What are the nuances of scope prototypal prototypical inheritance in AngularJS

Question

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

User · Answer

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

User · Answer

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   lt input type  text  ng-model  someObj prop1  gt    lt  --rather than  lt input type  text  ng-model  prop1  gt   -- gt     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    lt script type  text ng-template  id   tpl1 html  gt   lt input ng-model  myPrimitive  gt   lt  script gt   lt div ng-include src    tpl1 html   gt  lt  div gt    lt script type  text ng-template  id   tpl2 html  gt   lt input ng-model  myObject aNumber  gt   lt  script gt   lt div ng-include src    tpl2 html   gt  lt  div gt    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    lt input ng-model   parent myPrimitive  gt    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    lt ul gt  lt li ng-repeat  num in myArrayOfPrimitives  gt          lt input ng-model  num  gt       lt  li gt   lt ul gt   lt ul gt  lt li ng-repeat  obj in myArrayOfObjects  gt          lt input ng-model  obj num  gt       lt  li gt   lt ul gt    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   amp   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   lt div my-directive gt  and scope    localProp    parentProp      An attribute must be used to specify each parent property that the directive wants to bind to   lt div my-directive the-Parent-Prop parentProp gt  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    lt my-directive interpolated    parentProp1    twowayBinding  parentProp2  gt  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   amp   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

User · Answer

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

[javascript] What are the nuances of scope prototypal / prototypical inheritance in AngularJS?

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