Executive Summary:

In AngularJS, a child scope normally prototypically inherits from its parent scope. One exception to this rule is a directive that uses scope: { ... } -- this creates an "isolate" scope that does not prototypically inherit.（and directive with transclusion） This construct is often used when creating a "reusable component" directive. In directives, the parent scope is used directly by default, which means that whatever you change in your directive that comes from the parent scope will also change in the parent scope. If you set scope:true (instead of scope: { ... }), then prototypical inheritance will be used for that directive.

Scope inheritance is normally straightforward, and you often don't even need to know it is happening... until you try 2-way data binding (i.e., form elements, ng-model) to a primitive (e.g., number, string, boolean) defined on the parent scope from inside the child scope. It doesn't work the way most people expect it should work. What happens is that the child scope gets its own property that hides/shadows the parent property of the same name. This is not something AngularJS is doing – this is how JavaScript prototypal inheritance works. New AngularJS developers often do not realize that ng-repeat, ng-switch, ng-view, ng-include and ng-if all create new child scopes, so the problem often shows up when these directives are involved. (See this example for a quick illustration of the problem.)

This issue with primitives can be easily avoided by following the "best practice" of always have a '.' in your ng-models – watch 3 minutes worth. Misko demonstrates the primitive binding issue with ng-switch.

Having a '.' in your models will ensure that prototypal inheritance is in play. So, use
<input type="text" ng-model="someObj.prop1"> rather than
<input type="text" ng-model="prop1">.

If you really want/need to use a primitive, there are two workarounds:

1. Use $parent.parentScopeProperty in the child scope. This will prevent the child scope from creating its own property.
2. Define a function on the parent scope, and call it from the child, passing the primitive value up to the parent (not always possible)

The Details:

• JavaScript Prototypal Inheritance
• Angular Scope Inheritance
  • ng-include
  • ng-switch
  • ng-repeat
  • ng-view
  • ng-controller
  • directives

JavaScript Prototypal Inheritance

It is important to first have a solid understanding of JavaScript prototypal inheritance, especially if you are coming from a server-side background and you are more familiar with classical inheritance. So let's review that first.

Suppose parentScope has properties aString, aNumber, anArray, anObject, and aFunction. If childScope prototypically inherits from parentScope, we have:

(Note that to save space, I show the anArray object as a single blue object with its three values, rather than an single blue object with three separate gray literals.)

If we try to access a property defined on the parentScope from the child scope, JavaScript will first look in the child scope, not find the property, then look in the inherited scope, and find the property. (If it didn't find the property in the parentScope, it would continue up the prototype chain... all the way up to the root scope). So, these are all true:

childScope.aString === 'parent string'
childScope.anArray[1] === 20
childScope.anObject.property1 === 'parent prop1'
childScope.aFunction() === 'parent output'

Suppose we then do this:

childScope.aString = 'child string'

The prototype chain is not consulted, and a new aString property is added to the childScope. This new property hides/shadows the parentScope property with the same name. This will become very important when we discuss ng-repeat and ng-include below.

Suppose we then do this:

childScope.anArray[1] = 22
childScope.anObject.property1 = 'child prop1'

The prototype chain is consulted because the objects (anArray and anObject) are not found in the childScope. The objects are found in the parentScope, and the property values are updated on the original objects. No new properties are added to the childScope; no new objects are created. (Note that in JavaScript arrays and functions are also objects.)

Suppose we then do this:

childScope.anArray = [100, 555]
childScope.anObject = { name: 'Mark', country: 'USA' }

The prototype chain is not consulted, and child scope gets two new object properties that hide/shadow the parentScope object properties with the same names.

Takeaways:

• If we read childScope.propertyX, and childScope has propertyX, then the prototype chain is not consulted.
• If we set childScope.propertyX, the prototype chain is not consulted.

One last scenario:

delete childScope.anArray
childScope.anArray[1] === 22  // true

We deleted the childScope property first, then when we try to access the property again, the prototype chain is consulted.

Here is a jsfiddle where you can see the above javascript prototypical inheritance examples being modified and their result (open up your browser's console to see the output. The console output can be viewed as what the 'RootScope' would see).

Angular Scope Inheritance

The contenders:

• The following create new scopes, and inherit prototypically: ng-repeat, ng-include, ng-switch, ng-view, ng-controller, directive with scope: true, directive with transclude: true.
• The following creates a new scope which does not inherit prototypically: directive with scope: { ... }. This creates an "isolate" scope instead.

Note, by default, directives do not create new scope -- i.e., the default is scope: false.

ng-include

Suppose we have in our controller:

$scope.myPrimitive = 50;
$scope.myObject    = {aNumber: 11};

And in our HTML:

<script type="text/ng-template" id="/tpl1.html">
    <input ng-model="myPrimitive">
</script>
<div ng-include src="'/tpl1.html'"></div>
<script type="text/ng-template" id="/tpl2.html">
    <input ng-model="myObject.aNumber">
</script>
<div ng-include src="'/tpl2.html'"></div>

Each ng-include generates a new child scope, which prototypically inherits from the parent scope.

Typing (say, "77") into the first input textbox causes the child scope to get a new myPrimitive scope property that hides/shadows the parent scope property of the same name. This is probably not what you want/expect.

Typing (say, "99") into the second input textbox does not result in a new child property. Because tpl2.html binds the model to an object property, prototypal inheritance kicks in when the ngModel looks for object myObject -- it finds it in the parent scope.

We can rewrite the first template to use $parent, if we don't want to change our model from a primitive to an object:

<input ng-model="$parent.myPrimitive">

Typing (say, "22") into this input textbox does not result in a new child property. The model is now bound to a property of the parent scope (because $parent is a child scope property that references the parent scope).

For all scopes (prototypal or not), Angular always tracks a parent-child relationship (i.e., a hierarchy), via scope properties