I know that top-level functions are members of the window object, and it's consistent with the fact that this inside them refers to the window.
But for non-top level functions, this is still window, but they are not members of window!
Here's some code:
function topLevel1() {
alert(this)
}
function topLevel2() {
function inner() {
alert(this)
}
inner()
}
topLevel1() // alerts DOMWindow
alert(window.topLevel1) // alerts text of topLevel1, as expected
topLevel2() // again, alerts DOMWindow
alert(window.inner) // undefined
alert(window.topLevel2.inner) // undefined
If inner is neither a member of window nor of topLevel2, whose member is it?
How can it be that it's this is window, but it's not a window's member?
Isn't it a law in Javascript that if this == owner, then the current method was called via owner.? (except special cases like constructors, apply, call, etc)
Again, the this value of a function is decided of how the function is invoked, not where it is located or how it was designed.
If you just call a function
foobar()
anywhere in your code, just like that, its this value will always be window (non-strict mode) or undefined (strict). Now there are plenty of ways to modify the this, like calling the function with .apply(), .call() or .bind(). All of those methods give your the opportunity to modify the this value for a given function. Also, if you call a function with the new keyword, this referes to a newly created object (which is also returned).
So, location of a function tells you zero about its context or this.
Now to answer your specific questions:
inner is a member of the Activation Object from topLevel2 (ES3)
described above
I guess thats also covered above
A fantastic reference of the this keyword is: http://www.quirksmode.org/js/this.html
The inner() function inside topLevel2() can only be visible to topLevel2. It's a scope issue. That's why you get undefined.
Related
Just trying to clear some misconceptions...
function outer(){
function inner(){
}
}
Does a function being a member of a particular object decides it's contextual this ?
Like, is it the reason that outer function is member of window object therefore the contextual this of outer function refers to window object ?
If yes then whos member is the inner function ?
Is inner member of outer function's object ?
And why the inner function's contextual this is referring to the window object ?
Is any function(global or nested) invoked without new operator will have contextual this referring to window object ?
For an ordinary function (that is, not an arrow or member function), this is populated dynamically, at the call time. this depends on how the function is being called, not on where it's been declared.
this can be populated by 1) a property accessor (object.func) or 2) new as in new func() or 3) an indirection like call/apply. If the engine cannot populate this for a particular function call, it's set to the global/window in the sloppy mode and undefined in the strict mode. This applies to both global and inner functions.
Global functions being members of the global/window is an orthogonal concept and doesn't affect this in any way.
Inner functions, like local variables, are not members of any object.
If you open your console and type Function, it auto completes it for you, indicating that the indentifier Function is part of the winddow object. It appears as though the Function identifier refers to an empty anonymous function.
What is the point of the Function identifier on window?
Function is a global variable. Global variables are properties of the global object. In browsers, the global object is window.
If you are asking what the purpose of Function itself is: It's a constructor function (just like Object, Array or RegExp) to create new function objects. It lets you create a new function from code contained in a string (almost like eval). E.g.:
var myFunc = new Function('return 42;');
console.log(myFunc()); // 42
Functions created this way behave like they had been declared in global scope, i.e. they do not close over the scope they actually have been created in.
This can be useful for browser tools which evaluate user provided JS code provided, like the Babel REPL.
Consider:
function Thing() {
this.prop = null
}
Thing.prototype.whoIsThis = function() {
console.log(this)
}
a = new Thing()
a.whoIsThis() // logs '> Thing {...}'
f = a.whoIsThis
f() // logs '> Window {...}'
So this is not bound to the Thing in the second call. How does this work in this situation? Isn't a.whoIsThis a "method" of a Thing regardless of any variable its assigned to?
When you say a.whoIsThis, it will refer the function object only. The function object will have no reference to the object on which it is attached. But when you invoke the function, JavaScript dynamically decides the current object and sets that as this inside the function.
This dynamicity allows us to use any object as the current object in the runtime.
But when you simply invoke a function object, without any object reference, by default, JavaScript will set this as the global object (window object in browser) and in Strict mode, this will be set to undefined.
A function's this keyword behaves a little differently in JavaScript compared to other languages. In most cases, the value of 'this' is determined by how a function is called, when 'this' is inside a function.
1) When a function is called as a method of an object, its this is set to the object the method is called on.
2) When a function is called directly, the value of this is not set by the call. Since the code is not in strict mode, the value of this must always be an object so it defaults to the global object. In strict mode, the value of this remains at whatever it's set to when entering the execution context, so 'undefined'.
More info: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this
This binding is decided at run time and not at author time
having f = a.whoIsThis is just another reference to the function WhoIsThis
now Imagine If you have the function declared in the global scope..which is really what f is now..and when calling f JS at run time run the function as if it was declared in the global scope(as I mentioned) so It asks ..what this refers to if I'm in the global scope => default binding rule would answer..it's simply the global(window) object
When you invoke a top-level function in Javascript, the this keyword inside the function refers to the default object (window if in a browser). My understanding is that it's a special case of invoking the function as method, because by default it is invoked on window (as explained in John Resig's book, Secrets of the JavaScript Ninja, page 49). And indeed both invocations in the following code are identical.
function func() {
return this;
}
// invoke as a top-level function
console.log(func() === window); // true
// invoke as a method of window
console.log(window.func() === window); // true
So far so good... Now here is the part I don't understand:
When a function is nested in another function and invoked without specifying an object to invoke on, the this keyword inside the function also refers to window. But the inner function cannot be invoked on window (see code below).
function outerFunc() {
function innerFunc() {
return this;
}
// invoke without window.* - OK
console.log(innerFunc() === window); // true
// invoke on window
//window.innerFunc(); - error (window has no such method)
console.log(window.innerFunc) // undefined
}
outerFunc();
It makes perfect sense that the nested function isn't available on window, as it is after all nested... But then I don't understand why the this keyword refers to window, as if the function was invoked on window. What am I missing here?
EDIT
Here is a summary of the great answers below and some of my follow up research.
It is incorrect to say that invoking a function "normally" is the same as invoking it as a method of window. This is only correct if the function is defined globally.
The function context (the value of the this keyword) does not depend on where / how the function is defined, but on how it is being invoked.
Assuming that the code is not running in in strict mode, Invoking a function "normally" will have the function context set to to window (when running in a browser, or the corresponding global object in other environments).
An exception to the above rules is the use of bind to create a function. In this case even if the function is invoked "normally", it could have a context other than window. That is, in this case the context is determined by how you create the function, rather than how you invoke it. Although strictly speaking this isn't accurate, because bind creates a new function that internally invokes the given function using apply. The context of that new function will still be determined by the way it's invoked, but it shields the context of the function it internally invokes by using apply.
By invoking "normally" I refer to the following simple way of invocation:
myFunction();
To complete the picture, here is a brief coverage of other ways of invocation and the corresponding context:
As a property of an object (method) - the context is the object
Using apply or call - the context is specified explicitly
With the new operator (as a constructor) - the context is a newly created object
Feel free to update the above as necessary, for the benefit of people with similar questions. Thanks!
You can call any function that is in scope with functionName(). Since you haven't called it on an object, it will be called in the context of the default object (window). (IIRC, it will be called in the context of undefined if you are in strict mode).
The default object for context has nothing to do with where a function is defined or what scope that function appears in. It is simply the default object.
If a function is a property of an object, you can call it as reference.to.object.function(), and it will be called in the context of object instead of the default object.
Other things that change the context are the new keyword and the apply, call, and bind methods.
In JavaScript, when a function is invoked without an explicit context, the context is the global object. In the case of web browsers, the global object is window.
Additionally, JavaScript has functional scope, so any variables or functions within a function are not accessible in a scope outside of that function. This is why you can't access window.innerFunc.
Whether a function is nested inside another one has nothing to do with the value of this when the function is called. The only things that matter are:
If the function is "found" by traversing a property on an object, then the value of this will be a reference to that object:
someObject.prop( whatever );
It doesn't matter how the function was declared.
If you use call() or apply() to invoke a function, then the value of this is taken from the first argument to whichever of those functions you use.
If you've created a bound wrapper for the function with bind(), then the value of this will be as requested when bind() was called.
If you're calling a function as a constructor with new, then this will refer to the newly-created object instance.
Otherwise, this is either a reference to the global context, or else it's undefined (in "strict" mode or in an ES5-compliant runtime).
The "location" in the code where a function is defined does matter, of course, in that the scope includes whatever symbols it includes, and those are available to the function regardless of how a reference to it is obtained.
It does not depend where the function is declared but how it is called:
var obj = {
f: function() {
return this;
}
}
var f = obj.f;
console.log(obj.f()) // obj
console.log(f()) // window/default obj
Or in other words. The syntax obj.f() executes the function with this=obj while f() executes the function with this=window. In JavaScript the caller specifies the value of this.
When you define func in the global scope, it actually is assigned as a property of the window object. That is, the window object holds all globally scoped variables. (*) Therefore, func and window.func represent the same thing. innerFunc is defined inside a function scope and is not available outside of that scope. Therefore, window.innerFunc is (still) undefined.
However, the this context is determined by how you call the function. When you call a method like obj.method(), the this context is set to obj. On the other hand, you can also call the method on its own:
var f = obj.func;
f(); // in this call: this === window
In this case, you're not calling a function on an object and thus the this context is set to the default. The default however is the global scope and as stated above, this is represented by window.
You can always override the this context by using Function.prototype.call() or Function.prototype.apply() which take a this context as first argument. For example:
var f = obj.func;
f.call(obj); // in this call: this == obj
(*) Note that this only applies to JavaScript running inside a browser. In other environments, this may differ. For example, in Node.js the GLOBAL variable holds the global scope.
This is taken from John Resig`s Learning Advanced Javascript #25, called changing the context of a function.
1) in the line fn() == this what does this refer to? is it referring to the this inside the function where it says return this?
2) although I understand the purpose of the last line (to attach the function to a specific object), I don't understand how the code does that. Is the word "call" a pre-defined JavaScript function? In plain language, please explain "fn.call(object)," and explicitly tell me whether the object in parens (object) is the same object as the var object.
3). After the function has been assigned to the object, would you call that function by writing object.fn(); ?
var object = {};
function fn(){
return this;
}
assert( fn() == this, "The context is the global object." );
assert( fn.call(object) == object, "The context is changed to a specific object."
call is a function defined for a Function object. The first parameter to call is the object that this refers to inside the function being called.
When fn() is called without any particular context, this refers to the global context, or the window object in browser environments. Same rules apply for the value of this in the global scope. So in fn() == this), this refers to the global object as well. However, when it is called in the context of some other object, as in fn.call(object), then this inside fn refers to object.
fn.call(object) does not modify or assign anything to object at all. The only thing affected is the this value inside fn only for the duration of that call. So even after this call, you would continue calling fn() as regular, and not as object.fn().
The example simply demonstrates that the this value inside a function is dynamic.