Develop Reference

Why do objects have to be initialized first before use ? Constructor function vs Object.create [duplicate]

This code always works, even in different browsers:
function fooCheck() {
alert(internalFoo()); // We are using internalFoo() here...
return internalFoo(); // And here, even though it has not been defined...
function internalFoo() { return true; } //...until here!
}
fooCheck();
I could not find a single reference to why it should work, though.
I first saw this in John Resig's presentation note, but it was only mentioned. There's no explanation there or anywhere for that matter.
Could someone please enlighten me?

The function declaration is magic and causes its identifier to be bound before anything in its code-block* is executed.
This differs from an assignment with a function expression, which is evaluated in normal top-down order.
If you changed the example to say:
var internalFoo = function() { return true; };
it would stop working.
The function declaration is syntactically quite separate from the function expression, even though they look almost identical and can be ambiguous in some cases.
This is documented in the ECMAScript standard, section 10.1.3. Unfortunately ECMA-262 is not a very readable document even by standards-standards!
*: the containing function, block, module or script.

It is called HOISTING - Invoking (calling) a function before it has been defined.
Two different types of function that I want to write about are:
Expression Functions & Declaration Functions
Expression Functions:
Function expressions can be stored in a variable so they do not need function names. They will also be named as an anonymous function (a function without a name).
To invoke (call) these functions they always need a variable name. This kind of function won't work if it is called before it has been defined which means Hoisting is not happening here. We must always define the expression function first and then invoke it.
let lastName = function (family) {
console.log("My last name is " + family);
};
let x = lastName("Lopez");
This is how you can write it in ECMAScript 6:
lastName = (family) => console.log("My last name is " + family);
x = lastName("Lopez");
Declaration Functions:
Functions declared with the following syntax are not executed immediately. They are "saved for later use" and will be executed later, when they are invoked (called upon). This type of function works if you call it BEFORE or AFTER where is has been defined. If you call a declaration function before it has been defined Hoisting works properly.
function Name(name) {
console.log("My cat's name is " + name);
}
Name("Chloe");
Hoisting example:
Name("Chloe");
function Name(name) {
console.log("My cat's name is " + name);
}

The browser reads your HTML from beginning to end and can execute it as it is read and parsed into executable chunks (variable declarations, function definitions, etc.) But at any point can only use what's been defined in the script before that point.
This is different from other programming contexts that process (compile) all your source code, perhaps link it together with any libraries you need to resolve references, and construct an executable module, at which point execution begins.
Your code can refer to named objects (variables, other functions, etc.) that are defined further along, but you can't execute referring code until all the pieces are available.
As you become familiar with JavaScript, you will become intimately aware of your need to write things in the proper sequence.
Revision: To confirm the accepted answer (above), use Firebug to step though the script section of a web page. You'll see it skip from function to function, visiting only the first line, before it actually executes any code.

Some languages have the requirement that identifiers have to be defined before use. A reason for this is that the compiler uses a single pass on the sourcecode.
But if there are multiple passes (or some checks are postponed) you can perfectly live without that requirement.
In this case, the code is probably first read (and interpreted) and then the links are set.

I have only used JavaScript a little. I am not sure if this will help, but it looks very similar to what you are talking about and may give some insight:
http://www.dustindiaz.com/javascript-function-declaration-ambiguity/

The body of the function "internalFoo" needs to go somewhere at parsing time, so when the code is read (a.k.a parsing) by the JS interpreter, the data structure for the function is created and the name is assigned.
Only later, then the code is run, JavaScript actually tries to find out if "internalFoo" exists and what it is and whether it can be called, etc.

For the same reason the following will always put foo in the global namespace:
if (test condition) {
var foo;
}

Javascript: Create a sub-class within a class [duplicate]

I thought I had a reasonable understanding of the this object in JavaScript. When dealing with objects, callbacks, and both events and handlers, I haven't had a problem with it since time immemorial. Now, however, all has changed.
I've fallen head over heels in love with JavaScript. Pure JS, that is, not jQuery, prototype.js, dojo... So naturally, I've taken to using closures. In some cases, though, this is catching me off guard here. Take this snippet for one:
function anyFunc(par)
{
//console.log(par);
console.log(this);
}
function makeClosure(func)
{
return function(par)
{
return func(par);
}
}
var close = makeClosure(anyFunc);
close('Foo');
var objWithClosure = {cls:makeClosure(anyFunc),prop:'foobar'};
objWithClosure.cls(objWithClosure.prop);
var scndObj = {prop:'Foobar2'};
scndObj.cls = makeClosure;
scndObj.cls = scndObj.cls(anyFunc);
scndObj.cls(scndObj.prop);
In all three cases, this logs as the window object. It's an easy fix, of course:
function makeClosure(func)
{
return function(par)
{
return func.call(this,par);
}
}
This fix works, I put it here to avoid people answering this, without explaining what I need to know: why is this behaving the way it does here?
ensures the caller is effectively the object that the closure belongs to. What I fail to understand is this:
Sure enough, this points to the window object in the first case, but in other cases, it shouldn't. I tried logging this in the makeClosure function just before returning, and it did log the object itself, not the window object. But when the actual closure is used, this is back to pointing to the window object. Why?
The only thing I can think of is that, by passing the anyFunc function as an argument, I'm actually passing window.anyFunc. So I tried this quick fix:
function makeClosure(func)
{
var theFunc = func;
return function(par)
{
theFunc(par);
}
}
With the expected results, this now points to the objects, but again: Why? I have a few idea's (theFunc is a reference to the function in the local scope [this > private: theFunc]?), but I'm sure there are people here with a lot more know-how when it comes to JS, so I was hoping to get some more explanation or links to articles worth reading from them...
Thanks
Update
Here's a fiddle, may be I left something out, but here this logs all sorts of things ;)
Edit/Update 2
The case that confuses me is here.
Final Edit
Ok, This is getting a rather messy post. So to clarify: What I was expecting was a behaviour similar to this:
function makeClosure()
{
function fromThisFunc()
{
console.log(this);
}
return fromThisFunc;
}
var windowContext = makeClosure();
windowContext();
var objectContext = {cls:makeClosure()};
objectContext.cls();
What caught me, was that the function anyFunc wasn't declared within the correct scope, and therefore, this pointed to the window object. I found this out by reading an ancient scroll I found somewhere on the web.
But something a little more complicated has happened because the function object now referred to by globalVar was created with a [[scope]] property referring to a scope chain containing the Activation/Variable object belonging to the execution context in which it was created (and the global object). Now the Activation/Variable object cannot be garbage collected either as the execution of the function object referred to by globalVar will need to add the whole scope chain from its [[scope]] property to the scope of the execution context created for each call to it.
So what I needed to do, was simplify rather then complicate things:
function fromThisFunc()
{
console.log(this);
}
function makeClosure(funcRef)
{
//some code here
return funcRef;
}
That should work, right?
PS: I'll except Alnitak's answer, but special thanks goes to Felix Kling for all the patience and info.

As soon as you call:
return func(par);
You're creating a new scope (with its own this) and in this case because you haven't specified an object, this === window usually or undefined in strict mode. The called function does not inherit whatever this was in the calling scope.
Ways to set a value for this are:
myobj.func(par); // this === myobj
or
func.call(myobj, ...) // this === myobj
There are also:
apply
bind
arrow functions, where this is set to the same value as the outer execution context (also see MDN:Arrow functions )

The value of this depends only on whether you call the function as a method or as a function.
If you call it as a method, this will be the object that the method belongs to:
obj.myFunction();
If you call it as a function, this will be the window object:
myFunction();
Note that even if you are in a method that belongs to an object, you still have to call other methods in the object using the method syntax, otherwise they will be called as functions:
this.myOtherFunction();
If you put a method reference in a variable, you will detach it from the object, and it will be called as a function:
var f = obj.myFunction;
f();
The call and apply methods are used to call a function as a method even if it's not a method in the object (or if it's a method in a different object):
myFunction.call(obj);

Can you get the property name through which a function was called?

I've done a lot of searching and some playing around, and I'm pretty sure the answer to this question is no, but I'm hoping a JavaScript expert might have a trick up his sleeve that can do this.
A JavaScript function can be referenced by multiple properties, even on completely different objects, so there's no such thing as the object or property that holds the function. But any time you actually call a function, you must have done so via a single object (at the very least, the window object for global function calls) and property on that object.
(A function can also be called via a function-local variable, but we can consider the function-local variable to be a property of the activation object of the scope, so that case is not an exception to this rule.)
My question is, is there a way to get that property name that was used to call the function, from inside the function body? I don't want to pass in the property name as an argument, or closure around a variable in an enclosing scope, or store the name as a separate property on the object that holds the function reference and have the function access that name property on the this object.
Here's an example of what I want to do:
var callName1 = function() { var callName = /* some magic */; alert(callName); };
var obj1 = {'callName2':callName1, 'callName3':callName1 };
var obj2 = {'callName4':callName1, 'callName5':callName1 };
callName1(); // should alert 'callName1'
obj1.callName2(); // should alert 'callName2'
obj1.callName3(); // should alert 'callName3'
obj2.callName4(); // should alert 'callName4'
obj2.callName5(); // should alert 'callName5'
From my searching, it looks like the closest you can get to the above is arguments.callee.name, but that won't work, because that only returns the name that was fixed to the function object when it was defined, and only if it was defined as a named function (which the function in my example is not).
I also considered that maybe you could iterate over all properties of the this object and test for equality with arguments.callee to find the property whose value is a reference to the function itself, but that won't work either (in the general case), because there could be multiple references to the function in the object's own (or inherited) property set, as in my example. (Also, that seems like it would be kind of an inefficient solution.)
Can this be done?

Short answer:
No, you cannot get "the property name" used to call your function.
There may be no name at all, or multiple names across different scopes, so "the property name" is pretty ill defined.
arguments.callee is deprecated and should not be used.
There exists no solution that does not use arguments or closure.
Long answer:
As thefourtheye commented, you should rethink what you are trying to do and ask that instead in a new question. But there are some common misconceptions, so I will try to explain why you cannot get the "simple property name".
The reason is because it is not simple.
Before we go ahead, let us clarify something. Activation Objects are not objects at all.
The ECMAScript 5.1 specification calls them Environment Records (10.2.1), but a more common term is Scope chain.
In a browser the global scope is (often) the window object, but all other scopes are not objects.
There may be an object that you use to call a function, and when you call a function you must be in some scope.
With few exceptions, scopes are not objects, and objects are not scopes.
Then, there are many names.
When you call a function, you need to reference it, such as through an object property. This reference may have a name.
Scope chain has declarations, which always have a name.
A Function (the real function, not reference) may also have a function name - your arguments.callee.name - which is fixed at declaration.
Not only are they different names, they are not (always) the "the property name" you are seeking.
var obj = { prop : function f(){} }, func = obj.prop;
// "obj" and "func" are declarations.
// Function name is "f" - use this name instead of arguments.callee
// Property name is "prop"
func(); // Reference name is "func"
obj.prop(); // Reference names are "obj" and "prop"
// But they are the same function!
// P.S. "this" in f is undefined (strict mode) or window (non-strict)
So, a function reference may comes from a binding (e.g. function declaration), an Object (arguments.callee), or a variable.
They are all References (8.7). And reference does have a name (so to speak).
The catch is, a function reference does not always come from an object or the scope chain, and its name is not always defined.
For example a common closure technique:
(function(i){ /* what is my name? */ })(i)
Even if the reference does have a name, a function call (11.2.3) does not pass the reference or its name to the function in any way.
Which keeps the JavaScript engine sane. Consider this example:
eval("(new Function('return function a(){}'))()")() // Calls function 'a'.
The final function call refers the eval function, which refers the result of a new global scope (in strict mode, anyway), which refers a function call statement, which refers a group, which refers an anonymous Function object, and which contains code that expresses and returns a function called 'a'.
If you want to get the "property name" from within a, which one should it get? "eval"? "Function"? "anonymous"? "a"? All of them?
Before you answer, consider complications such as function access across iframes, which has different globals as well as cross origin restriction, or interaction with native functions (Function.prototype.bind for example), and you will see how it quickly becomes hell.
This is also why arguments.caller, __caller__, and other similar techniques are now all deprecated.
The "property name" of a function is even more ill defined than the caller, almost unrealistic.
At least caller is always an execution context (not necessary a function).
So, not knowing what your real problem is, the best bet of getting the "property name" is using closure.

there is no reflection, but you can use function behavior to make adding your own fairly painless, and without resorting to try/catch, arguments.callee, Function.caller, or other strongly frowned-upon behavior, just wasteful looping:
// returning a function from inside a function always creates a new, unique function we can self-identify later:
function callName() {
return function callMe(){
for(var it in this) if(this[it]===callMe) return alert(it);
}
};
//the one ugly about this is the extra "()" at the end:
var obj1 = {'callName2':callName(), 'callName3':callName() };
var obj2 = {'callName4':callName(), 'callName5':callName() };
//test out the tattle-tale function:
obj1.callName2(); // alerts 'callName2'
obj2.callName5(); // alerts 'callName5'
if you REALLY want to make it look like an assignment and avoid the execution parens each time in the object literal, you can do this hacky routine to create an invoking alias:
function callName() {
return function callMe(){
for(var it in this) if(this[it]===callMe) return alert(it);
}
};
//make an alias to execute a function each time it's used :
Object.defineProperty(window, 'callNamer', {get: function(){ return callName() }});
//use the alias to assign a tattle-tale function (look ma, no parens!):
var obj1 = {'callName2': callNamer, 'callName3': callNamer };
var obj2 = {'callName4': callNamer, 'callName5': callNamer };
//try it out:
obj1.callName2(); // alerts 'callName2'
obj2.callName5(); // alerts 'callName5'
all that aside, you can probably accomplish what you need to do without all the looping required by this approach.
Advantages:
works on globals or object properties
requires no repetitive key/name passing
uses no proprietary or deprecated features
does not use arguments or closure
surrounding code executes faster (optimized) than
a try/catch version
is not confused by repeated uses
can handle new and deleted (renamed) properties
Caveats:
doesn't work on private vars, which have no property name
partially loops owner object each access
slower computation than a memorized property or code-time repetition
won't survive call/bind/apply
wont survive a setTimeout without bind() or a wrapper function
cannot easily be cloned
honestly, i think all the ways of accomplishing this task are "less than ideal", to be polite, and i would recommend you just bite the coding bullet and pass extra key names, or automate that by using a method to add properties to a blank object instead of coding it all in an object literal.

Yes.
Sort Of.
It depends on the browser. (Chrome=OK, Firefox=Nope)
You can use a factory to create the function, and a call stack parsing hack that will probably get me arrested.
This solution works in my version of Chrome on Windows 7, but the approach could be adapted to other browsers (if they support stack and show the property name in the call stack like Chrome does). I would not recommend doing this in production code as it is a pretty brittle hack; instead improve the architecture of your program so that you do not need to rely on knowing the name of the calling property. You didn't post details about your problem domain so this is just a fun little thought experiment; to wit:
JSFiddle demo: http://jsfiddle.net/tv9m36fr/
Runnable snippet: (scroll down and click Run code snippet)
function getCallerName(ex) {
// parse the call stack to find name of caller; assumes called from object property
// todo: replace with regex (left as exercise for the reader)
// this works in chrome on win7. other browsers may format differently(?) but not tested.
// easy enough to extend this concept to be browser-specific if rules are known.
// this is only for educational purposes; I would not do this in production code.
var stack = ex.stack.toString();
var idx = stack.indexOf('\n');
var lines = ex.stack.substring(idx + 1);
var objectSentinel = 'Object.';
idx = lines.indexOf(objectSentinel);
var line = lines.substring(idx + objectSentinel.length);
idx = line.indexOf(' ');
var callerName = line.substring(0, idx);
return callerName;
}
var Factory = {
getFunction: function () {
return function () {
var callName = "";
try {
throw up; // you don't *have* to throw to get stack trace, but it's more fun!
} catch (ex) {
callName = getCallerName(ex);
}
alert(callName);
};
}
}
var obj1 = {
'callName2': Factory.getFunction(),
'callName3': Factory.getFunction()
};
var obj2 = {
'callName4': Factory.getFunction(),
'callName5': Factory.getFunction()
};
obj1.callName2(); // should alert 'callName2'
obj1.callName3(); // should alert 'callName3'
obj2.callName4(); // should alert 'callName4'
obj2.callName5(); // should alert 'callName5'

JavaScript closure and the this object

I thought I had a reasonable understanding of the this object in JavaScript. When dealing with objects, callbacks, and both events and handlers, I haven't had a problem with it since time immemorial. Now, however, all has changed.
I've fallen head over heels in love with JavaScript. Pure JS, that is, not jQuery, prototype.js, dojo... So naturally, I've taken to using closures. In some cases, though, this is catching me off guard here. Take this snippet for one:
function anyFunc(par)
{
//console.log(par);
console.log(this);
}
function makeClosure(func)
{
return function(par)
{
return func(par);
}
}
var close = makeClosure(anyFunc);
close('Foo');
var objWithClosure = {cls:makeClosure(anyFunc),prop:'foobar'};
objWithClosure.cls(objWithClosure.prop);
var scndObj = {prop:'Foobar2'};
scndObj.cls = makeClosure;
scndObj.cls = scndObj.cls(anyFunc);
scndObj.cls(scndObj.prop);
In all three cases, this logs as the window object. It's an easy fix, of course:
function makeClosure(func)
{
return function(par)
{
return func.call(this,par);
}
}
This fix works, I put it here to avoid people answering this, without explaining what I need to know: why is this behaving the way it does here?
ensures the caller is effectively the object that the closure belongs to. What I fail to understand is this:
Sure enough, this points to the window object in the first case, but in other cases, it shouldn't. I tried logging this in the makeClosure function just before returning, and it did log the object itself, not the window object. But when the actual closure is used, this is back to pointing to the window object. Why?
The only thing I can think of is that, by passing the anyFunc function as an argument, I'm actually passing window.anyFunc. So I tried this quick fix:
function makeClosure(func)
{
var theFunc = func;
return function(par)
{
theFunc(par);
}
}
With the expected results, this now points to the objects, but again: Why? I have a few idea's (theFunc is a reference to the function in the local scope [this > private: theFunc]?), but I'm sure there are people here with a lot more know-how when it comes to JS, so I was hoping to get some more explanation or links to articles worth reading from them...
Thanks
Update
Here's a fiddle, may be I left something out, but here this logs all sorts of things ;)
Edit/Update 2
The case that confuses me is here.
Final Edit
Ok, This is getting a rather messy post. So to clarify: What I was expecting was a behaviour similar to this:
function makeClosure()
{
function fromThisFunc()
{
console.log(this);
}
return fromThisFunc;
}
var windowContext = makeClosure();
windowContext();
var objectContext = {cls:makeClosure()};
objectContext.cls();
What caught me, was that the function anyFunc wasn't declared within the correct scope, and therefore, this pointed to the window object. I found this out by reading an ancient scroll I found somewhere on the web.
But something a little more complicated has happened because the function object now referred to by globalVar was created with a [[scope]] property referring to a scope chain containing the Activation/Variable object belonging to the execution context in which it was created (and the global object). Now the Activation/Variable object cannot be garbage collected either as the execution of the function object referred to by globalVar will need to add the whole scope chain from its [[scope]] property to the scope of the execution context created for each call to it.
So what I needed to do, was simplify rather then complicate things:
function fromThisFunc()
{
console.log(this);
}
function makeClosure(funcRef)
{
//some code here
return funcRef;
}
That should work, right?
PS: I'll except Alnitak's answer, but special thanks goes to Felix Kling for all the patience and info.

As soon as you call:
return func(par);
You're creating a new scope (with its own this) and in this case because you haven't specified an object, this === window usually or undefined in strict mode. The called function does not inherit whatever this was in the calling scope.
Ways to set a value for this are:
myobj.func(par); // this === myobj
or
func.call(myobj, ...) // this === myobj
There are also:
apply
bind
arrow functions, where this is set to the same value as the outer execution context (also see MDN:Arrow functions )

The value of this depends only on whether you call the function as a method or as a function.
If you call it as a method, this will be the object that the method belongs to:
obj.myFunction();
If you call it as a function, this will be the window object:
myFunction();
Note that even if you are in a method that belongs to an object, you still have to call other methods in the object using the method syntax, otherwise they will be called as functions:
this.myOtherFunction();
If you put a method reference in a variable, you will detach it from the object, and it will be called as a function:
var f = obj.myFunction;
f();
The call and apply methods are used to call a function as a method even if it's not a method in the object (or if it's a method in a different object):
myFunction.call(obj);

How do you explain this structure in JavaScript?

(function()
{
//codehere
}
)();
What is special about this kind of syntax?
What does ()(); imply?

The creates an anonymous function, closure and all, and the final () tells it to execute itself.
It is basically the same as:
function name (){...}
name();
So basically there is nothing special about this code, it just a 'shortcut' to creating a method and invoking it without having to name it.
This also implies that the function is a one off, or an internal function on an object, and is most useful when you need to the features of a closure.

It's an anonymous function being called.
The purpose of that is to create a new scope from which local variables don't bleed out. For example:
var test = 1;
(function() {
var test = 2;
})();
test == 1 // true
One important note about this syntax is that you should get into the habit of terminating statements with a semi-colon, if you don't already. This is because Javascript allows line feeds between a function name and its parentheses when you call it.
The snippet below will cause an error:
var aVariable = 1
var myVariable = aVariable
(function() {/*...*/})()
Here's what it's actually doing:
var aVariable = 1;
var myVariable = aVariable(function() {/*...*/})
myVariable();
Another way of creating a new block scope is to use the following syntax:
new function() {/*...*/}
The difference is that the former technique does not affect where the keyword "this" points to, whereas the second does.
Javascript 1.8 also has a let statement that accomplishes the same thing, but needless to say, it's not supported by most browsers.

That is a self executing anonymous function. The () at the end is actually calling the function.
A good book (I have read) that explains some usages of these types of syntax in Javascript is Object Oriented Javascript.

This usage is basically equivalent of a inner block in C. It prevents the variables defined inside the block to be visible outside. So it is a handy way of constructing a one off classes with private objects. Just don't forget return this; if you use it to build an object.
var Myobject=(function(){
var privatevalue=0;
function privatefunction()
{
}
this.publicvalue=1;
this.publicfunction=function()
{
privatevalue=1; //no worries about the execution context
}
return this;})(); //I tend to forget returning the instance
//if I don't write like this

See also Douglas Crockford's excellent "JavaScript: The Good Parts," available from O'Reilly, here:
http://oreilly.com/catalog/9780596517748/
... and on video at the YUIblog, here:
http://yuiblog.com/blog/2007/06/08/video-crockford-goodstuff/

The stuff in the first set of brackets evaluates to a function. The second set of brackets then execute this function. So if you have something that want to run automagically onload, this how you'd cause it to load and execute.

John Resig explains self-executing anonymous functions here.