this confusion

[Stevo]

Simplified example code which can be copy/pasted and will exhibit the
problem.

var ob={id:2};
ob.alertid=func tion(){alert("i d="+this.id); }
ob.funcs=[ob.alertid];
ob.testalert=fu nction(){
this.alertid(); //this works
this.funcs[0](); //this doesn't
}

ob.testalert();

I have an object with an id property. The alertid function alerts that
id property if it's called directly, but not if it's called via an array
of function pointers. I don't understand the difference. Why is 'this'
different in those two cases. It's the same object method isn't it.

The output is:

id=2
id=undefined

[RobG]

On Aug 9, 7:23 am, Stevo <[e-mail]:

Simplified example code which can be copy/pasted and will exhibit the
problem.

var ob={id:2};
ob.alertid=func tion(){alert("i d="+this.id); }
ob.funcs=[ob.alertid];
ob.testalert=fu nction(){
this.alertid(); //this works
this.funcs[0](); //this doesn't

}

ob.testalert();

I have an object with an id property. The alertid function alerts that
id property if it's called directly, but not if it's called via an array
of function pointers. I don't understand the difference. Why is 'this'
different in those two cases. It's the same object method isn't it.

I think the best post I've seen about the this keyword is the one here
by Mike Winter:

<URL:
http://groups.google.com.au/group/co...26cd885e01c292

>

The first part is about closures, the second part on this is
excellent.
--
Rob

[David Mark]

On Aug 8, 5:23 pm, Stevo <[e-mail]:

Simplified example code which can be copy/pasted and will exhibit the
problem.

var ob={id:2};
ob.alertid=func tion(){alert("i d="+this.id); }
ob.funcs=[ob.alertid];
ob.testalert=fu nction(){
this.alertid(); //this works

Right. Because you called a method of the ob object. In this case,
"this" is ob, which has an id property.

this.funcs[0](); //this doesn't

funcs[0] holds a reference to an anonymous function. The fact that ob
has a property (alertid method) that references it as well is
meaningless. In this case, "this" refers to the anonymous function,
which has no id property.

[Stevo]

David Mark wrote:

In this case, "this" refers to the anonymous function,
which has no id property.

Thanks David, Randy, Thomas and Rob. You all said the same thing
basically, this differs depending on how it's called. Excellent link
Rob, that's one to bookmark.

I've altered the alertid function to add 'that' and it now works.

var ob={id:2};
ob.alertid=func tion(){var that=ob;alert(" id="+that.id); }
ob.funcs=[ob.alertid];
ob.testalert=fu nction(){
this.alertid();
this.funcs[0]();
}
ob.testalert();

That has the name ob hard-coded in the alertid function though. I wonder
if it's possible to create 'that' without knowing the name of the
object? Take the following example with ??? where I don't know what to set:

var ob_a={id:"a"};
var ob_b={id:"b"};
var alertid=functio n(){var that=???;alert( "id="+that.id); }
ob_a.alertid=al ertid;
ob_b.alertid=al ertid;

I'm guessing that there's no way to define 'that' which would work for
both objects there, and I'd have to define an anonymous function for
each object, with hard-coded references to ob_a and ob_b in them like so:

var ob_a={id:"a"};
var ob_b={id:"b"};
ob_a.alertid=fu nction(){var that=ob_a;alert ("id="+that.id) ;};
ob_b.alertid=fu nction(){var that=ob_b;alert ("id="+that.id) ;}

[Thomas 'PointedEars' Lahn]

Stevo wrote:

[...] I wonder if it's possible to create 'that' without knowing the name of the
object? Take the following example with ??? where I don't know what to set:

var ob_a={id:"a"};
var ob_b={id:"b"};
var alertid=functio n(){var that=???;alert( "id="+that.id); }
ob_a.alertid=al ertid;
ob_b.alertid=al ertid;

I'm guessing that there's no way to define 'that' which would work for
both objects there, and I'd have to define an anonymous function for
each object, with hard-coded references to ob_a and ob_b in them like so:

var ob_a={id:"a"};
var ob_b={id:"b"};
ob_a.alertid=fu nction(){var that=ob_a;alert ("id="+that.id) ;};
ob_b.alertid=fu nction(){var that=ob_b;alert ("id="+that.id) ;}

var alertid = function(that){ alert("id=" + that.id); };
// ...
ob_a.alertid = function() { alertid(this); };
ob_b.alertid = function() { alertid(this); };

or

var alertid = function(){ alert("id=" + this.id); }
// ...

and then either

ob_a.alertid = function() { alertid.call(th is); };
ob_b.alertid = function() { alertid.call(th is); };

or

ob_a.alertid = function() { alertid.apply(t his); };
ob_b.alertid = function() { alertid.apply(t his); };
HTH

PointedEars
--
Prototype.js was written by people who don't know javascript for people
who don't know javascript. People who don't know javascript are not
the best source of advice on designing systems that use javascript.
-- Richard Cornford, cljs, <f8************ *******@news.de mon.co.uk>

[Thomas 'PointedEars' Lahn]

Stevo wrote:

[...] I wonder if it's possible to create 'that' without knowing the name of the
object? Take the following example with ??? where I don't know what to set:

var ob_a={id:"a"};
var ob_b={id:"b"};
var alertid=functio n(){var that=???;alert( "id="+that.id); }
ob_a.alertid=al ertid;
ob_b.alertid=al ertid;

I'm guessing that there's no way to define 'that' which would work for
both objects there, and I'd have to define an anonymous function for
each object, with hard-coded references to ob_a and ob_b in them like so:

var ob_a={id:"a"};
var ob_b={id:"b"};
ob_a.alertid=fu nction(){var that=ob_a;alert ("id="+that.id) ;};
ob_b.alertid=fu nction(){var that=ob_b;alert ("id="+that.id) ;}

var alertid = function(that){ alert("id=" + that.id); };
// ...
ob_a.alertid = function() { alertid(this); };
ob_b.alertid = function() { alertid(this); };

or

var alertid = function(){ alert("id=" + this.id); }
// ...

and then either

ob_a.alertid = function() { alertid.call(th is); };
ob_b.alertid = function() { alertid.call(th is); };

or

ob_a.alertid = function() { alertid.apply(t his); };
ob_b.alertid = function() { alertid.apply(t his); };
HTH

PointedEars
--
Prototype.js was written by people who don't know javascript for people
who don't know javascript. People who don't know javascript are not
the best source of advice on designing systems that use javascript.
-- Richard Cornford, cljs, <f8************ *******@news.de mon.co.uk>

[Thomas 'PointedEars' Lahn]

Stevo wrote:

[...] I wonder if it's possible to create 'that' without knowing the name of the
object? Take the following example with ??? where I don't know what to set:

var ob_a={id:"a"};
var ob_b={id:"b"};
var alertid=functio n(){var that=???;alert( "id="+that.id); }
ob_a.alertid=al ertid;
ob_b.alertid=al ertid;

I'm guessing that there's no way to define 'that' which would work for
both objects there, and I'd have to define an anonymous function for
each object, with hard-coded references to ob_a and ob_b in them like so:

var ob_a={id:"a"};
var ob_b={id:"b"};
ob_a.alertid=fu nction(){var that=ob_a;alert ("id="+that.id) ;};
ob_b.alertid=fu nction(){var that=ob_b;alert ("id="+that.id) ;}

var alertid = function(that){ alert("id=" + that.id); };
// ...
ob_a.alertid = function() { alertid(this); };
ob_b.alertid = function() { alertid(this); };

or

var alertid = function(){ alert("id=" + this.id); }
// ...

and then either

ob_a.alertid = function() { alertid.call(th is); };
ob_b.alertid = function() { alertid.call(th is); };

or

ob_a.alertid = function() { alertid.apply(t his); };
ob_b.alertid = function() { alertid.apply(t his); };
HTH

PointedEars
--
Prototype.js was written by people who don't know javascript for people
who don't know javascript. People who don't know javascript are not
the best source of advice on designing systems that use javascript.
-- Richard Cornford, cljs, <f8************ *******@news.de mon.co.uk>

[Thomas 'PointedEars' Lahn]

Stevo wrote:

[...] I wonder if it's possible to create 'that' without knowing the name of the
object? Take the following example with ??? where I don't know what to set:

var ob_a={id:"a"};
var ob_b={id:"b"};
var alertid=functio n(){var that=???;alert( "id="+that.id); }
ob_a.alertid=al ertid;
ob_b.alertid=al ertid;

I'm guessing that there's no way to define 'that' which would work for
both objects there, and I'd have to define an anonymous function for
each object, with hard-coded references to ob_a and ob_b in them like so:

var ob_a={id:"a"};
var ob_b={id:"b"};
ob_a.alertid=fu nction(){var that=ob_a;alert ("id="+that.id) ;};
ob_b.alertid=fu nction(){var that=ob_b;alert ("id="+that.id) ;}

var alertid = function(that){ alert("id=" + that.id); };
// ...
ob_a.alertid = function() { alertid(this); };
ob_b.alertid = function() { alertid(this); };

or

var alertid = function(){ alert("id=" + this.id); }
// ...

and then either

ob_a.alertid = function() { alertid.call(th is); };
ob_b.alertid = function() { alertid.call(th is); };

or

ob_a.alertid = function() { alertid.apply(t his); };
ob_b.alertid = function() { alertid.apply(t his); };
HTH

PointedEars
--
Prototype.js was written by people who don't know javascript for people
who don't know javascript. People who don't know javascript are not
the best source of advice on designing systems that use javascript.
-- Richard Cornford, cljs, <f8************ *******@news.de mon.co.uk>

[Thomas 'PointedEars' Lahn]

Stevo wrote:

[...] I wonder if it's possible to create 'that' without knowing the name of the
object? Take the following example with ??? where I don't know what to set:

var ob_a={id:"a"};
var ob_b={id:"b"};
var alertid=functio n(){var that=???;alert( "id="+that.id); }
ob_a.alertid=al ertid;
ob_b.alertid=al ertid;

I'm guessing that there's no way to define 'that' which would work for
both objects there, and I'd have to define an anonymous function for
each object, with hard-coded references to ob_a and ob_b in them like so:

var ob_a={id:"a"};
var ob_b={id:"b"};
ob_a.alertid=fu nction(){var that=ob_a;alert ("id="+that.id) ;};
ob_b.alertid=fu nction(){var that=ob_b;alert ("id="+that.id) ;}

var alertid = function(that){ alert("id=" + that.id); };
// ...
ob_a.alertid = function() { alertid(this); };
ob_b.alertid = function() { alertid(this); };

or

var alertid = function(){ alert("id=" + this.id); }
// ...

and then either

ob_a.alertid = function() { alertid.call(th is); };
ob_b.alertid = function() { alertid.call(th is); };

or

ob_a.alertid = function() { alertid.apply(t his); };
ob_b.alertid = function() { alertid.apply(t his); };
HTH

PointedEars
--
Prototype.js was written by people who don't know javascript for people
who don't know javascript. People who don't know javascript are not
the best source of advice on designing systems that use javascript.
-- Richard Cornford, cljs, <f8************ *******@news.de mon.co.uk>

[Henry]

On Aug 16, 3:50 pm, ron.h.h...@gmai l.com wrote:
<snip>

In other words, a function invoked as f(...) will have a
"this" value of the global object,

function f(){
var x = (this+'');
}

with(
{
f:f,
toString:functi on(){alert('I am not the global object');}
}
){
f(); // alerts: I am not the global object
// So the - this - value when - f() - is
// executed is _not_ the global object.
}

You appear to have failed to take into account the role of variable
objects and the scope chain in determining the - this - value.

<snip>

So the second problem with Mike's post is that it over-complicates
the description of what one should expect of the "this" value to
be by involving the variable object, scope chain and the "new"
operator[1] in the description.

<snip>