WITH (this) {a;b;c}

[SamJs]

WITH (this) {
a = 5
}

I'm finding that this.a = 5 will come from this expression. I would
have thought that the search for a would come up undefined and leave it
at that.

Is JavaScript explicit about the behavior of creating variables on a
WITH (object) ?

[VK]

SamJs wrote:

Is JavaScript explicit about the behavior of creating variables on a
WITH (object) ?

That is irrelevant to with() construct. If you assign a value to a
non-existing object property, the engine creates new property first and
then assign to it.

[Duncan Booth]

"SamJs" <Sa*******@gmai l.comwrote:

WITH (this) {
a = 5
}

I'm finding that this.a = 5 will come from this expression. I would
have thought that the search for a would come up undefined and leave it
at that.

How strange, I would have thought you would get a syntax error from this
expression. Javascript doesn't have a 'WITH' statement (although it does
have a 'with' statement).

>
Is JavaScript explicit about the behavior of creating variables on a
WITH (object) ?

Try this and you will find that assigning to 'a' doesn't create an
attribute on this:

f = function() {
with(this) {
a = 42;
}
}
var g = new f();
alert("g.a:"+g. a+", window.a:"+wind ow.a);
// displays: g.a:undefined, window.a:42
but the following code will update the attribute on 'this':

window.a = undefined;
f = function() {
this.a = 'hello';
with(this) {
a = 42;
}
}
var g = new f();
alert("g.a:"+g. a+", window.a:"+wind ow.a);
// displays: g.a:42, window.a:undefi ned

[Michael Winter]

SamJs wrote:

WITH (this) {
a = 5
}

I'm finding that this.a = 5 will come from this expression.

Syntax error aside, that observation depends on the value of the this
operator. The this operator may, and often does, refer to the global object.

As long as there is no variable, a, within the scope chain, the
assignment expression above will create a new property on the global
object. If the this operator does indeed refer to that object, then the
expression

this.a

will evaluate to the number 5.

Compare to:

var object = {
method : function() {
with (this) {
a = 5;
}
alert(this.a);
}
};

object.method() ;

Here, the this operator refers to the object assigned to the variable,
object. As before, the assignment expression within the with statement
creates a property on the global object, but because the this operator
value is different, the expression evaluates to undefined.

[snip]

Is JavaScript explicit about the behavior of creating variables on a
WITH (object) ?

Yes: it never happens. Only existing properties can be read or assigned.

Mike

[RobG]

Duncan Booth wrote:

"SamJs" <Sa*******@gmai l.comwrote:

[...]

Is JavaScript explicit about the behavior of creating variables on a
WITH (object) ?

Try this and you will find that assigning to 'a' doesn't create an
attribute on this:

Yes it does...

>
f = function() {
with(this) {
a = 42;
}
}
var g = new f();
alert("g.a:"+g. a+", window.a:"+wind ow.a);
// displays: g.a:undefined, window.a:42

Because the value of this was the window object, so an a property was
added to window and assigned a value of 42.

but the following code will update the attribute on 'this':

window.a = undefined;
f = function() {
this.a = 'hello';
with(this) {
a = 42;
}
}
var g = new f();
alert("g.a:"+g. a+", window.a:"+wind ow.a);
// displays: g.a:42, window.a:undefi ned

Because when you call f as a constructor, its this value is set to the
newly created object: this.<propertyN ameassigns properties to the new
object.
--
Rob

[Duncan Booth]

"RobG" <rg***@iinet.ne t.auwrote:

Duncan Booth wrote:

>"SamJs" <Sa*******@gmai l.comwrote:

[...]

Is JavaScript explicit about the behavior of creating variables on a
WITH (object) ?

Try this and you will find that assigning to 'a' doesn't create an
attribute on this:

Yes it does...

Assignment never creates a new attribute on the object referred to in the
'with' statement.

>>
f = function() {
with(this) {
a = 42;
}
}
var g = new f();
alert("g.a:"+g .a+", window.a:"+wind ow.a);
// displays: g.a:undefined, window.a:42

Because the value of this was the window object, so an a property was
added to window and assigned a value of 42.

Hello? Did you read the code?

f is called with the new operator. The value of 'this' inside the function
is the newly created object, not the window object. Neither has an 'a'
attribute to the assignment creates a new value on the global object (which
in this case happens to be the window object).

>

>but the following code will update the attribute on 'this':

window.a = undefined;
f = function() {
this.a = 'hello';
with(this) {
a = 42;
}
}
var g = new f();
alert("g.a:"+g .a+", window.a:"+wind ow.a);
// displays: g.a:42, window.a:undefi ned

Because when you call f as a constructor, its this value is set to the
newly created object: this.<propertyN ameassigns properties to the new
object.

Yes, 'this' is the same as in the first example. The difference this time
though is that this.a already exists so the assignment to plain 'a' updates
the existing attribute.

[VK]

Duncan Booth wrote:

Assignment never creates a new attribute on the object referred to in the
'with' statement.

Welcome to JavaScript :-) and you are wrong as it's already pointed out
:-(
If property doesn't exist yet, it will be conveniently created for you.

>
f = function() {
with(this) {
a = 42;
}
}
var g = new f();
alert("g.a:"+g. a+", window.a:"+wind ow.a);
// displays: g.a:undefined, window.a:42

Because the value of this was the window object, so an a property was
added to window and assigned a value of 42.

Hello? Did you read the code?

It seems that you (as well as OP) are semi-mesmerized by with()
construct so presuming some hidden magic out of it: while it's nothing
but a shortcut tool.
with (this) {a = 'foo';}
programmaticall y is absolutely the same as
this.a = 'foo';

Try this:

function f() {
this.a = foo;
}

f();
alert(window.a) ; // 'foo'
var foo = new f();
alert(foo.a) // 'foo'

1] If property doesn't exist yet, it will be created during the
assignment.
2] The call context is everything in JavaScript

[RobG]

Duncan Booth wrote:

"SamJs" <Sa*******@gmai l.comwrote:
Is JavaScript explicit about the behavior of creating variables on a
WITH (object) ?

[...]

Assignment never creates a new attribute on the object referred to in the
'with' statement.

So Mike said - maybe I should have believed him. :-)

[...]

Did you read the code?

Yes, but it didn't sink in at the time. I don't use with, and probably
never will - thanks to this discussion I have a better idea of how it
works.
--
Rob

[VK]

VK wrote:

Assignment never creates a new attribute on the object referred to in the
'with' statement.

Humply swallow my words back

<script>
function f() {
with(this) {
a = 'foo';
}
}

f();
alert(window.a) ; // foo
var foo = new f();
alert(foo.a); // undefined
</script>

Interesting...

[Richard Cornford]

VK wrote:

VK wrote:

>Duncan Booth wrote:

>>Assignment never creates a new attribute on the object referred to
in the 'with' statement.

Welcome to JavaScript :-) and you are wrong as it's already pointed
out :-(
If property doesn't exist yet, it will be conveniently created for you.

Humply swallow my words back

<script>
function f() {
with(this) {
a = 'foo';
}
}

f();
alert(window.a) ; // foo
var foo = new f();
alert(foo.a); // undefined
</script>

Interesting...

As you have never managed to grasp the difference between scope chain
Identifier resolution and property accessor resolution you may find
seeing the expected result "interestin g". In reality all you are doing
is again demonstrating that your ignorance of javascript, and fictional
perceptions of it lead you to repeatedly making false statements about
javascript, and wasting everyone's time reading them and correcting the
more coherent ones.

Richard.