Examine this code
var _class=function()
{
this.Test=100;
this.Callback=function(msg)
{
alert(msg+"\r\n"+this.Test);
}
}
function run(call)
{
call("Hello world");
}
var obj=new _class();
run(obj.Callback);
I got the result :
[Alert]
Hello world
undefined
but when i call obj.Callback("Hello world")
i got expected
[Alert]
Hello world
100
why ?
thank for help
There's no intrinsic relationship between an object and the functions defined "inside" it. The only thing that determines the value of this (called the "receiving" object) in a function call is the way in which the function is called.
object.func(), and this will be bound to object.this is determined by the first parameter.this won't refer to window (or whatever the global context is).The trick is that when you want to use a function as if it were a "method" on an object, such that the relationship remains intact even though the function reference has been yanked away from the object, you can pre-bind this in a couple of ways:
this in a closure.The first way would look like this:
run(function() { obj.Callback(); });
The second way would look like this:
run(obj.Callback.bind(obj));
JavaScript is quite different from languages like C# or Java in this respect. In those languages, a function is sort-of stuck forever in a relationship with its class (or instances of its class). Not JavaScript; it really doesn't matter at all, in fact, where a function is defined. Your "_class" function would be equivalent if it were written like this:
function helloThere() {
alert(msg + "\r\n" + this.Test);
}
var _class = function() {
this.Test = 100;
this.Callback = helloThere;
};
edit — @jamietre correctly notes that had your "_class" function contained some var declarations or local functions, then there most certainly would be a difference (though not with respect to the way this behaves when "Callback" is invoked).
edit again — Thanks @Koolinc
helloThere in your example would not be able to access them, because it was defined in a different scope. Example: jsfiddle.net/Bd886
var variables in the function. Had that been the case, then you're right. I'll edit the answer.
this would refer to the global object I'd say.window) object.this.Test is not defined. The scope of this in that context is the callback function.
This is a quick fix:
var _class=function()
{
var self = this;
this.Test=100;
this.Callback=function(msg)
{
console.log(msg+"\r\n"+self.Test);
}
}
