Develop Reference

Does ES6 const affect garbage collection?

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?

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.
I should add the even some const data can still be reduced in size and make the majority of its contents available for garbage collection. For example, if you had a really large 100,000 element array of objects (that you perhaps received from some external http call) in a const array:
const bigData = [really large number of objects from some API call];
You can still massively reduce the size of that data by simply clearing the array which potentially makes the large number of objects that was in the array eligible for garbage collection if nothing else had a reference to them:
bigData.length = 0;
Remember, that const prevents assignment to that variable name, but does not prevent mutating the contents that the variable points to.
You could do the same thing with other built-in collection types such as map.clear() or set.clear() or even any custom object/class that has methods for reducing its memory usage.

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.

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.

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.

What is the cost to convert a javascript variable to jQuery Object?

Sometimes I see in Javascript functions that, if the conversion of a variable to jQuery is used repeatedly, then it can be assigned to a local variable first:
$variable = $(variable);
Is this necessary, and how much is the cost of conversion?

No matter what, storing the object is faster than having to re-instantiate a jQuery object every time you want to use jQuery methods on it...even if it's miniscule for caching $(this) or $(anObject).
A term used to describe this method of "store now, use later" is "caching". The reason it's often called "caching" is because caching refers to storing a reference to something once and using that, without going back out to grab the same thing again, later (very non-technical, non-100% accurate description).
The major point is dealing with selectors. jQuery has to query the DOM every time, which is the expensive part. Generating the object and storing references isn't that expensive compared to DOM manipulation (and jQuery processing your selection in the first place).
If you're simply creating a jQuery object out of an object reference, it's not nearly as devastating, as the processing that takes place is the creation of the jQuery object...so it's really limited to whatever jQuery does for that. It's still good practice and still prevents some unnecessary processing. For example, this:
var element = document.getElementById("div_id");
$(element).someMethod();
// Later:
$(element).someOtherMethod();
is slightly inefficient, since a new jQuery object is created each time. It could easily be condensed to store a reference to a single jQuery object in a variable, and reference that.
The one caveat I can think of is that it isn't a live list of elements (if selecting DOM elements). For example, you may want to cache all elements with the class testing-class, like so:
var myelements = $(".testing-class");
But if another element is added to the DOM with the testing-class class, myelements will not be reflected. It will have the same, previous list. So in that case, the DOM will obviously need to be re-queried and update myelements.
To me, the best practice for caching is within a scope....not the entire page. If you are running a function, and it selects some elements, cache it at the beginning, and use that. But don't cache it globally and use it throughout your page; cache it for an execution cycle.
For example, I would do this:
function someFunc() {
var elements = $(".class-stuff");
// Use `elements` here
// code
// Use `elements` here
someOtherFunc(elements);
}
function someOtherFunc(el) {
// Use `el` here
}
someFunc();
// Some time later:
someFunc();
but I wouldn't do this:
var elements = $(".class-stuff");
function someFunc() {
// Use `elements`
}
function someOtherFunc() {
// Use `elements`
}
someFunc();
someOtherFunc();
// Some time later
someOtherFunc();

It depends on what the variable is. If the original variable is just a single DOM element then it's not particularly expensive - the DOM traversal has already been done so all you're doing is wrapping that element in the jQuery pseudo-array and attaching the prototype.
However if the original variable is a selector, then you absolutely should cache the result to avoid repeated conversions from DOM -> element list.
In any event, it's good practise not to repeat yourself, so caching $(variable) is just good code hygiene.

If the $(variable) is being called anyway this assignment has basically no cost -- this is only storing a reference to the object in memory.
Purists might point out that because the jQuery object is now stored it can't be garbage collected, and this is true. So I guess if you had lots of these it could cause a memory issue, but in itself it has no cost to speak of.
The reason it is done is because there is a cost associated with creating the object, that is the $(variable) part. That if done many times could be expensive. Store a reference to the object means only one needs to be created.
Another important point: The following statement
var $variable = $(variable);
could act different if it is done in a calling context of a closure. That is if there is a function defined in the scope of the var statement that variable will "stick around" for the function to use. This could have the same effects as described above (no gc and pointer memory) with the addition of a longer lifetime. (Because it will stay as long as the function has potential to be called.)

Anonymous functions and memory consumption

In terms of memory consumption, are these equivalent or do we get a new function instance for every object in the latter?
var f=function(){alert(this.animal);}
var items=[];
for(var i=0;i<10;++i)
{
var item={"animal":"monkey"};
item.alertAnimal=f;
items.push(item);
}
and
var items=[];
for(var i=0;i<10;++i)
{
var item={"animal":"monkey"};
item.alertAnimal=function(){alert(this.animal);};
items.push(item);
}
EDIT
I'm thinking that in order for closure to work correctly, the second instance would indeed create a new function each pass. Is this correct?

You should pefer the first method, since the second one creates a function every time the interpreter passes that line.
Regarding your edit: We are in the same scope all the time, since JavaScript has function scope instead of block scope, so this might be optimizable, but i did not encounter an implementation that doesn't create it every time. I would recommend not to rely on this (probably possible) optimization, since implementations that lack support could likely exceed memory limits if you use this technique extensively (which is bad, since you do not know what implementation will run it, right?).

I am not an expert, but it seems to me that different javascript engines could be handling this in different ways.
For example, V8 has something called hidden classes, which could affect memory consumption when accessing the same property. Maybe somebody can confirm or deny this.

Javascript global variables or object variables

I prefer to declare one Javascript file for my all website. I am trying to decrease the usage of global variables. My examples at the below, in both case each object has a myName field.
I would like to know when they are initialized?
And In terms of memory and efficiency which one is more effective?
For variable a, is declaring a.myName the same as global "var myName = Rebecca" ?
var a = {
myName : 'Rebecca' ,
sayHello : function() {
console.log(this.myName);
}
};
var b = {
myName : function() {
return 'Rebecca';
},
sayHello : function() {
console.log(this.myName());
}
};
Thanks

I believe these will be initialized identically (i.e. when the code is reached). What's different is what's happening when they are initialized and where the load is placed when their data is actually required.
To me, it would depend a lot on what you were expecting to have in myName. If it were just a string, I'd avoid the function and go with choice a. On the other hand, if there were a great deal of logic involved and that logic might not need to be invoked (for example, if it only gets executed when a user clicks on a button or the application reaches a certain state), I'd go with choice b. As I understand it, the function does consume memory and won't get garbage collected (which is a minus), but it also won't consume CPU resources until it's actually needed (which can be a huge plus).
I'm not sure I understand the question, but I'd say it's not the same. If the only member of a is myName then the two are equivalent (both are occupying the global namespace. But if you have multiple properties, the savings become obvious. From your examples, I think it's clear you understand this, so again I may not understand the question.

They will be initialized when the statements are first encountered. In a, 'Rebecca' is initialized as the value for the myName key. In b, it's just data internal to the myName (anonymous) function. a will be slightly more efficient because it avoids a function call. I also find it more readable in this simple example.
I find the choice to put everything in a single file questionable. In some cases, you want a modular design. And since you're worried about efficiency (albeit perhaps prematurely), note that having one big file can actually hurt performance if pages include code they don't need.

1) They are initialized when the script is processed in the browser, unless you declare the objects in an event handler. In that case the object is created when the event script is executed.
2) In terms of efficiency, a will probably be more efficient. Note though that in the first case you use a.myName and in the second b.myName() to get the value of the property.
3) No. If you assign a value to a property of an object, you always have to get that value through the object. In this case either a.myName or a['myName'].

a doesn't make any sense, because you're logging a function reference. B is the way to go, since you're actually invoking the method, using ().

When is an object in Javascript constructed?

Consider the following Javascript function (1):
function setData(domElement) {
domElement.myDataProperty = {
'suppose': 'this',
'object': 'is',
'static': 'and',
'pretty': 'big'
};
};
Now what I don't like about this function is that the exact same object is created every time the function is called. Since the object does not change I would rather create it just once. So we could make the following adjustments (2):
var dataObject = {
'suppose': 'this',
'object': 'is',
'static': 'and',
'pretty': 'big'
};
function setData(domElement) {
domElement.myDataProperty = dataObject;
};
Now the object is created once when the script is loaded and stored in dataObject. But let's assume that setData is called only occasionally -- most of the times that the script is loaded the function is not used. What I don't like about this function in that case is that the object is always created and held in memory, including many occasions in which it will never be used. I figured you could do something like this to strike the ideal balance (3):
var dataObject;
function setData(domElement) {
if (!dataObject) {
dataObject = {
'suppose': 'this',
'object': 'is',
'static': 'and',
'pretty': 'big'
};
}
domElement.myDataProperty = dataObject;
};
Would that make sense? I figure it depends on when the interpreter decides to create an object. Does it really wait until it passes the !dataObject condition, or does it enter the function, tries to be smart and decides to construct it in advance? Perhaps different Javascript engines have different policies with regard to this?
Then of course there is the question of whether these optimizations will ever matter in practice. Obviously this depends on factors like the size of the object, the speed of the engine, the amount of resources available, etc.. But in general, which one would you say is the more significant optimization: from (1) to (2) or from (2) to (3)?

The answer is, you're not supposed to know. The examples you showed have very little difference between them. The only way you'd ever reasonably worry about this is if you had actual evidence that one way or another was noticably harming performance or memory usage on a particular interpreter. Until then, it's the interpreter's job to worry about that stuff for you.
That said, if you really want to know... Try it and find out. call the different versions 1,000,000 times and see what difference it makes.
Make a giant version of the object and see if that makes a dent. Watch task manager. Try different browsers. Report back your results. It's a much better way to find out than just asking a bunch of jerks on the internet what they guess might be the case.
just keep in mind that object has to be in memory anyway, regardless ... as source text

A new object must be created -- it cannot not be, partially because the spec requires it, but mostly because alternative behaviour would be counter intuitive, take:
function f() {
return {a : "b", c: "d"};
}
o=f();
alert([o.c, o.e]); // Alerts "b,"
delete o.c;
o.e="f";
o=f();
alert([o.c, o.e]); // If the object was only created once this would produce ",f"
Do you really expect a new object expression to not actually produce the object you're asking for? Because that's what you seem to want.
Conceivably you just want to do:
var myFunction = (function(){
var object = {a: "b", c: "d"};
return function() { return object; }
})();
Which would get the effect you want, although you would have to realise that the object you're returning is a completely mutable object that can be changed, and everyone would be sharing that same mutating instance.

First, I'd implement it in situation #2 and load it once immediately after the page is loaded.
If there was a problem with page speed, I would measure the time taken for specific tasks within the page.
If it was very expensive to create the object (relatively speaking), then I would move to situation #3.
There's no point in adding the 'if' statement if it really doesn't buy you anything... and in this case, creating a simple/big object is no sweat off your CPU's back. Without measurements, you're not optimizing - you're just shooting blind.
It's actually a fairly common method of initializing things that I've personally used in C++ and Java.

First, this optimization will never matter in practice.
Second, the last function is exactly as good as the first function. Well, almost. In the first I suppose you're at the mercy of the garbage collector, which should destroy the old object when you reassign domElement.myDataProperty. Still, without knowing exactly how the garbage collector works on your target platform (and it can be very different across browsers), you can't be sure you're saving any work at all really.

Try all three of them in a couple of browsers and find out which is faster.