thanks
"Lasse Reichstein Nielsen" <lrn@hotpop.com> schreef in bericht
news:lluiibzy.fsf@hotpop.com...[color=blue]
> "Dan" <no@mail.xy> writes:
>
> Please don't top-post.
>[color=green]
> > I tried different ways about this and maybe you can help me understand[/color][/color]
some[color=blue][color=green]
> > differences:[/color]
>[color=green]
> > 1) this works ('ev' is used instead of 'event')
> > <IMG ID="myimg" SRC="bugs.gif" >
> > <script type="text/javascript">
> > var ev;[/color]
>
> This (global) variable declaration is not necessary. The (local) function
> parameter overshadows it inside the function, and you never assign[/color]
anything[color=blue]
> to the global variable.
>[color=green]
> > lap=document.getElementById("myimg")
> > function downtest(ev)
> > {
> > var strid = ev.target.id;
> > alert(strid)
> > }
> > lap.onmousedown=downtest // WHY NOT[/color]
>
> Why not what? It looks absolutely correct, although I would end my
> sentences with a semicolon. Mostly because I find it more readable.
>[color=green]
> > lap.onmousedown=downtest(ev) ??[/color]
>
> This is not what you want. You don't want to call the function now
> and assigne the result (which is "undefined" since there is no return
> statment in it) to "lap.onmousedown".
>
> If you have both of these lines, the latter overwrites the former,
> which might be why you don't see the result.
>[color=green]
> > </script>
> >
> > 2) this works too ('event' used)
> > <IMG ID="myimg" SRC="bugs.gif" onMouseDown=downtest(event)>[/color]
>
> You should have quotes around the "onmousedown" attribute value,
> since it contains "(" and ")". In practice, it is easier to always
> put quotes around than to worry which charaters are legal and
> which aren't.
>
> You call the "downtest" function with the value of the "event"
> variable. The javascript code in the onmousedown attribute value is
> evaluated in a context, where "event" refers to the current event
> (probably why Microsoft decided to just have one global event
> variable). The *value* of the variable "event" is used as argument
> to the downtest function.
>[color=green]
> > <script type="text/javascript">
> > lap=document.getElementById("myimg")
> > function downtest(event)[/color]
>
> The name of the function argument is irrelevant, you can call it "ev",
> "event", "foo" or even "body" without affecting how this function works.
> A function argument is local to the function, so
> ---
> var x=4;
> function foo(x){
> x=2;
> return x;
> }
> foo(12);
> alert(x);
> ---
> will alert "4". The variable "x" outside the function and the one inside
> are two different variables, and the following is *completely* equivalent:
> ---
> var x=4;
> function foo(y){
> y=2;
> return y;
> }
> foo(12);
> alert(x);
> ---
> (in programming language theory, that is a well known concept: renaming
> of "bound" variables doesn't change the behavior of the program).
>[color=green]
> > {
> > var strid = event.target.id;
> > alert(strid)
> > }
> > </script>[/color]
>
>[color=green]
> > 3) this doesn't work: why can 'event' not be replaces by 'ev'? error= ev[/color][/color]
has[color=blue][color=green]
> > no properties
> > <IMG ID="myimg" SRC="bugs.gif" onMouseDown=downtest(ev)>[/color]
>
> Here the code "downtest(ev)" is executed in an environment where
> there is a variable, called "event", referring to the current event.
> The "ev" variable is the one you declare below, and it only contains
> "undefined".
>[color=green]
> > <script type="text/javascript">
> > var ev;
> > lap=document.getElementById("myimg")
> > function downtest(ev)
> > {
> > var strid = ev.target.id;[/color]
>
> That means that at this point, the local variable (also called "ev")
> has the value "undefined", and you can't find the property "target"
> of a value that is "undefined".
>
> /L
> --
> Lasse Reichstein Nielsen -
lrn@hotpop.com
> Art D'HTML: <URL:http://www.infimum.dk/HTML/randomArtSplit.html>
> 'Faith without judgement merely degrades the spirit divine.'[/color]
