Understanding 'Type mismatch' in IE

[Andrew Poulos]

I have a constructor with this IE specific code in it:

this.doc.onreadystatechange = this.ready;

in a subsequent method I have:

alert(this.doc.readyState);
this.doc.onreadystatechange = null;

The alert displays 4 so it 'must' be recognising this.doc yet IE tells
me that the next line has Error: Type mismatch.

Could someone please explain why the error is occurring?

Andrew Poulos

[Duncan Booth]

Andrew Poulos wrote:

in a subsequent method I have:

alert(this.doc.readyState);
this.doc.onreadystatechange = null;

The alert displays 4 so it 'must' be recognising this.doc yet IE tells
me that the next line has Error: Type mismatch.

Could someone please explain why the error is occurring?

I can't, but there is a bit about this on quirksmode:

http://www.quirksmode.org/blog/archi...notes_a_1.html

(see the heading 'Clear readystatechange!')

Not an explanation, but a workaround and some discussion.

[VK]

Andrew Poulos wrote:

I have a constructor with this IE specific code in it:

this.doc.onreadystatechange = this.ready;

in a subsequent method I have:

alert(this.doc.readyState);
this.doc.onreadystatechange = null;

The alert displays 4 so it 'must' be recognising this.doc yet IE tells
me that the next line has Error: Type mismatch.

Could someone please explain why the error is occurring?

Some IE versions do not allow you to "kill" event listener in such way.
Overall it's a very rude habit :-)

Use instead:

this.doc.attachEvent('onreadystatechange', myListener);
....
this.doc.detachEvent('onreadystatechange', myListener);

and
addEventListener / removeEventListener respectively for Firefox

[VK]

Andrew Poulos wrote:

I have a constructor with this IE specific code in it:

this.doc.onreadystatechange = this.ready;

in a subsequent method I have:

alert(this.doc.readyState);
this.doc.onreadystatechange = null;

The alert displays 4 so it 'must' be recognising this.doc yet IE tells
me that the next line has Error: Type mismatch.

Could someone please explain why the error is occurring?

Andrew Poulos

Or (much less academical but more universal and simple I guess) use an
event dumper:

function foo() {
/* NOP */
}
....
obj.onreadystatechange = someUsefulFunction;
....
obj.onreadystatechange = foo;