The API Reference Scope page says:
A scope can inherit from a parent scope.
The Developer Guide Scope page says:
A scope (prototypically) inherits properties from its parent scope.
So, does a child scope always prototypically inherit from its parent scope?
Are there exceptions?
When it does inherit, is it always normal JavaScript prototypal inheritance?
Quick answer:
A child scope normally prototypically inherits from its parent scope, but not always. One exception to this rule is a directive with scope: { ... } -- this creates an "isolate" scope that does not prototypically inherit. This construct is often used when creating a "reusable component" directive.
As for the nuances, scope inheritance is normally straightfoward... until you need 2-way data binding (i.e., form elements, ng-model) in the child scope. Ng-repeat, ng-switch, and ng-include can trip you up if you try to bind to a primitive (e.g., number, string, boolean) in the parent scope from inside the child scope. It doesn't work the way most people expect it should work. The child scope gets its own property that hides/shadows the parent property of the same name. Your workarounds are
define objects in the parent for your model, then reference a property of that object in the child: parentObj.someProp
use $parent.parentScopeProperty (not always possible, but easier than 1. where possible)
define a function on the parent scope, and call it from the child (not always possible)
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">`
-->
L-o-n-g answer:
JavaScript Prototypal Inheritance
Also placed on the AngularJS wiki: https://github.com/angular/angular.js/wiki/Understanding-Scopes
It is important to first have a solid understanding of prototypal inheritance, especially if you are coming from a server-side background and you are more familiar with class-ical 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.
Angular Scope Inheritance
The contenders:
The following create new scopes, and inherit prototypically: ng-repeat, ng-include, ng-switch, 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 $parent, $$childHead and $$childTail. I normally don't show these scope properties in the diagrams.
For scenarios where form elements are not involved, another solution is to define a function on the parent scope to modify the primitive. Then ensure the child always calls this function, which will be available to the child scope due to prototypal inheritance. E.g.,
// in the parent scope
$scope.setMyPrimitive = function(value) {
$scope.myPrimitive = value;
}
Here is a sample fiddle that uses this "parent function" approach. (The fiddle was written as part of this answer: https://stackoverflow.com/a/14104318/215945.)
See also https://stackoverflow.com/a/13782671/215945 and https://github.com/angular/angular.js/issues/1267.
ng-switch
ng-switch scope inheritance works just like ng-include. So if you need 2-way data binding to a primitive in the parent scope, use $parent, or change the model to be an object and then bind to a property of that object. This will avoid child scope hiding/shadowing of parent scope properties.
See also AngularJS, bind scope of a switch-case?
ng-repeat
Ng-repeat works a little differently. Suppose we have in our controller:
$scope.myArrayOfPrimitives = [ 11, 22 ];
$scope.myArrayOfObjects = [{num: 101}, {num: 202}]
And in our HTML:
<ul><li ng-repeat="num in myArrayOfPrimitives">
<input ng-model="num">
</li>
<ul>
<ul><li ng-repeat="obj in myArrayOfObjects">
<input ng-model="obj.num">
</li>
<ul>
For each item/iteration, ng-repeat creates a new scope, which prototypically inherits from the parent scope, but it also assigns the item's value to a new property on the new child scope. (The name of the new property is the loop variable's name.) Here's what the Angular source code for ng-repeat actually is:
childScope = scope.$new(); // child scope prototypically inherits from parent scope
...
childScope[valueIdent] = value; // creates a new childScope property
If item is a primitive (as in myArrayOfPrimitives), essentially a copy of the value is assigned to the new child scope property. Changing the child scope property's value (i.e., using ng-model, hence child scope num) does not change the array the parent scope references. So in the first ng-repeat above, each child scope gets a num property that is independent of the myArrayOfPrimitives array:
This ng-repeat will not work (like you want/expect it to). Typing into the textboxes changes the values in the gray boxes, which are only visible in the child scopes. What we want is for the inputs to affect the myArrayOfPrimitives array, not a child scope primitive property. To accomplish this, we need to change the model to be an array of objects.
So, if item is an object, a reference to the original object (not a copy) is assigned to the new child scope property. Changing the child scope property's value (i.e., using ng-model, hence obj.num) does change the object the parent scope references. So in the second ng-repeat above, we have:
(I colored one line gray just so that it is clear where it is going.)
This works as expected. Typing into the textboxes changes the values in the gray boxes, which are visible to both the child and parent scopes.
See also Difficulty with ng-model, ng-repeat, and inputs and
https://stackoverflow.com/a/13782671/215945
ng-controller
Nesting controllers using ng-controller results in normal prototypal inheritance, just like ng-include and ng-switch, so the same techniques apply.
However, "it is considered bad form for two controllers to share information via $scope inheritance" -- http://onehungrymind.com/angularjs-sticky-notes-pt-1-architecture/
A service should be used to share data between controllers instead.
(If you really want to share data via controllers scope inheritance, there is nothing you need to do. The child scope will have access to all of the parent scope properties.
See also Controller load order differs when loading or navigating)
directives
default (scope: false) - the directive does not create a new scope, so there is no inheritance here. This is easy, but also dangerous because, e.g., a directive might think it is creating a new property on the scope, when in fact it is clobbering an existing property. This is not a good choice for writing directives that are intended as reusable components.
scope: true - the directive creates a new child scope that prototypically inherits from the parent scope. If more than one directive (on the same DOM element) requests a new scope, only one new child scope is created. Since we have "normal" prototypal inheritance, this is like ng-include and ng-switch, so be wary of 2-way data binding to parent scope primitives, and child scope hiding/shadowing of parent scope properties.
scope: { ... } - the directive creates a new isolate/isolated scope. It does not prototypically inherit. This is usually your best choice when creating reusable components, since the directive cannot accidentally read or modify the parent scope. However, such directives often need access to a few parent scope properties. The object hash is used to set up two-way binding (using '=') or one-way binding (using '#') between the parent scope and the isolate scope. There is also '&' to bind to parent scope expressions. So, these all create local scope properties that are derived from the parent scope.
Note that attributes are used to help set up the binding -- you can't just reference parent scope property names in the object hash, you have to use an attribute. E.g., this won't work if you want to bind to parent property parentProp in the isolated scope: <div my-directive> and scope: { localProp: '#parentProp' }. An attribute must be used to specify each parent property that the directive wants to bind to: <div my-directive the-Parent-Prop=parentProp> and scope: { localProp: '#theParentProp' }.
Isolate scope's __proto__ references Object.
Isolate scope's $parent references the parent scope, so although it is isolated and doesn't inherit prototypically from the parent scope, it is still a child scope.
For the picture below we have
<my-directive interpolated="{{parentProp1}}" twowayBinding="parentProp2"> and
scope: { interpolatedProp: '#interpolated', twowayBindingProp: '=twowayBinding' }
Also, assume the directive does this in its linking function: scope.someIsolateProp = "I'm isolated"
For more information on isolate scopes see http://onehungrymind.com/angularjs-sticky-notes-pt-2-isolated-scope/
transclude: true - the directive creates a new "transcluded" child scope, which prototypically inherits from the parent scope. The transcluded and the isolated scope (if any) are siblings -- the $parent property of each scope references the same parent scope. When a transcluded and an isolate scope both exist, isolate scope property $$nextSibling will reference the transcluded scope. I'm not aware of any nuances with the transcluded scope.
For the picture below, assume the same directive as above with this addition: transclude: true
This fiddle has a showScope() function that can be used to examine an isolate and transcluded scope. See the instructions in the comments in the fiddle.
Summary
There are four types of scopes:
normal prototypal scope inheritance -- ng-include, ng-switch, ng-controller, directive with scope: true
normal prototypal scope inheritance with a copy/assignment -- ng-repeat. Each iteration of ng-repeat creates a new child scope, and that new child scope always gets a new property.
isolate scope -- directive with scope: {...}. This one is not prototypal, but '=', '#', and '&' provide a mechanism to access parent scope properties, via attributes.
transcluded scope -- directive with transclude: true. This one is also normal prototypal scope inheritance, but it is also a sibling of any isolate scope.
For all scopes (prototypal or not), Angular always tracks a parent-child relationship (i.e., a hierarchy), via properties $parent and $$childHead and $$childTail.
Diagrams were generated with graphviz "*.dot" files, which are on github. Tim Caswell's "Learning JavaScript with Object Graphs" was the inspiration for using GraphViz for the diagrams.
I in no way want to compete with Mark's answer, but just wanted to highlight the piece that finally made everything click as someone new to Javascript inheritance and its prototype chain.
Only property reads search the prototype chain, not writes. So when you set
myObject.prop = '123';
It doesn't look up the chain, but when you set
myObject.myThing.prop = '123';
there's a subtle read going on within that write operation that tries to look up myThing before writing to its prop. So that's why writing to object.properties from the child gets at the parent's objects.
I would like to add an example of prototypical inheritance with javascript to #Scott Driscoll answer. We'll be using classical inheritance pattern with Object.create() which is a part of EcmaScript 5 specification.
First we create "Parent" object function
function Parent(){
}
Then add a prototype to "Parent" object function
Parent.prototype = {
primitive : 1,
object : {
one : 1
}
}
Create "Child" object function
function Child(){
}
Assign child prototype (Make child prototype inherit from parent prototype)
Child.prototype = Object.create(Parent.prototype);
Assign proper "Child" prototype constructor
Child.prototype.constructor = Child;
Add method "changeProps" to a child prototype, which will rewrite "primitive" property value in Child object and change "object.one" value both in Child and Parent objects
Child.prototype.changeProps = function(){
this.primitive = 2;
this.object.one = 2;
};
Initiate Parent (dad) and Child (son) objects.
var dad = new Parent();
var son = new Child();
Call Child (son) changeProps method
son.changeProps();
Check the results.
Parent primitive property did not change
console.log(dad.primitive); /* 1 */
Child primitive property changed (rewritten)
console.log(son.primitive); /* 2 */
Parent and Child object.one properties changed
console.log(dad.object.one); /* 2 */
console.log(son.object.one); /* 2 */
Working example here http://jsbin.com/xexurukiso/1/edit/
More info on Object.create here https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/create
This question is inspired by:
ng-model in numeric input does not affect scope
In that question I was surprised to learn that somehow I have nested controllers even though I have exactly one controller per template and I have no custom directives. This might have something to do with Ionic.
I had a problem that I couldn't access primitive $scope properties without wrapping them in objects. I had this problem regardless what I named the primitive property. I could only access it from my template if I wrapped it in an object or referred to $parent.
How come I can't access the primitives regardless of name? I would understand that there can be a naming conflict, but no name worked. And if no name works, then why do primitives wrapped in objects work? Why do I not need to write $parent.object in my template to access an object attached to my $scope?
Accessing vs Defining values
When accessing a value (such as using {{ value }}, or method()), the application will climb up parent scopes looking for the value until it reaches $rootScope: the first version found, if any, is then used.
When defining a value, however (such as using value = 'foo'), it applies it to the current scope, even if a value exists with the same key under a parent scope.
Defining primitives vs defining object properties
When setting a primitive value such as 'value', you're simply defining what value is:
value = 123;
But when defining the property of an object, the object has to be accessed before the property change can be applied to it:
object.value = 123;
The above code looks for object, and applies the change to its value property. If object doesn't exist in the current scope, the application will search up the parent scopes to find object, so it can access it. Then, the change to value is applied to that instance of object. If object doesn't exist at all, the code will throw an error because it is essentially the below:
null.value = 123;
Example
Similar functionality can be seen in vanilla Javascript through prototypal inheritance.
var parent = { primitive: 123, object: { value: 123 } };
var child = Object.create(parent);
console.log(child.primitive); // 123, from parent
console.log(child.object.value); // 123, from parent
child.primitive = 456; // modifies own primitive key
console.log(child.primitive); // 456, from self
console.log(parent.primitive); // still 123
child.object.value = 456; // modifies parent's object's value
console.log(child.object.value); // 456, from parent
console.log(parent.object.value); // 456, since it was modified above
JavaScript objects are dynamic "bags" of properties (referred to as
own properties). JavaScript objects have a link to a prototype object.
When trying to access a property of an object, the property will not
only be sought on the object but on the prototype of the object, the
prototype of the prototype, and so on until either a property with a
matching name is found or the end of the prototype chain is reached.
Read more here
You need to know the prototype inheritance of javascript first.
Then understand how scope works with this very helpful guide.
Today I wondered after some strange behavior of angularjs. I used console.log to log $scope, there were no key attached to scope named val1, but when I use console.log($scope.val1) its returning value as a object. After digging out the reason, I found that it was accessing parent scope as there is no kay named val1 in $scope. Now my question is that,
Which is good practice? Can you please justify?
Using $scope.$parent.val1
Using $scope.val1
You should generally never use $scope.$parent and instead rely on Javascripts prototypal inheritance. By accessing the parent directly, you run the risk of the code breaking if you move the directive/controller a step up in the scope hierarchy, and the data is now on $scope.$parent.$parent instead.
Instead, never write to properties directly on the scope but to objects on the scope object instead.
Say you do:
$scope.$parent.value = 'x';
Then you do:
console.log($scope.value);
That'll print x to the console. If you then do:
$scope.value = 'y';
You're not modifying the value property on the parent scope, but introducing a new property on the child scope. So $scope.$parent.value will still contain x.
To get around that, you do:
$scope.$parent.data = {value: 'x'};
console.log($scope.data.value); // prints x
$scope.data.value = 'y';
console.log($scope.data.value); // prints y
console.log($scope.$parent.data.value); // also prints y
The reason this works is that instead of creating a new value property on the child scope, it first needs to lookup the data property. And that's not on the child scope, so it goes up the prototype chain ($parent) to find the data object. The data object is found on the parent, and the value property is set on that object instead.
It depends on the type of val1, in case it is a primitive, you will have to access it explicitly via $scope.$parent.
In case it is an object, you can take advantage of the prototypal inheritance that exists between parent and child scope in Angular and just reference it regularly, as I am sure you are aware of, objects are passed by reference, so any change to the object will change it on the parent scope.
More info here
How are scope attributes being inherited when scope is not set in Angular directive definition?
In Angular docs (under 'Directive Definition Object') there are two cases: when scope is true or when scope is an object ({}). What if scope is not set or is false. How are attributes inherited in this case for the scope and for it's children?
When scope is set to false(also default value) in directive definition, behavior of scope differs based on whether you are doing transclusion or not.
If you do not transclude, it pretty much uses parent scope. Just as if you did not used a directive and written the linked template directly in the page.
Fiddle
Transclusion always creates a child scope but it's bound to parent scope in a strange way when the directive scope is false. You can "read" parent scope but as soon as you "write" to a property it's not bound to parent scope's same property anymore but now a property on child scope unless that property is an object.
Transcluded Fiddle
Child scope is the ones in green border.
Try changing the parent scope first. Enter something in the "var" input field, you will see that the child scope's var will also change. But when you change var in child scope it's not changing the parent scope, and changing the var in parent scope does not affect child scope as the bond is broken when you wrote to var from child scope. This does not apply to the objects (try the same on sp.var, you will see that you cannot break the "bond"). According to developers this is the expected and/or intended behavior.
If the scope is set to false (which is the default) then the directive has the same scope as the parent- no new scope is created. Since they share a scope any changes in the parent will be reflected in the directive and vice-versa.
Since this isn't great from an encapsulation standpoint, many recommend using an isolate scope whenever possible (an isolate scope being when you set the scope to {})
I'm experimenting with the Google closure library and in particluar its 'inherits' method.
I have one question. When I extend a base class, if I do not set the base class's properties in its constructor, all the children end up sharing the properties, for example, if I add items to an array the array just keeps getting bigger.
Could someone explain why I need to set the properties in the constructor?
Here's my example, if you comment out "this.list = [9,8,7];" in the 'Person' constructor, the children share the persons list and keep adding to it.
http://jsbin.com/imujoy/1/edit
Thanks for any help.
When you reference "this.list" from the child, it first looks at the object itself to find that property. When it cannot find it, it looks at its prototype and finds the "list" array. That list, however, is a reference to the same list you created on the prototype of the Person. If you assign a new "this.list" in the constructor, you are just assigning a property on the object rather than the prototype. Generally speaking, you should not assign non-primitive types (Arrays, Objects) on the prototype if you intend to use an inheritance model because the children will share the same reference.
Are you copying parent defined variables in the child using Parent.call(this) or Parent.apply
function Child(){
Parent.apply(this,arguments);
}
goog.inherits(Child, Parent);
list becomes a property of any Child instance when defined in the Parent in the following manner:
function Parent(){
this.list=[];
}
Basic prototype behavior:
Prototypical inheritance - writing up
Good reference dealing with constructor parameters but to apply you have to implement _init and change goog.base:
How to "properly" create a custom object in JavaScript?