instance without the `new` operator.

## Tests

`$ npm test`   
`$ npm test -- --watch` during development

## License

[MIT](https://www.opensource.org/licenses/mit-license.php)

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