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Message-ID: <3506285.IpskKQNfjB@PointedEars.de>
From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Reply-To: Thomas 'PointedEars' Lahn <cljs@PointedEars.de>
Organization: PointedEars Software (PES)
Date: Sat, 22 Feb 2014 10:54:43 +0100
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Subject: Re: How to detect key pressed plus mouse click, e.g., ctrl + left click
Newsgroups: comp.lang.javascript
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Xref: csiph.com comp.lang.javascript:23191

Andrew Poulos wrote:

> On 22/02/2014 6:32 AM, Evertjan. wrote:
>> document.onmousedown = function(e) {
>>    var e = e || window.event;
>>    if (e.shiftKey || e.altKey || e.ctrlKey)
>>      alert ( "shiftKey=" + e.shiftKey + "\naltKey="
>>      + e.altKey + "\nctrlKey=" + e.ctrlKey );
>> };
> 
> I notice that you put a "var" before the "e". Is that really necessary?

No.  As “e” is in the formal parameter list, it is treated as as if it was a 
local variable.  Redeclaring a local variable has no effect.  Initializing 
it like that also has not.

A local variable can come in handy if the original value of the argument of 
the function call is needed later.  The local variable SHOULD have a 
different identifier then (otherwise it would prevent accesses to the 
argument value by parameter name).  Only insofar development tools (like 
Eclipse JSDT) that warn about parameter assignment are useful here, because 
ECMAScript implementations support only call-by-value (references to objects 
are values; properties and variables cannot be referenced in terms of their 
memory locations): Accidental assignments to the parameter itself will not 
change a value in the calling execution contexts.

> With
> 
> 
> var e = "1";
> 
> function foo(e) {
>    window.alert(e);
> }
> 
> foo("2");
> 
> 
> wouldn't the alert display "2"?

No, it would.  However, the whole operation is bogus.  It should be at least

  if (!e) e = window.event;

so that the assignment only takes place when necessary.  Further feature 
tests are recommended, see jsx.dom.createEventListener() [1].

[1] <http://PointedEars.de/wsvn/JSX/trunk/dom/events.js>

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PointedEars

Twitter: @PointedEars2
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