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Message-ID: <1350222.i9fspTd0WE@PointedEars.de>
From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Reply-To: Thomas 'PointedEars' Lahn <cljs@PointedEars.de>
Organization: PointedEars Software (PES)
Date: Sat, 11 Feb 2012 22:06:25 +0100
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Subject: Re: Simple parameter passing within a setInternal() call?
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.javascript:11708

Andreas Bergmaier wrote:

> Tuxedo schrieb:
>> How can a simple string parameter be passed between two functions while
>> inside a setInterval call? For example:
>>
>> function initialFunction(x){
>> alert(x);
>> setInterval('repeatingStuff(x)',5000);
>> }
>>
>> function repeatingStuff(x){
>> alert(x);
>> }
>> ...
>>
>> <a href="..." onclick="initialFunction('param_x');">stuff</a>
>>
>> But the error "x is not defined" results from ..Stuff(x) in
>> setInterval('repeatingStuff(x)',5000). How can the parameter between the
>> two functions be passed within the setInteval bit?
> 
> x must be in the scope of the second function.

In the scope _chain_.

> Therefore, you should define repeatingStuff inside the initialFunction:
> 
> function initialFunction(x){
>     alert(x);            ^
            ^--------------'
>     function repeatingStuff(x){
                              ^
>        alert(x);            |
               ^--------------'
>     }
>     setInterval(repeatingStuff,5000);
> }
> 
> or shorter:
> 
> function initialFunction(x){
>     alert(x);            ^
            ^--------------'
>     setInterval( function(x){
>        alert(x);          ^
               ^------------'
>     }, 5000);
> }

This does not work, because in the context of the function (referred by 
`repeatingStuff'), `x' is bound to the inner function's argument `x' (which 
is not passed a useful value from the DOM implementation anyway) and not to 
that of the inner function's definition context.  Please test before you 
post, or clearly mark code as untested.

> You should pass a function to setIntervall/Timeout, instead of a code to
> be evaluated each time (slow & buggy).

The function code also will have to be evaluated each time.  But it is true 
that evaluating the string value as a Program is less efficient and more 
error-prone than that.
 
> Another alternative is to use bind() or other functions, to return a
> function which calls repeatingStuff with the needed parameters:
> 
> function repeatingStuff(){...}
> function initialFunction(x){
>     alert(x);
>     setInterval( repeatingStuff.bind(this, x), 5000);
> }

This works in implementations of ECMAScript 5.x or where you have a suitable 
scripting framework that emulates Function.prototype.bind(), but it is 
unnecessarily inefficient by comparison.


PointedEars
-- 
> If you get a bunch of authors […] that state the same "best practices"
> in any programming language, then you can bet who is wrong or right...
Not with javascript. Nonsense propagates like wildfire in this field.
  -- Richard Cornford, comp.lang.javascript, 2011-11-14