Does Javascript closure always retain scope variables?

Question

I understand that code in closure can access variables & methods & arguments up the scope chain, but what happens if it doesn't use any of them ? do those variables still retained ?

Consider this case :

function f(){

  var a=[];
  for(var i=0;i<1000000;i++) a.push({});

  return function(){
     alert('Hi');
  };

}

var x = f();

Does variable a retained in memory even though the closure does not use it ?

Thanks

UPDATE: Seems there's no answer about 'trivial' closures. So is it fair to assume that each and every closure ( even if it does nothing at all ) may retain in memory all the methods up the scope chain including their arguments , variables and inner functions ( until the closure is garbage collected )?

Also, about the 'possibly duplicate' question about node.js - to my knowledge node.js runs only on a dedicated environment that based on google's v8 JS engine. Here I'm talking about web-apps that will run in any modern browser ( in most cases )

You never can really know… The JS implementation can do as it want. Clever garbage collectors that do statical code analysis can collect the values, but there is no guarantee. — Bergi
– Bergi, Commented Sep 19, 2013 at 0:19
possible duplicate of garbage collection with node.js (and its related questions) — Bergi
– Bergi, Commented Sep 19, 2013 at 0:28
Rule of thumb: For (old) engines with dumb garbage collectors better set a=null explicitly. — Bergi
– Bergi, Commented Sep 19, 2013 at 0:41
If we are to suppose only relatively new browsers , It this an issue to worry about ? — TheZver
– TheZver, Commented Sep 19, 2013 at 0:44

Eric · Accepted Answer · 2013-09-19 00:25:36Z

5

When the interpreter chooses to free the memory it occupied is an implementation detail - there is no single javascript interpreter.

Note that it's not always possible for the interpreter to know the variable is unused:

function f() {
    var a = 123

    return function(x) {
        alert(eval(x));  // if there's an eval, we have to hold onto all local variables
    };

}

f()('a')

Experimenting in the chrome console

var e = eval

var f = function(){
    var a = 123;
    
    return function() {
        return eval('a');
    };
};

var g = function(){
    var a = 123;

    return function() {
        return e('a');
    };
};


f()()  // 123
g()()  // ReferenceError

It appears that V8 is making optimizations based on the present of eval

edited Sep 19, 2013 at 0:25

answered Sep 19, 2013 at 0:18

Eric

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3 Comments

Eric Over a year ago

this answer sheds a bit more light on what's happening. It'd be nice to see the spec for that.

Bergi Over a year ago

No, it does not only look innocent but is innocent. g('a') and eval('a') do evaluate differently, EcmaScript specially introduced the concept of a "direct call" for this. g will eval the code in the global scope.

TheZver Over a year ago

That's fair. but I'm talking about specifically trivial functions of few lines that only use arguments and/or local variables. Does the new browsers know to handle that and act like It's a global-scope-method ? It's of big importance because I've tones of tiny closures used for quick event-handlers setup ( specifically talking - ExtJs component listeners )

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