javascript - garbage collection with node.js

Question

I was curious about how the node.js pattern of nested functions works with the garbage collector of v8. here's a simple example

readfile("blah", function(str) {
   var val = getvaluefromstr(str);
   function restofprogram(val2) { ... } (val)
})

if restofprogram is long-running, doesn't that mean that str will never get garbage collected? My understanding is that with node you end up with nested functions a lot. Does this get garbage collected if restofprogram was declared outside, so str could not be in scope? Is this a recommended practice?

EDIT I didn't intend to make the problem complicated. That was just carelessness, so I've modified it.

Answer 1

Simple answer: if value of the str is not referenced from anywhere else (and str itself is not referenced from restofprogram) it will become unreachable as soon as the function (str) { ... } returns.

Details: V8 compiler distinguishes real local variables from so called context variables captured by a closure, shadowed by a with-statement or an eval invocation.

Local variables live on the stack and disappear as soon as function execution completes.

Context variables live in a heap allocated context structure. They disappear when the context structure dies. Important thing to note here is that context variables from the same scope live in the same structure. Let me illustrate it with an example code:

function outer () {
  var x; // real local variable
  var y; // context variable, referenced by inner1
  var z; // context variable, referenced by inner2

  function inner1 () {
    // references context 
    use(y);
  }

  function inner2 () {
    // references context 
    use(z);
  }

  function inner3 () { /* I am empty but I still capture context implicitly */ } 

  return [inner1, inner2, inner3];
}

In this example variable x will disappear as soon as outer returns but variables y and z will disappear only when both inner1, inner2 and inner3 die. This happens because y and z are allocated in the same context structure and all three closures implicitly reference this context structure (even inner3 which does not use it explicitly).

Situation gets even more complicated when you start using with-statement, try/catch-statement which on V8 contains an implicit with-statement inside catch clause or global eval.

function complication () {
  var x; // context variable

  function inner () { /* I am empty but I still capture context implicitly */ }

  try { } catch (e) { /* contains implicit with-statement */ }

  return inner;
}

In this example x will disappear only when inner dies. Because:

• try/catch-contains implicit with-statement in catch clause
• V8 assumes that any with-statement shadows all the locals

This forces x to become a context variable and inner captures the context so x exists until inner dies.

In general if you want to be sure that given variable does not retain some object for longer than really needed you can easily destroy this link by assigning null to that variable.