How JavaScript closures are garbage collected

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As far as I can tell, this is not a bug but the expected behavior.

From Mozilla’s Memory management page: “As of 2012, all modern browsers ship a mark-and-sweep garbage-collector.” “Limitation: objects need to be made explicitly unreachable“.

In your examples where it fails some is still reachable in the closure. I tried two ways to make it unreachable and both work. Either you set some=null when you don’t need it anymore, or you set window.f_ = null; and it will be gone.

Update

I have tried it in Chrome 30, FF25, Opera 12 and IE10 on Windows.

The standard doesn’t say anything about garbage collection, but gives some clues of what should happen.

  • Section 13 Function definition, step 4: “Let closure be the result of creating a new Function object as specified in 13.2”
  • Section 13.2 “a Lexical Environment specified by Scope” (scope = closure)
  • Section 10.2 Lexical Environments:

“The outer reference of a (inner) Lexical Environment is a reference to the Lexical Environment that logically
surrounds the inner Lexical Environment.

An outer Lexical Environment may, of course, have its own outer
Lexical Environment. A Lexical Environment may serve as the outer environment for multiple inner Lexical
Environments. For example, if a Function Declaration contains two nested Function Declarations then the Lexical
Environments of each of the nested functions will have as their outer Lexical Environment the Lexical
Environment of the current execution of the surrounding function.”

So, a function will have access to the environment of the parent.

So, some should be available in the closure of the returning function.

Then why isn’t it always available?

It seems that Chrome and FF is smart enough to eliminate the variable in some cases, but in both Opera and IE the some variable is available in the closure (NB: to view this set a breakpoint on return null and check the debugger).

The GC could be improved to detect if some is used or not in the functions, but it will be complicated.

A bad example:

var someClass = function() {};

function f() {
  var some = new someClass();
  return function(code) {
    console.log(eval(code));
  };
}

window.f_ = f();
window.f_('some');

In example above the GC has no way of knowing if the variable is used or not (code tested and works in Chrome30, FF25, Opera 12 and IE10).

The memory is released if the reference to the object is broken by assigning another value to window.f_.

In my opinion this isn’t a bug.

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