How long local(function) variables live in javascript

Question

I came across to this example:

function countMyself() {
// Check to see if the counter has been initialized
if ( typeof countMyself.counter == 'undefined' ) {
    // It has not... perform the initialization
    countMyself.counter = 0;
}

// Do something stupid to indicate the value
alert(++countMyself.counter);
}

The snippet above demonstrate "how to implement static local variable in javascript"

I know that function variables are stored in the stack. Having C background I know that variables in the stack can be easily overwritten by consequent function calls.

This doesn't seem to be the case with javascript.

What rule specifies how long a local(function) variable live in the lifetime of the program? I mean stack in Javascript must bear different semantic than stack in C, C++?

That's not really a local variable; it's a property of the function object itself. Functions are objects, and their properties last until something changes or removes them. — Pointy
– Pointy, Commented Aug 13, 2018 at 15:07
"Having C background I know that variables in the stack can be easily overwritten by consequent function calls." I'm not really sure what that means; it's not that they're "overwritten", which would imply a consistent memory location, it's that there's a new stack frame. In that sense JS is exactly the same, the difference being what Pointy said: this isn't a variable "local to the function" in the way you think it is. "Local function variables" in JS look and act roughly as they do in C. — Dave Newton
– Dave Newton, Commented Aug 13, 2018 at 15:10
@DaveNewton In C to return reference to a local variable(object) is idiotic. In JS this i common practis. — Hairi
– Hairi, Commented Aug 13, 2018 at 15:12
@Hairi setting a property of a function is not common practice in JavaScript. — epascarello
– epascarello, Commented Aug 13, 2018 at 15:17

Jonas Wilms · Accepted Answer · 2021-06-22 11:44:05Z

2

Local variables at least exist as long as they are accessible. Through closures they could even stay forever:

   function noClosure() {
     let local = 3;
     //...
   } // local gets recycled here

   function closure() {
     let local = 3;
     return function inner() {
       return local; // <- closured
    }
  }

  var closured = closure();
  // local exists here:
  console.log(closured()); // 3
  // but now it might get recycled:
  closured = undefined;

In your snippet that isn't actually a local variable but rather the property of a global function object, which exists until it gets deleted, becomes unreferencable, or the engine stops executing:

  delete countMyself.counter; // property deletion
  countMyself = somethingNew; // unreferencable

edited Jun 22, 2021 at 11:44

answered Aug 13, 2018 at 15:26

Jonas Wilms

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1 Comment

Dave Newton Over a year ago

(I added "unreferenceable" to cover when the function declaration is overwritten.)

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