I am new to JavaScript. I have the following script working,
var navRef = this.navigator;
function onSearch( templateName) {
navRef.onSearch();
}
but not the one below and I am trying to understand why? Any help is appreciated. (navigator is sent as an argument to this object).
function onSearch( templateName) {
this.navigator.onSearch();
}
'this' points to different objects in the two cases. In the first case 'this' most likely refers to the windows object that has navigator as a property. In the second case 'this', most likely, refers to whatever object invoked the function. It's hard to be exact since you didn't give the context. But this should be enough to understand what's going on.
You need to read up on javascript scopes. In the first example the 'this' is referencing the current scope (that has the navigator property). In your second example, the 'this' is actually referencing the current function scope it is in.
You might want to check out this article on the this keyword. Essentially, this is used within a function to refer to the context in which that function is executed. When the function in question is called as a method on an object, this refers to that object. When the function is called in the global scope and has no instance to refer to, this refers to the window object.
Javascript has lexical scope so variables (like your navRef) can be used inside inner functions
var navRef = this.navigator;
function onSearch(){
navRef.onShow();
}
the this "variable", however, is an evil exception. Each function gets its own this (and the value it has depends on how the function was called) so if you want to access an outer this (or one of its properties) you need to use an intermediate variable:
var that = this;
function onSearch(){
that.navigator.onSearch();
}
Related
so my question is when using an object literal inside a method I see people doing 'this.something = ...'
And then referencing it throughout their object. What's the name for this?
For e.g. Here in the example below if you look in the 'CacheDom' method 'this.button = document.getElementById('submit')' gives us a reference we can use later.
I understand the basics of the 'this' keyword and inside an object it will reference to that object, but I found it strange to be able to store elements etc and reference them later.
Essentially what is the official term for this?
Thanks
https://jsfiddle.net/rvs6ymqj/
HTML
<body>
<button id="submit" type="submit">Submit</button>
</body>
JS
var obj = {
init: function() {
this.cacheDom();
this.bindEvents();
},
cacheDom: function() {
this.button = document.getElementById('submit');
},
bindEvents: function() {
this.button.addEventListener("click", function() {
console.log("we clicked the button");
})
}
}
obj.init();
It varies from language to language. In Javascript, it's called a property. In Java, they're fields. C++ also calls them fields, though they may also be called data members.
JS is notably different from the other two in that Java and C++ force you to declare your fields, while JS members are defined on the fly. Really, JS objects are just maps from keys to values: the properties are the keys.
In Object-Oriented Programming, "this" refers to the object you are inside. You aren't storing properties in "this". It is a way to differentiate between global variables/constructor parameters and instance variables of the same name. I don't think there's a specific name for the concept of "this" but this phrase sums it up nicely:
'this' is usually an immutable reference or pointer which refers to the current object
https://en.wikipedia.org/wiki/This_(computer_programming)
Javascript-Specific Explanation:
JavaScript
When used outside any function, in global space, this refers to the
enclosing object, which in this case is the enclosing browser window,
the window object. When used in a function defined in the global
space, what the keyword this refers to depends on how the function is
called. When such a function is called directly (e.g. f(x)), this will
refer back to the global space in which the function is defined, and
in which other global functions and variables may exist as well (or in
strict mode, it is undefined). If a global function containing this is
called as part of the event handler of an element in the document
object, however, this will refer to the calling HTML element. When a
method is called using the new keyword (e.g. var c = new Thing()) then
within Thing this refers to the Thing object itself. When a function
is attached as a property of an object and called as a method of that
object (e.g. obj.f(x)), this will refer to the object that the
function is contained within. It is even possible to manually
specify this when calling a function, by using the .call() or .apply()
methods of the function object. For example, the method call
obj.f(x) could also be written as obj.f.call(obj, x). To work around
the different meaning of this in nested functions such as DOM event
handlers, it is a common idiom in JavaScript to save the this
reference of the calling object in a variable (commonly called that or
self), and then use the variable to refer to the calling object in
nested functions.
From Javascript-Garden:
Foo.method = function() {
function test() {
//this is set to the global object
}
test();
}
In order to gain access to Foo from within test, it is necessary to create a local variable inside of method that refers to Foo:
Foo.method = function() {
var that = this;
function test(){
//Use that instead of this here
}
test();
}
Could anyone explain this? As far as I understood, this refers to the global object if it's called in the global scope. But here it's called inside of a function, which is inside a method (first example). Why exactly does it refer to the global object, while the second example doesn't?
As far as I understood, this refers to the global object if it's called in the global scope.
No. this will refer to the default object if the function is called without explicit context. The scope is irrelevant. (In strict mode it will refer to undefined instead).
Why exactly does it refer to the global object
We can't tell what it refers to. The value of this is determined by how the function is called, not how it is defined.
Now you have updated the example, we can see that it is called without context, so this (in the inner function) will be the default object, which in a web browser is window (it would be undefined in strict mode).
while the second example doesn't?
In the second example, the inner function doesn't use this (which will have the same value as the previous example).
The second example uses that instead. that is defined in the scope of the outer function and is set to whatever the value of this is when that function is called.
Assuming that function is called as Foo.method() then (outer) this (and hence that) will be Foo because that is the context on which method was called.
this in a function isn't set when you define the function. It's only dynamically defined to the receiver of the function call.
If you call foo.test(), this in test will be foo.
But if you do
var f = foo.test;
f();
then this in f (which is foo.test) will be the external object (window if you're executing it at the root level).
It would be the same with
foo.test.call(window);
The second example uses a closure to place the outer functions variables on the scope chain of the inner function.
Foo.method = function() {
var that = this;
function test(){
//This function has access to the outer variables scope chain.
}
}
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.
What is the calling context of an invoked method or function within another function?
In browsers, the default calling context is the window object. In various situations, how do I avoid this?
If a function is invoked -- for example, by theFunction(); -- within a containing function, is the invoked function's calling context the containing function?
In these two examples
(function ()
{
something.initialize();
}());
and
(function ()
{
something.initialize.call(this);
}());
..., is the calling context the same?
They are not the same. In the following I assume you were talking about this when you mentioned context.
In the first example, inside initialize, this will refer to something. In the second one, it will refer to the global object, which is window in browsers.
What this refers to is determined by how the function was called. There are five cases:
func(), calling a function "standalone": this refers to the global object.
new func(), calling a function as constructor method: this will refer to an empty object which inherits from func.prototype.
obj.func(), calling a function as property of an object: this will refer to the object obj.
func.apply(foo), func.call(foo), invoking a function with apply or call: this refers to the object passed as first argument.
ECMAScript 5 also introduced .bind() [MDN] which enables you to bind this to a certain object, without immediately calling the function.
Now you understand why in your second example, inside initialize, this will refer to window:
The outer function is called "standalone" (first case), so this inside of it will refer to window. Next you are passing this to call, which sets this inside initialize to window (fourth case).
Further reading:
MDN - this, explains all I write above with some examples.
No, they are not the same. The first parameter to the call method sets the value of this inside that function; example 1's this should theoretically contain a reference to something; example 2's this corresponds to the this of your self executing function.
The environment available to a function when it is invoked is based on several things.
the this binding which is different for the initialize method in your example
the actual parameter bindings which are not and a binding for a special arguments object
a binding for the function name, if any
the environment chain at the time the function was created. This is the "close" part of the term "closure". A javascript function "closes over" all the variables that are in-scope when the function object comes into existence.
In your example, only 1 is affected by the choice to use call which has the effect of passing this instead of something as the this value in the method body.
The relevant part of the spec starts at section 10.4.3
The following steps are performed when control enters the execution context for function code contained in function object F, a caller provided thisArg, and a caller provided argumentsList: ...
Functions don't create context, unless it's a constructor (a function called with new).
In your second example the context is undefined (which gets transformed into the global object , window). In ES5 strict mode it won't be transformed into anything.
I recommend John Resig's interactive JavaScript tutorial on the topic of context.
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.