If you are not using a .(dot) in your AngularJS models you are doing it wrong?

The question:

I remember seeing this famous quote from a video on AngularJS saying that should be always using a . (dot) in your models.

Well I am trying to follow this say I have

   var item = {}
   item.title = "Easy Access to support";
   item.available = true;
   item.price = 31.67;

So this works great in my view i do

  {{ item.title }}
  {{ item.available }}

I am using a dot so I think this is good.

But I have some properties that I don’t consider part of the model but maybe I am wrong. For example I have a property I use to enable or disable a button using the ng-disable, I have entered this using dot format. Its basically entered like so

 $scope.disableButton = true;

and I use it like


Should I make this part of the model “item” ? or create another js object just so i can hold this property using a dot ?

Anybody know if this acceptable or should I be doing everything (even these simple properties) with a .dot ??


The Solutions:

Below are the methods you can try. The first solution is probably the best. Try others if the first one doesn’t work. Senior developers aren’t just copying/pasting – they read the methods carefully & apply them wisely to each case.

Method 1

The “there should always be a dot in your model” refers to ngModel. This directive does two-way binding. If you two-way bind to a primitive (such as a Boolean in your case), the setter will set it on the current scope rather than the scope on which it is defined, which can cause a headache when you have a large user-interface with a lot of child scopes. It does not refer to other directives such as ngDisable. See this explanation for more details on this specific issue.

Sample scenario: a parent scope with $scope.foo = "bar", and a child scope with a <input type="text" data-ng-model="foo">. It will display bar initially, but once the user changes the value, a foo will be created on the child scope and the binding will read and write that value. The parent’s foo will remain bar. Hope that summarises it well.

So for ngModel purposes, you might have to create an object to work around such binding issues, but for any other directive you should have the regular, logical grouping.

Method 2

Here’s a situation where a dot is needed.

When you have a $scope value that you want to use as a ngModel value (let’s call it selectedItem), you might be tempted to just create $scope.selectedItem and pass that to the template. However, this is dangerous if the template creates child scopes.

There are certain AngularJS directives that create child scopes:

  • ngRepeat
  • ngIf
  • ngController
  • …and others (their doc page will say “This directive creates new scope” under the Usage heading).

Child scopes are dangerous because of how scope inheritance works. The relationship is this:

-- $parent scope
   ├ selectedItem
   └─ $child scope

As a child scope, the $child object prototypically inherits from $parent. That’s a javascript term that basically means you can get any $parent property by getting $child.<property>. But you cannot set values, which is the problem. This is just how javascript works.

So a template can access $parent.selectedItem by reading $child.selectedItem. But if the template sets $child.selectedItem, it sets it on $child not $parent, so now you have two versions of selectedItem:

-- $parent scope
   ├ selectedItem
   └─ $child scope
      └ selectedItem

And ngModel directives both get and set the scope value. The getting works, but the setting breaks things (ss others have explained).

Why using a dot solves the problem

When you store the selectedItem value with a dot on the parent scope (e.g. $scope.vm.selectedItem, then the child scope template only ever gets the vm object.

Using vm, the relationship looks like this:

-- $parent scope
   ├ selectedItem
   │ └─ vm
   │     └─ selectedItem
   └─ $child scope

The $child scope only ever reads the vm object; it never writes a new vm object (in JS, objects are references not values). And prototypical inheritance is only involved in accessing vm. After a scope gets the vm object, it can directly use its values without any prototypical inheritance.

Think of it as passing around an object between scopes.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

Leave a Comment