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Backbone.js - Model validation in constructor

I've started to look at various Design Patterns in JavaScript and one of most popular Design Patterns in any language is the MVC Pattern. In JavaScript, there are various libraries out there which helps in modularizing the application. However, I've heard a lot about Backbone.js and I thought this would be the right time to give it a try. I've not learnt Backbone completely, but I've gained a good understanding of the Model part of the MVC in Backbone.

To build a Model, you would extend Backbone.Model and provide instance properties like - defaults, validate, initialize etc,. I assumed that if the validate function is defined, it would be invoked whenever an instance of the Model is created, but this was not true. To overcome this, I tried to call validate method in initialize but this did not suffice the purpose, since this function (initialize) was called after the Model was created. On reading further and on trying out some options I understood that the validate function is called whenever a user tries to set a property on the Model (carObj.set('property',value)):

As seen in the above code snippet, an instance of Car is created even though the 'color' attribute is not specified. Here's the console output:

Inside initialize
d {attributes: Object, _escapedAttributes: Object, cid: "c0", changed: Object, _silent: Object…}
_callbacks: Object
_escapedAttributes: Object
_pending: Object
_previousAttributes: Object
_silent: Object
attributes: Object
brand: "Ford"
model: "Figo"
__proto__: Object
changed: Object
cid: "c0"
__proto__: x 
Is Model valid: false 
Error Occurred 

Although you can call isValid function on the object (carObj.isValid()) and validate the Model once it has been created, but I wanted the Model to validate the data before returning an instance of it. I added a constructor property to the object and was able to validate the data there:

As you can see, I'm overriding the constructor function. Here I'm calling the validate method in constructor and checking whether the data is valid, if any of the required arguments are not present then the validate method would return a string 'Error Occurred'. The constructor would then return an empty object.

It is important to note that since we are overriding the constructor property and providing our own implementation, we need to call the parent constructor (Backbone.Model) explicitly to initialize the Model, this would then construct the object and invoke the initialize method.

Comments

  1. I am trying to override the constructor, the thing is, everytime i try to console.log arguments I pass to the constructor, it always get undefined, even if it's not and if the model get instanciated with valid parameters.

    ReplyDelete
  2. Can you paste the code here. In the example that I have mentioned, the 'attributes' parameter to the constructor should contain the data that you have passed. Would like to know in which scenario this fails.

    ReplyDelete

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