Does ES6 const affect garbage collection?

问题:

In Kyle Simpson's new title, You don't know JS: ES6 and beyond, I find the following snippet:

WARNING Assigning an object or array as a constant means that value will not be able to be garbage collected until that constant’s lexical scope goes away, as the reference to the value can never be unset. That may be desirable, but be careful if it’s not your intent!

(Excerpt From: Simpson, Kyle. “You Don’t Know JS: ES6 & Beyond.” O'Reilly Media, Inc., 2015-06-02. iBooks. This material may be protected by copyright.)

As far as I can see, he doesn't expand on this, and 10 minutes on Google turns up nothing. Is this true, and if so, what does "the reference to the value can never be unset" mean exactly? I have got into the habit of declaring variables that won't be changed as const, is this a bad habit in real concrete performance/memory terms?

回答1:

No, there are no performance implications. This note refers to the practise of helping the garbage collector (which is rarely enough needed) by "unsetting" the variable:

{
    let x = makeHeavyObject();
    window.onclick = function() {
        // this *might* close over `x` even when it doesn't need it
    };
    x = null; // so we better clear it
}

This is obviously not possibly to do if you had declared x as a const.

The lifetime of the variable (when it goes out of scope) is not affected by this. But if the garbage collector screws up, a constant will always hold the value it was initialised with, and prevent that from being garbage-collected as well, while a normal variable might no more hold it.

回答2:

WARNING Assigning an object or array as a constant means that value will not be able to be garbage collected until that constant’s lexical scope goes away, as the reference to the value can never be unset. That may be desirable, but be careful if it’s not your intent!

That note sounds a bit more of a warning than is necessary (perhaps even a bit silly) and tries to make some sort of special case out of this situation.

With a const variable declaration, you can't assign to the variable something little like "" or null to clear its contents. That's really the only difference in regard to memory management. Automatic garbage collection is not affected at all by whether it is declared const or not.

So, if you would like to be able to change the contents of the variable in the future for any reason (including to manually remove a reference to something to allow something to be garbage collected sooner), then don't use const. This is the same as any other reason for using or not using const. If you want to be able to change what the variable contains at any time in the future (for any reason), then don't use const. This should be completely obvious to anyone who understand what const is for.

Calling out garbage collection as a special case for when not to use const just seems silly to me. If you want to be able to clear the contents of a variable, then that means you want to modify the variable so duh, don't use const. Yes, manually enabling garbage collection on a large data structure that might be caught in a lasting scope/closure is one reason that you might want to change the variable in the future. But, it's just one of millions of reasons. So, I repeat one more time. If you ever want to change the contents of the variable for any reason in the future, then don't declare it as const.

The garbage collector itself doesn't treat a const variable or the contents it points to any different than a var or let variable. When it goes out of scope and is no longer reachable, its contents will be eligible for garbage collection.

const has a number of advantages. It allows the developer to state some intent that the contents this variable points to are not to be changed by code and may allow the runtime to make some optimizations because it knows the contents of the variable cannot be changed. And, it prevents rogue or accidental code from ever changing the contents of that variable. These are all good things when used in an appropriate case. In general, you SHOULD use const as much as practical.

回答3:

The way garbage collectors (GC) work is when something is referenced by nothing ("cannot be reached"), the GC can safely say that something isn't used anymore and reclaim the memory used by that something.

Being able to replace the value of a variable allows one to remove a reference to the value. However, unlike var, const cannot be reassigned a value. Thus, one can't remove that constant from referencing the value.

A constant, like a variable, can be reclaimed when the constant goes "out of scope", like when a function exits, and nothing inside it forms a closure.

回答4:

That note in my book was referring to cases like this, where you'd like to be able to manually make a value GC'able earlier than the end of life of its parent scope:

var cool = (function(){
   var someCoolNumbers = [2,4,6,8,....1E7]; // a big array

   function printCoolNumber(idx) {
      console.log( someCoolNumbers[idx] );
   }

   function allDone() {
      someCoolNumbers = null;
   }

   return {
      printCoolNumber: printCoolNumber,
      allDone: allDone
   };
})();

cool.printCoolNumber( 10 ); // 22
cool.allDone();

The purpose of the allDone() function in this silly example is to point out that there are times when you can decide you are done with a large data structure (array, object), even though the surrounding scope/behavior may live on (via closure) indefinitely in the app. To allow the GC to pick up that array and reclaim its memory, you unset the reference with someCoolNumbers = null.

If you had declared const someCoolNumbers = [...]; then you would be unable to do so, so that memory would remain used until the parent scope (via the closure that the methods on cool have) goes away when cool is unset or itself GCd.

Update

To make absolutely clear, because there's a lot of confusion/argument in some comment threads here, this is my point:

const absolutely, positively, undeniably has an effect on GC -- specifically, the ability of a value to be GCd manually at an earlier time. If the value is referenced via a const declaration, you cannot unset that reference, which means you cannot get the value GCd earlier. The value will only be able to be GCd when the scope is torn down.

If you'd like to be able to manually make a value eligible for GC earlier, while the parent scope is still surviving, you'll have to be able to unset your reference to that value, and you cannot do that if you used a const.

Some seem to have believed that my claim was const prevents any GC ever. That was never my claim. Only that it prevented earlier manual GC.