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Re: How to detect key pressed plus mouse click, e.g., ctrl + left click

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Thomas 'PointedEars' Lahn wrote:

> 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.

Apparently nobody has objected to this yet, so I correct myself:

Assignment to a formal parameter changes the value of that parameter as 
copied from the argument of the call *for the local execution context*.  
This was probably obvious as one would implement optional named arguments 
this way.

What may not be obvious: When *not* in strict mode (this is the default), 
the corresponding element of the array-like “arguments” object is changed as 
well.

<http://ecma-international.org/ecma-262/5.1/#sec-C>
<http://ecma-international.org/ecma-262/5.1/#sec-10.6>

This can have unwanted side-effects.  Either initializing a local variable  
with the parameter value or enabling strict mode (where supported) prevents 
that.

It might be prudent then, even if the result may be distracting at times, to 
enable the “Parameter assignment” warning in the JSDT Validator.  (I know 
there are recommendations on the Web not to do that when using certain 
libraries; people might want to reconsider.)

As a current case in point, today I had modified my

    getFeature: function (obj) {
      for (var i = 1, len = arguments.length; i < len; i++)
      {
        var component = arguments[i];
        if (_isObject(obj)
            && typeof obj[component] != "undefined" && obj[component])
        {
          obj = obj[component];
        }
        else
        {
          return void 0;
        }
      }

      return obj;
    }

so that it also accepted a second Array argument instead of the remaining 
argument list (see also <news:2932926.BXQvjz4BgK@PointedEars.de>¹):

    getFeature: function (obj, path) {
      var start = 0;

      if (!_isArray(path))
      {
        path = arguments;
        start = 1;
      }

      for (var i = start, len = path.length; i < len; i++)
      {
        var component = path[i];
        if (_isObject(obj)
            && typeof obj[component] != "undefined" && obj[component])
        {
          obj = obj[component];
        }
        else
        {
          return void 0;
        }
      }

      return obj;
    }

This modification, although syntactically and semantically sound, proved to 
be a show-stopper: When the method was not passed an Array, the second 
element of the “arguments” object (“arguments[1]”) became a reference to the 
“arguments” object itself, causing the unit test to fail.

Since I cannot rely on strict mode support for now, I have modified it as 
follows:

    getFeature: function (obj, path) {
      var realPath = path;
      var start = 0;

      if (!_isArray(realPath))
      {
        realPath = arguments;
        start = 1;
      }

      for (var i = start, len = realPath.length; i < len; i++)
      {
        var component = realPath[i];
        if (_isObject(obj)
            && typeof obj[component] != "undefined" && obj[component])
        {
          obj = obj[component];
        }
        else
        {
          return void 0;
        }
      }

      return obj;
    }

Now it passes the unit test again.

IOW: Declaring a local “e” variable in the code on the top is not necessary 
and *here* it has none of the other side-effects that I mentioned; but it 
does have a noticeable positive effect.

_______
¹ On a side note, I have also noticed that it already supports false-values.
  It does not support “undefined” and “null” values yet, but I do not think
  this is a problem: I for one would consider “undefined” and “null” to be
  an indication that the feature on that property path is not available.
-- 
PointedEars

Twitter: @PointedEars2
Please do not Cc: me. / Bitte keine Kopien per E-Mail.

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How to detect key pressed plus mouse click, e.g., ctrl + left click "Joey@still_Learning.invalid" <Joey@still_Learning.invalid> - 2014-02-21 10:45 -0800
  Re: How to detect key pressed plus mouse click, e.g., ctrl + left click "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2014-02-21 20:04 +0100
    Re: How to detect key pressed plus mouse click, e.g., ctrl + left click "Joey@still_Learning.invalid" <Joey@still_Learning.invalid> - 2014-02-21 11:21 -0800
      Re: How to detect key pressed plus mouse click, e.g., ctrl + left click "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2014-02-21 20:32 +0100
        Re: How to detect key pressed plus mouse click, e.g., ctrl + left click "Joey@still_Learning.invalid" <Joey@still_Learning.invalid> - 2014-02-21 14:24 -0800
          Re: How to detect key pressed plus mouse click, e.g., ctrl + left click "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2014-02-22 00:31 +0100
            Re: How to detect key pressed plus mouse click, e.g., ctrl + left click "Joey@still_Learning.invalid" <Joey@still_Learning.invalid> - 2014-02-21 17:41 -0800
        Re: How to detect key pressed plus mouse click, e.g., ctrl + left click Andrew Poulos <ap_prog@hotmail.com> - 2014-02-22 12:48 +1100
          Re: How to detect key pressed plus mouse click, e.g., ctrl + left click "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2014-02-22 10:36 +0100
          Re: How to detect key pressed plus mouse click, e.g., ctrl + left click Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-02-22 10:54 +0100
            Re: How to detect key pressed plus mouse click, e.g., ctrl + left click "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2014-02-22 11:19 +0100
              Re: How to detect key pressed plus mouse click, e.g., ctrl + left click Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-02-22 11:27 +0100
                Re: How to detect key pressed plus mouse click, e.g., ctrl + left click "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2014-02-22 11:39 +0100
                Re: How to detect key pressed plus mouse click, e.g., ctrl + left click Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-02-22 12:50 +0100
                Re: How to detect key pressed plus mouse click, e.g., ctrl + left click Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-02-22 12:56 +0100
                Re: How to detect key pressed plus mouse click, e.g., ctrl + left click John Harris <niam@jghnorth.org.uk.invalid> - 2014-02-23 14:00 +0000
            Re: How to detect key pressed plus mouse click, e.g., ctrl + left click Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-02-23 23:48 +0100
              Re: How to detect key pressed plus mouse click, e.g., ctrl + left click Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2014-02-23 20:04 -0300
            Re: How to detect key pressed plus mouse click, e.g., ctrl + left click Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-02-23 23:39 +0100

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