why manipulating DOM in controller is a bad thing?
Its common knowledge that DOM manipulations should not be performed in AngularJS Controller, its very hard however to find why exactly is it a bad thing. All the sources say its difficult to test and because controller should be used for communication between directives, but fail to illustrate with code why this is a bad thing.
From my understanding I would think that controller, unlike directive, is not associated with any specific HTML, hence all the DOM modifications controller would do would very possibly fail. This would sure complicate development and testing.
Controller in directives being executed before link functions of child directives would also fail as the controller might not be aware of what the actual HTML of the child directives is. Link is executed after controller function and might modify HTML structure.
I hope I make sense here and if someone could clarify why manipulating DOM from controller is a bad thing, maybe some code exampleor link that explains it well that would be great.
The reason it is more difficult to prove their point with a code sample is that the reason can't really be represented by a short code snippet (short enough for Stack Overflow). It is really a maintainability precaution. Over the long term, you want to be able to independently alter the logic behind controllers and views independently, because otherwise a coupled controller and view pair tend to stay that way and limit each other in their ability to change their functionality without breaking the other. As soon as you decide to change anything about the view, you have the chance of making your controller code break without even touching it.
Testing becomes easier over time because the more tests you have, the more you wish that things were more modular and dependent on as little variables and parameters as possible.
Again, it is maintenance that drives this suggestion. The problems listed above might not be that bad starting out. But imagine adopting a project that you didn't build from the ground up and know all the intricacies behind the coupling between controller and view that hold this application together. What if your application reaches so many thousands of lines of code that it would be impossible for you to know all these intricacies even if you DID build it from the ground up?
For a more general understanding of why design patterns like the one you have alluded to are necessary, you can refer to this google search that will take you on a journey as long as you are willing to take. And for a general understanding of why design patterns even exist and why many people end up suggesting the same thing over and over again, you can refer to one of the catalysts to the introduction of design patterns, Christopher Alexander. He shows us that patterns are what they are because they work well and people repeat what works well.
If you look at the ever so popular question "Thinking in AngularJS" if I have a jQuery background? you will get some hints.
One of the biggest factor that i think DOM manipulation is neither needs nor done is because Angular uses declarative approach when it comes to DOM linking as against the imperative approach that you would use with direct DOM manipulation. Some of the answers detail this difference between declarative and imperative approach.
With jQuery you tell the DOM what needs to happen, step by step. With AngularJS you describe what results you want but not how to do it. More on this here. Also, check out Mark Rajcok's answer.
A more comprehensive treatment of this topic is also available here What is the difference between declarative and imperative programming
With such an approach the controller implementation is simplified, and we start to get real value as code base size grows and complexity increases.
My perspective is bit different and covers more than testing. It is the ability of the designer to have control over the HTML layout that otherwise would be taken over by writing code inside of the Angular Controller. Consider the following snippet (it is just a simple example)
<div ng-repeat="article in articles">
<p class="article-body">{{article.text}}</p>
</div>
This example just iterates over a collection and prints article in a separate paragraph tag. While it is certainly possible to loop over the collection in an Angular Controller and dynamically generate paragraph tags with text inside it. It will take away designer's capability to modify look & feel. Hence, If there is a requirement to show the article title, instead of doing this
<div ng-repeat="article in articles">
<span class="article-title">{{article.title}}</span>
<p class="article-body">{{article.text}}</p>
</div>
the designer will now have to locate Angular Controller code to modify DOM manipulation. Just compare above two approaches and guess which one will provide more flexibility.
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