babel-plugin-object-initializer-dragons-included

This is a Babel plugin which allows constructing objects and immediately assigning/defining properties and methods, using something resembling the object literal syntax.

Caveat : This package uses private APIs and hacks to register a syntax extension into Babylon (Babel's parser) and therefore may be broken by any Babel update. Here be dragons !

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Usage

To install this package, run :

npm install --save-dev babel-plugin-object-initializer-dragons-included

Then, to enable it, add object-initializer-dragons-included to the plugins array of your .babelrc file.

Examples (a bit contrived ...)

Assigning some properties to an object :

class User {
    constructor(username, email) {
        this.username = username;
        this.email = email;
        this.firstName = null;
        this.lastName = null;
        // ... potentially a lot of other attributes with defaults ...
        this.phone = null;
        this.birthday = null;
    }
    async save() {
        // ... some calls to a persistence layer ...
    }
}

function createDemoUser() {
    return new User('Exter-N', '[email protected]') {
        firstName: 'Nicolas',
        birthday: new Date(1991, 10, 29)
    };
}

function createDemoUserNoOI() {
    let user = new User('Exter-N', '[email protected]');
    user.firstName = 'Nicolas';
    user.birthday = new Date(1991, 10, 29);
    return user;
}

Overriding a method just for one instance :

class ListFactory {
    createList() {
        return document.createElement('ul');
    }
}

let blueListFactory = new ListFactory {
    createList() {
        let list = super.createList();
        list.style.color = 'blue';
        return list;
    }
};

let blueListFactoryNoOI = new ListFactory;
blueListFactoryNoOI.createList = function createList() {
    let list = ListFactory.prototype.createList.call(this);
    list.style.color = 'blue';
    return list;
};

Property/method shadowing

In most cases, using the object initializer syntax is equivalent to assigning the properties "the classic way", as shown above (except that you can use super in overridden methods). It will therefore invoke the properties' setters where appropriate. But there are cases where you may want to define a new property which will shadow the one defined in the class. There are two ways to do that in the initializer.

Automatic shadowing of get/set properties

If you declare a get/set property, it will automatically shadow any existing one :

class Greeter {
    constructor(recipient) {
        this.recipient = recipient;
    }
    get message() {
        return 'Hello, ' + this.recipient + '!';
    }
    greet() {
        console.log(this.message);
    }
}

// This will print "Bonjour, monde !" ("Hello, world!" in French)
// Again, quite the contrived example ...
(new Greeter('world') {
    get message() {
        return 'Bonjour, ' + this.recipient + ' !';
    },
    recipient: 'monde'
}).greet();

Explicit shadowing

You can apply the @shadow ambient decorator to a property or method to explicitly shadow the existing one, for example to force-set a read-only property :

class ProtectedObject {
    get someProperty() {
        return "some read-only, maybe computed value";
    }
}

function thisWouldThrow() {
    return new ProtectedObject {
        someProperty: "some other value"
    };
}

function thisWouldNot() {
    return new ProtectedObject {
        @shadow someProperty: "some other value"
    };
}

License

This package is released by Nicolas "Exter-N" L. under the MIT License. See LICENSE.md.