JavaScript pass scope to another function

Question

Is it possible to somehow pass the scope of a function to another?

For example,

function a(){
   var x = 5;
   var obj = {..};
   b(<my-scope>);
}

function b(){
   //access x or obj....
}

I would rather access the variables directly, i.e., not using anything like this.a or this.obj, but just use x or obj directly.

Answer 1

No.

You're accessing the local scope object. The [[Context]].

You cannot publicly access it.

Now since it's node.js you should be able to write a C++ plugin that gives you access to the [[Context]] object. I highly recommend against this as it brings proprietary extensions to the JavaScript language.

Answer 2

The only way to truly get access to function a's private scope is to declare b inside of a so it forms a closure that allows implicit access to a's variables.

Here are some options for you.

Direct Access

1. Declare b inside of a.

   function a() {
      var x = 5,
         obj = {};
      function b(){
         // access x or obj...
      }
      b();
   }
   
   a();
   

2. If you don't want b inside of a, then you could have them both inside a larger container scope:

   function container() {
      var x, obj;
      function a(){
         x = 5;
         obj = {..};
         b();
      }
      function b(){
         // access x or obj...
      }
   }
   
   container.a();
   

These are the only ways you're going to be able to use a's variables directly in b without some extra code to move things around. If you are content with a little bit of "help" and/or indirection, here are a few more ideas.

Indirect Access

1. You can just pass the variables as parameters, but won't have write access except to properties of objects:

   function a() {
      var x = 5,
         obj = {};
      b(x, obj);
   }
   
   function b(x, obj){
      // access x or obj...
      // changing x here won't change x in a, but you can modify properties of obj
   }
   
   a();
   

   As a variation on this you could get write access by passing updated values back to a like so:

   // in a:
   var ret = b(x, obj);
   x = ret.x;
   obj = ret.obj;
   
   // in b:
   return {x : x, obj : obj};
   

2. You could pass b an object with getters and setters that can access a's private variables:

   function a(){
      var x = 5,
         obj = {..},
         translator = {
            getX : function() {return x;},
            setX : function(value) {x = value;},
            getObj : function() {return obj;},
            setObj : function(value) {obj = value;}
         };
      b(translator);
   }
   
   function b(t){
      var x = t.getX(),
         obj = t.getObj();
   
      // use x or obj...
      t.setX(x);
      t.setObj(obj);
   
      // or you can just directly modify obj's properties:
      obj.key = value;
   }
   
   a();
   

   The getters and setters could be public, assigned to the this object of a, but this way they are only accessible if explicitly given out from within a.

3. And you could put your variables in an object and pass the object around:

   function a(){
      var v = {
         x : 5,
         obj : {}
      };
      b(v);
   }
   
   function b(v){
      // access v.x or v.obj...
      // or set new local x and obj variables to these and use them.
   }
   
   a();
   

   As a variation you can construct the object at call time instead:

   function a(){
      var x = 5,
         obj = {};
      b({x : x, obj: obj});
   }
   
   function b(v){
      // access v.x or v.obj...
      // or set new local x and obj variables to these and use them.
   }
   
   a();
   

Answer 3

Have you tried something like this:

function a(){
   var x = 5;
   var obj = {..};
   b(this);
}
function b(fnA){
   //access x or obj....
   fnA.obj = 6;
}

If you can stand function B as a method function A then do this:

function a(){
   var x = 5;
   var obj = {..};
   b(this);

   this.b = function (){
      // "this" keyword is still === function a
   }
}

Answer 4

As others have said, you cannot pass scope like that. You can however scope variables properly using self executing anonymous functions (or immediately executing if you're pedantic):

(function(){
    var x = 5;
    var obj = {x:x};
    module.a = function(){
        module.b();
    };
    module.b = function(){
        alert(obj.x);    
    };
}());

a();

Answer 5

function a(){
   this.x = 5;
   this.obj = {..};
   var self = this;
   b(self);
}
function b(scope){
   //access x or obj....

}

Answer 6

I think the simplest thing you can do is pass variables from one scope to a function outside that scope. If you pass by reference (like Objects), b has 'access' to it (see obj.someprop in the following):

function a(){
   var x = 5;
   var obj = {someprop : 1};
   b(x, obj);
   alert(x); => 5
   alert(obj.someprop); //=> 'otherval'
}
function b(aa,obj){
   x += 1; //won't affect x in function a, because x is passed by value
   obj.someprop = 'otherval'; //change obj in function a, is passed by reference
}