Reputation: 38942
I get confused about 'this' keyword in the following codes, there are two 'this':
var Foo = function(string){
this.name=string // 1st-this
}
Foo.prototype.get_name = function(){
return this.name // 2nd-this
}
var myFoo = new Foo('John')
the_name=myFoo.get_name()
'the_name' is equal to 'John', the prototype method get the name by return this.name. But can anyone explain to me the 1st-this and 2nd-this, what do they stand for?
Upvotes: 5
Views: 534
Reputation: 13632
In Javascript, the value of this
is dependent on the way you call the function.
There are 5 ways to call a function in JS, and they all have effect on this
:
new Foo();
<= here, you’re creating a new object, and this
will reflect that new objectFoo();
<= here, you're calling the function as-is, and this
will be the global object(!)var obj = { foo: Foo };
obj.foo();
<= here, you're calling the function as a method of obj
; this
will be obj
Foo.call(thisObject, arg1, arg2);
<= here, you can specify the value of this
in the first argumentFoo.apply(thisObject, [args]);
<= here, you can specify the value of this
in the first argumentIn 4 and 5, the difference between call and apply is that with call
, you need to pass all the arguments separately, whereas with apply
, you can pass an array containing all the arguments.
Note that in my example 2 above, the function should have been called foo
instead of Foo
. Since it’s impossible to know off-hand whether a function is supposed to be called with new
or not, the consensus is to start the function name with a capital letter if it’s a constructor (and should be used with new
); otherwise, it should start with lowercase.
Upvotes: 15
Reputation: 5219
wow all of these different answers, and only one of them even used the word "context"! Here is the straightforward answer to your question:
the this
keyword is an object reference that points to the context object.
In your example, this
refers to an instance of the Foo
object.
Upvotes: 1
Reputation: 340055
The answer is "it depends", since the meaning of this
depends on the context in which it is invoked.
For example in things like callbacks, this
no longer refers to the current object, but more typically to the DOM element on which the event occurred:
So this identical-looking function:
Foo.prototype.handleSomeEvent = function(e) {
return this.name;
}
if used as an event callback would try to resolve the name
attribute of the element, and not your object's name.
Upvotes: 2
Reputation: 866
The "this" keyword always refers to the owner of the function. So for instance if you click on a button that accesses that function, this refers to that button. So if you have button A that has an onclick that calls that function then this is A.
In this case this would be John since in both cases they reference myFoo which is John since Myfoo is used in calling them
Upvotes: 0
Reputation: 944568
When you use the new
keyword this
is the instance object that you are creating.
var foo = new Bar();
i.e. the instance of Bar
being assigned to foo
When you don't, this
is the object on which the method you are calling lives.
var baz = foo.thing();
var boz = thing();
i.e. foo
in the first example and window
in the second (window
is the default object).
You can also fritz with it using apply
var baz = foo.thing.apply(bar);
Here this
(still inside the thing
method) is bar
)
Upvotes: 2
Reputation: 14467
In your example they are the same, when you call this
from a method you override in the prototype chain you are referring to the same thing as when calling this
from within the constructor function.
It gets trickier when working with callbacks or when defining member variables in prototype methods.
Upvotes: 1
Reputation: 50215
this
refers to the object invoking the function, which in this case is myFoo
. When you construct myFoo
as a new Foo('John')
the this
keyword enables you to set myFoo.name = 'John'
, so when you call myFoo.get_name()
it will also let you return myFoo.name
which equals John
.
Upvotes: 1
Reputation: 176956
this link make you understand : The this keyword
function doSomething() {
this.style.color = '#cc0000';
}
In JavaScript this always refers to the “owner” of the function we're executing, or rather, to the object that a function is a method of. When we define our faithful function doSomething() in a page, its owner is the page, or rather, the window object (or global object) of JavaScript. An onclick property, though, is owned by the HTML element it belongs to.
Upvotes: 2