Comments on: Is JavaScript’s “Global” Scope Really Just A Closure? http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/ Better Than Yesterday Fri, 24 May 2013 06:39:00 +0000 hourly 1 http://wordpress.org/?v=3.4.2 By: Derick Bailey http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/#comment-2080 Derick Bailey Sun, 01 Jan 2012 00:49:00 +0000 http://lostechies.com/derickbailey/?p=688#comment-2080 yeah, in this version you're replacing the "write" function on the global "document" object.  another way to do this is like I said in the comment for your other version... use the "call" method to set the context of your function and then call "this.document.write" in that... var w = {   document: {     write: function(){       // ...     }   } } function myFunc(){   this.document.write("foo"); } myFunc.call(w); or just pass in an argument to the function: function func2(w2){ w2.document.write("bar"); } func2(w); yeah, in this version you’re replacing the “write” function on the global “document” object. 

another way to do this is like I said in the comment for your other version… use the “call” method to set the context of your function and then call “this.document.write” in that…

var w = {
  document: {
    write: function(){
      // …
    }
  }
}

function myFunc(){
  this.document.write(“foo”);
}

myFunc.call(w);

or just pass in an argument to the function:

function func2(w2){
w2.document.write(“bar”);
}

func2(w);

]]> By: Derick Bailey http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/#comment-2079 Derick Bailey Sun, 01 Jan 2012 00:47:00 +0000 http://lostechies.com/derickbailey/?p=688#comment-2079 in this version, you never override anything. you're creating an object with a "write" method, but never calling it. you've set the context of your immediate function by using the 'call' method, but you're still using a closure (scope chain) up to the global function to find the "document.write" call from within that. if you wanted to use your object instead of the global, you would need to call the version of "write" that is attached to the function's scope: "this.document.write(...)". in this version, you never override anything. you’re creating an object with a “write” method, but never calling it.

you’ve set the context of your immediate function by using the ‘call’ method, but you’re still using a closure (scope chain) up to the global function to find the “document.write” call from within that.

if you wanted to use your object instead of the global, you would need to call the version of “write” that is attached to the function’s scope: “this.document.write(…)”.

]]> By: Jason Sebring http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/#comment-2078 Jason Sebring Sun, 01 Jan 2012 00:06:00 +0000 http://lostechies.com/derickbailey/?p=688#comment-2078 Revised version:             (function(){                 var window = { // define a fake window object                     document : {                         write : function(str) {                           // fake write implementation                           alert(str);                         }                     }                 },                 document = window.document;                 (function() { // wrap pre-existing script with this fakey closure                     document.write('global');                 })();             })(); Looks like you have to explicitly override the global vars, property by property. This would be cumbersome to fake the window object seems. Please advise on this one as I'm interested on how to do that. John Resig? Douglas Crockford? Revised version:

            (function(){
                var window = { // define a fake window object
                    document : {
                        write : function(str) {
                          // fake write implementation
                          alert(str);
                        }
                    }
                },
                document = window.document;
                (function() { // wrap pre-existing script with this fakey closure
                    document.write(‘global’);
                })();
            })();

Looks like you have to explicitly override the global vars, property by property. This would be cumbersome to fake the window object seems. Please advise on this one as I’m interested on how to do that. John Resig? Douglas Crockford?

]]> By: Jason Sebring http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/#comment-2077 Jason Sebring Sat, 31 Dec 2011 23:52:00 +0000 http://lostechies.com/derickbailey/?p=688#comment-2077 Apparently NOT. I would think the same but WTF is this example I cooked up. It defies my understanding. Please help.                     var myFakeWindow = { // define a fake window object                 document : {                     write : function(str) {                       // fake write implementation                       alert(str);                     }                 }             };             (function(window) { // wrap pre-existing script with this fakey closure this.document.write('does alert'); window.document.write('does alert');                 document.write('global');             }).call(myFakeWindow,myFakeWindow); That doesn't play by the rules of closures. Meaning "window" is set to be "this" and also have it explicitly overridden yet the "document.write" still calls the global window object. huh? Apparently NOT. I would think the same but WTF is this example I cooked up. It defies my understanding. Please help.
       

            var myFakeWindow = { // define a fake window object
                document : {
                    write : function(str) {
                      // fake write implementation
                      alert(str);
                    }
                }
            };
            (function(window) { // wrap pre-existing script with this fakey closure
this.document.write(‘does alert’);
window.document.write(‘does alert’);
                document.write(‘global’);
            }).call(myFakeWindow,myFakeWindow);

That doesn’t play by the rules of closures. Meaning “window” is set to be “this” and also have it explicitly overridden yet the “document.write” still calls the global window object. huh?

]]> By: website hosting india http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/#comment-2016 website hosting india Mon, 05 Dec 2011 12:37:00 +0000 http://lostechies.com/derickbailey/?p=688#comment-2016 I follow above the steps to implement global variable in java script.Most of the points are explained very clearly. I follow above the steps to implement global variable in java script.Most of the points are explained very clearly.

]]> By: developingchris http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/#comment-2011 developingchris Sat, 03 Dec 2011 03:22:00 +0000 http://lostechies.com/derickbailey/?p=688#comment-2011 I find it interesting too, if you think of the dom as being all inside a self run function like this. new function (){  var window = this;   // running before the html tag. and the closing brace   <html> </html> } Then now you can see, there is no global scope, just everything as a closure scoped to window. I find it interesting too, if you think of the dom as being all inside a self run function like this.

new function (){  var window = this;   // running before the html tag. and the closing brace  
<html>
</html>
}

Then now you can see, there is no global scope, just everything as a closure scoped to window.

]]> By: Derick Bailey http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/#comment-2010 Derick Bailey Thu, 01 Dec 2011 18:02:00 +0000 http://lostechies.com/derickbailey/?p=688#comment-2010 this popped into my head a moment ago: when a variable is accessed, whether or read or assignment, the runtime has to find the location of that variable. it first checks the local scope. if not found, it moved up to the next outer scope. if not found, it moves up again, until it finally reaches the global scope. at any point in time, if the variable declaration is found in the scope that is currently being examined, the runtime uses that variable. it would make sense that this same basic process works for both reads and writes, with one small exception: when reading a variable, the runtime will reach the top of the global scope and if not found, it will throw an error saying the variable is not defined.  but when assigning something to a variable, when the runtime reaches the very top of the global scope and doesn't find any declaration for the variable, the runtime shrugs and says "whatever. i'll just create a variable for you, here". so... it makes sense in my head, then, that you actually can create a variable in a scope outside of your current scope... but the only scope where this ever happens is the global scope, because the runtime just keeps searching up the scope stack until it either finds the declaration, or reaches the top. ... at least, that's how I would do it if i were building JavaScript. it's the simplest way to make this work, in my head. though I'm sure there are many nuances, subtleties, and other cases that my simple line of thinking hasn't covered. :) this popped into my head a moment ago:

when a variable is accessed, whether or read or assignment, the runtime has to find the location of that variable. it first checks the local scope. if not found, it moved up to the next outer scope. if not found, it moves up again, until it finally reaches the global scope.

at any point in time, if the variable declaration is found in the scope that is currently being examined, the runtime uses that variable.

it would make sense that this same basic process works for both reads and writes, with one small exception:

when reading a variable, the runtime will reach the top of the global scope and if not found, it will throw an error saying the variable is not defined. 

but when assigning something to a variable, when the runtime reaches the very top of the global scope and doesn’t find any declaration for the variable, the runtime shrugs and says “whatever. i’ll just create a variable for you, here”.

so… it makes sense in my head, then, that you actually can create a variable in a scope outside of your current scope… but the only scope where this ever happens is the global scope, because the runtime just keeps searching up the scope stack until it either finds the declaration, or reaches the top.

… at least, that’s how I would do it if i were building JavaScript. it’s the simplest way to make this work, in my head. though I’m sure there are many nuances, subtleties, and other cases that my simple line of thinking hasn’t covered. :)

]]> By: Avi Block http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/#comment-2009 Avi Block Thu, 01 Dec 2011 17:02:00 +0000 http://lostechies.com/derickbailey/?p=688#comment-2009 Really... "It looks like global scope is nothing more than the bi-product of the closure support that JavaScript has built into it, in combination with the outermost scope of the runtime (a DOMWindow or some other scope for CommonJS implementations)." Really…

“It looks like global scope is nothing more than the bi-product of the closure support that JavaScript has built into it, in combination with the outermost scope of the runtime (a DOMWindow or some other scope for CommonJS implementations).”

]]> By: Jason Mulligan http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/#comment-2008 Jason Mulligan Thu, 01 Dec 2011 16:08:00 +0000 http://lostechies.com/derickbailey/?p=688#comment-2008 True, but the context of the copy appears to be a browser so it's a safe assumption. True, but the context of the copy appears to be a browser so it’s a safe assumption.

]]> By: Joey Vano http://lostechies.com/derickbailey/2011/11/30/is-javascripts-global-scope-really-just-a-closure/#comment-2007 Joey Vano Thu, 01 Dec 2011 15:57:00 +0000 http://lostechies.com/derickbailey/?p=688#comment-2007 In a browser environment, yes. In a browser environment, yes.

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