Entity JS

A Javascript entity with support a events and export toJSON very simple that can be extended using prototype chain.

Motivation

We have tons of libraries that try to work in JS like a Oriented Object language, this is made to work as prototype, what makes it light weight, fast, simple and extensible.

How to use

You can create a new entity,add, change, remove or get attributes and listen for events.

Example:

var myEntity = new Entity();

function logEntityEvents(event){
  console.log(event.type, event.attribute, event);
}

myEntity.addEventListener('change', logEntityEvents);
myEntity.addEventListener('changed', logEntityEvents);
myEntity.addEventListener('remove', logEntityEvents);
myEntity.addEventListener('removed', logEntityEvents);

myEntity.setAttribute('foo', 'bar');
// change foo EntityEvent{/*...*/}
// changed foo EntityEvent{/*...*/}

myEntity.setAttribute('zaz', 'traz');
// change zaz EntityEvent{/*...*/}
// changed zaz EntityEvent{/*...*/}

myEntity.removeAttribute('zaz');
// remove zaz EntityEvent{/*...*/}
// removed zaz EntityEvent{/*...*/}

console.log(myEntity.getAttribute('foo'));
// bar

myEntity.setAttribute('abc', 'cde');
// change abc EntityEvent{/*...*/}
// changed abc EntityEvent{/*...*/}

console.log(JSON.stringify(myEntity));
//{"foo":"bar","abc":"cde"}

Constructor

Accept a object to be parsed as attributes from the entity, and will try to load any prototype with getPrototype method, that allows to access the super constructor.

Methods

Public methods:

• getAttribute( name ): Return the attribute content
• setAttribute( name, value ): Update the attribute value (if doesn't exist, it will be created)
• removeAttribute( name ): Return true if the attribute was removed
• parse( object ): Add all the properties in a object as attribute and remove the attributes in the model that isn't present in the object
• clone(): Create a new instance with the same attributes of the actual, it will use the constructor method to do it.
• toJSON(): Export the Entity to a object that can be stringfy by JSON.stringfy method.

Internal use methods:

• constructor(): Stores the function that was used to construct the object.
• getPrototype(): Will expose the prototype instance, to allow super constructors.

Properties

** Read-only properties:

• attributes: this object represent the attributes, if you change the attribute here, it will not trigger the events

Events

Events are providen by ObjectEventTarget prototype, means that you can use addEventListener, removeEventListener or dispatchEvent in any instance of Entity.

Cancelable Events:

• change: Dispatched when is about to change a attribute value.
• remove: Dispatched when is about to remove a attribute value.

Non-cancelable Events:

• changed: When the attribute value has changed.
• removed: When the attribute value has been removed.

EntityEvent

This is injected as first argument of any EntityEvent, and has all the properties from ObjectEvent and some properties refered to the Entity.

• attribute: The name of the attribute that is been manipulated.
• value: Will return the value of an attribute change, also if you try to remove the attribute this will have the final value of the operation.
• oldValue: The value of the attribute, before any applied change.

Example to filter changes only in a specific attribute:

myEntity.addEventListener('changed', function (event){
  if(event.attribute !== 'foo') return;
  /* my code to be executed only if foo is changed */
});

Prototype

To prototype you can call from your new constructor passing your context.

Example:

function FancyEntity( name ){
    Entity.call(this);
    this.name = name;

    // When you are customizing the prototype, the constructor will be overloaded by the prototype
    // Make sure to expose it, to the `clone` method works
    this.constructor = FancyEntity;
}
FancyEntity.prototype = Entity.prototype;

Architecture

There is no event for a attribute added to your entity, this happens because a entity is not normalized representation.

Also you can overload the EntityAttributes object constructor, creating a default value in the prototype chain, when you do that, if you use the method removeAttribute the getAttribute will start to return the next value of the attribute in the prototype chain. So if you don't want to return the default value, you need to use setAttribute with undefined value.

If your browser has support to Object.defineProperties, it will add all methods as not enumerable, if you do a for in, they will not be iterated. But if you plan to give support to IE-9, use hasOwnProperty or a shim to ensure that behaviour.