处理setTimeout（javascript）中的对象方法函数调用？

Question

Code:

​var a = function() {
   this.message = "hello";
    this.shout = function(){
        alert(this.message); // alerted undefined
    }
    this.Timer = setTimeout(this.shout, 3000);
}

var b = new a();

I get undefined in the alert dialog. I've tried "this.shout()" in setTimeout but then there is a DOM error on finding shout. How do I deal with this?

Answer 1

this.message 

needs to be inside the this.shout function, since at the moment it's out of scope.

Then it will work :)

var a = function() {

    this.shout = function(){
        this.message = "hello";
        alert(this.message); // alerted undefined
    }
    this.Timer = setTimeout(this.shout, 3000);
}

var b = new a();

Answer 2

this in your function shout refers to the function shout , rather than the function a

If you define your variable in the scope of a instead of using this, you can refer to it later on and get it's value:

var a = function() {
    var message = "hello";
    this.shout = function(){
        alert(message); // Alerts hello
    }
    this.Timer = setTimeout(this.shout, 3000);
}

var b = new a();

Or if you'd like you can store a reference instead, so that you can use self to refer to a :

var a = function() {
    this.message = "hello";
    var self = this;

    this.shout = function(){
        alert(self.message); // Alerts hello
    }
    this.Timer = setTimeout(this.shout, 3000);
}

var b = new a();

Answer 3

"this"inside settimeout is the settimeout instance you must get previously in a var the outside "this"

var a = function()
{
   this.message = "hello";
   this.shout = function()
   {
        alert(this.message); // alerted undefined
   }
   var t = this;
   this.Timer = window.setTimeout(function()
  {
    t.shout();
  }, 3000);
}

var b = new a();