How to create own inheritable class with underscore

问题

I want to built a Backbone.Model like object/class.

I looked through the Backbone docs and found that they create a inheritable object/class with its inheritable attributes, methods in two steps:

I. Create a function with some attributes

var Model = Backbone.Model = function(attributes, options) {
    var defaults;
    attributes || (attributes = {}); //?
    if (options && options.parse) attributes = this.parse(attributes); //?
    if (defaults = getValue(this, 'defaults')) {
      attributes = _.extend({}, defaults, attributes); // ?
    }
    if (options && options.collection) this.collection = options.collection;
    this.attributes = {};
    this._escapedAttributes = {};
    this.cid = _.uniqueId('c');
    this.changed = {};
    this._silent = {};
    this._pending = {};
    this.set(attributes, {silent: true});

    this.changed = {};
    this._silent = {};
    this._pending = {};
    this._previousAttributes = _.clone(this.attributes);
    this.initialize.apply(this, arguments);
};

II. use underscore's extend to give it some functions

_.extend(Model.prototype, Events, { // Events?
   changed: null,

   _silent: null,

   _pending: null,

   idAttribute: 'id',

   initialize: function(){},

   toJSON: function(options) {
       return _.clone(this.attributes);
   }

   // other Model methods...
};

I have some questions about this behaviour:

What does line 3-4 in the first part do
What happens in line 6?
Why do we overgive "_.extend" the Events object, what else could I give as parameter?

Is there anything else I have to pay attention about?

Regards

回答1:

Line 3, attributes || (attributes = {}); is an example of short circuit evaluation. If attributes has a falsy valye, then javascript will evaluate the second part of the OR expression. That part assigns attributes a value of {}, an empty object. The end result is, if attributes is null or undefined (typical case) then, assign attributes an empty object.

That line is equivalent to saying:

if (attributes == false) { // attributes can be any value that evaluates to false
                           // attributes could be null, undefined, false, 0 etc
   attributes = {};
}

Same case with line 4, if (options && options.parse) attributes = this.parse(attributes);

If an options object exists, ie its not null or undefined, and the options object has a valid property called parse, then assign attributes the value of this.parse(attributes);

EDIT 1:

The _.extend lines can be explained by the Underscore.js documentation.

Its a standard pattern to create a new object that will contain properties of all passed in objects. In the first example attributes = _.extend({}, defaults, attributes); // ? the pattern is used to override the default values with what ever may have been passed. You would see this pattern followed in most plugins that allow you to pass in an options object. If you dont pass in anything, the default values will be used. If you pass in only a few properties, the remaining will get their values from the defaults.

http://api.jquery.com/jQuery.extend/ is NOT the exact same thing. But its very similar. Their documentation is much better. You will understand the concept better.

EDIT 2:

The Backbone Events class has some event handling related methods. The Backbone Model class also needs to be able to make use of event handling. It needs to raise events to tell the view to re-render itself, it needs to listen to events from the dom when the view changes the underlying data. This event handling functionality could either have been re-written from scratch when defining the Model class, or the Model class could be extended to get these methods from the Events class. That latter is what is happening.

If you read the documentation for the $.extend or _.extend, you will realize that the Model class is being extended not just with the properties under Events, but also with other properties like toJSON and initialize etc...

回答2:

As to what's going on on line 6 - the object is created that contains all properties of defaults object and all from attributes object. Any property of defaults object that is already defined in attributes object will get overwriten by the value of latter.

来源：https://stackoverflow.com/questions/11467108/how-to-create-own-inheritable-class-with-underscore

标签

javascript

backbone.js

underscore.js