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I've read extensively about Javascript prototypal inheritance, specifically in child scopes. My understanding is that we always ensure a "." is in our ng-model binding so that it references an object.

My question is ... why does prototypal inheritance create a primitive on its local scope but not an object? It seems like the very same thing.

In one case, we have a primitive. In another case we have an object - say an array. What's even more confusing is that I can pass an object to the child scope (without a ".") and by pushing data onto the object that was passed down within the local scope, the parent object is still modified and thus no child array object is created. Again, why? Prototypal inheritance seems to auto-create primitives ... what makes an object different?

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  • Prototypal inheritance doesn't create anything. Angular adds a property to the current scope. It also doesn't matter if the property is a primitive or an object. Commented Jan 10, 2015 at 22:12
  • It does matter. Objects are located on the parent scope and modified appropriately whereas primitives aren't. I'm trying to understand why this is the case. Commented Jan 10, 2015 at 23:02
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    Question is far too broad without some examples that may be causing you confusion. I disagree with @zeroflagL though with regard to doesn't matter as it sure does matter once scopes get turned into child scopes as to what stays bound and what doesn't Commented Jan 11, 2015 at 1:33
  • var $scope.name = "Daniel"; Commented Jan 11, 2015 at 5:49
  • Charles and @charlietfl, maybe I wasn't explicit enough or misinterpreted the question. The question was: why does prototypal inheritance create a primitive on its local scope but not an object. So if you have ng-model="something.text" then text will be added to scope.something. But if the object something doesn't already exist somewhere in the scope chain, then it will be created and added to the current scope. If you meant something different, Charles, please, rephrase the question. Commented Jan 11, 2015 at 8:42

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Here's a simplified (for the example) version of how child scope is created from parent scope. For more comprehensive view - go to the source

Parent scope is created:

function Scope(){ }; 
var scope = new Scope(); // scope instance is created

Suppose it has some primitive and some object properties:

scope.a = 1;
scope.obj = {a: 1};

Child scope instance is created and its prototype is "scope"

function ChildScope(){ };
ChildScope.prototype = scope;
var childScope = new ChildScope(); // an instance of the child scope

Now, let see:

console.log(scope.a);          // should be 1
console.log(childScope.a);     // should be 1
console.log(scope.obj.a);      // should be 1
console.log(childScope.obj.a); // should be 1

As expected, right?

If we now set values on properties exposed in childScope

childScope.a = 2;
childScope.obj.a = 3;

Then here's the output:

console.log(scope.a);          // should still be 1
console.log(childScope.a);     // should be 2 now
console.log(scope.obj.a);      // should be 3
console.log(childScope.obj.a); // should be 3

jsFiddle

So, when you are doing ng-repeat over array of primitives: item in ["a", "b",...] and within it use <input ng-model="item">, you are essentially doing childScope.item = "...".

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2 Comments

This answer doesn't really explain the why question of the OP, refer to this question for a more extensive answer: stackoverflow.com/questions/14049480/…
The why follows from the behavior of prototypical inheritance in JS, and I've showed how a child scope is created with the parent scope as its prototype. That there is a duplicate question and answer is a different story.

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