The stand-alone code can be found here : Var-Declaration-Test-Program

Load it down and play with it !  Just reading my crap won't bring you very far ! 



Here is what The Sandman already told you about variables : 



Local variables
are those variables that exist ONLY inside the function where it was created.
 Once program execution leaves the function then the variable(s) created inside
 this function are discarded and never used again. This makes good programming
 practice to use this kind of variable if it is not going to be needed elsewhere
within the script.

Global variables
once defined, exist within the entire script, they can therefore be changed
 or read by any function within the script itself and is often used as a kind
 of 'counter' and or to maintain the results from having a string or number
 processed by one or more functions.
 
 A short extension by me :   
 - Within a function, a local declared variable has more rights than a global
     variable with the same name. So you can - within a function - pass a value 
     to a global variable and change this local variable, under the condition
     that there was no local variable with the same name declared.
 - Within a function you cant't access variables, which are local variables in
    another function.

	

So far, so good (isn't it ? :)
Let me bring some light into this darkness (and show you some evil traps) : Javascript is a rather 'cool' 
 language concerning the strictness of the use of variables.  But  this 'coolness' has its prize : you
 gotta be extremely careful with the use of the variables.
Look at this code : (you should have a printing of _global.htm lying next to your console)


var TestOne='Global_One'
TestTwo='Global_Two'
TestThree=null
showglobal('Before');

var TestOne='Global_One'......:-- this is a DECLARATION
TestTwo='Global_Two'...........:-- this is an ASSIGNMENT
TestThree=null.....................:-- just another assignment

We declare one variable and we assign two variables. Check this out : 



Nothing sensational up to now. So let's dive into a function. 
Here's the code : 


function Local_Pub()
{
var TestThree='test_three'

go('Local_Test_One');

alert ('Local_Test_Two : '+ TestOne + ' * ' + TestTwo + ' * ' + TestThree + ' ')

var TestOne='Local_One'
TestTwo='Local_Two'
TestThree='Local_Three'

alert ('Local_Test_Three : '+ TestOne + ' * ' + TestTwo + ' * ' + TestThree + ' ')

go('Local_Test_Four');
}


Let's wade through this code step by step. 
We have a function : 

function Local_Pub()
{
var TestThree='test_three'.......and we declare a local variable 

go('Local_Test_One');............after this declaration we call another function, 
							            to show us the new values.
								  
The alert-box (Local_Test_One) gives us this :

TestOne :  Global_One *** TestTwo :  Global_Two *** TestThree : Null

Remember : TestThree is a local variable, which can't be shown in another function !


On with the code : 

alert ('Local_Test_Two : '+ TestOne + ' * ' + TestTwo + ' * ' + TestThree + ' ')


Local_Test_Two shows us : 

TestOne :  undefined *** TestTwo :  Global_Two *** TestThree : test_Three

This local alert-box shows us three things : 
1. The local variable 'TestThree' now has a value within the function   (surprise ;)
2. The global variable 'TestTwo' (which was assigned) shows its value  (surprise ;)
3. The global value 'TestOne' (which was declared) is undefined !!!!
   Don't ask me why! This is really strange ! (Or am I just making a mistake ??)


On with the code : 

var TestOne='Local_One'........:-- declaration of a local variable

TestTwo='Local_Two'............:---assignment of a local variable

TestThree='Local_Three'........:---just another assignment

Let's make some alert (within the function) : 

alert ('Local_Test_Three : '+ TestOne + ' * ' + TestTwo + ' * ' + TestThree + ' ')


This box (Local_Test_Three) shows us : 

TestOne :  Local_One *** TestTwo :  Local_Two *** TestThree : Local_Three

So this goes to show, that the local variables have more 'rights' than the global ones. 
(AND, don't forget : you can change the value of a global variable within a function, 
if there was no declaration of a local variable with the same name ! 


Next line : 	

go('Local_Test_Four');


Once again we call another function to show us some variables, and we get (Local_Test_Four) :

TestOne :  Global_One *** TestTwo :  Local_Two *** TestThree : null

Funny, isn'*t it ?
The value of the global variable 'TestTwo' has been changed (forever), just because this
global variable was assigned, but not declared !!


From the main-program we make a last test : 

.... onClick="showglobal('after')"


This alert ("after") gives us :

TestOne :  Global_One *** TestTwo :  Local_Two *** TestThree : null

and just goes to show that the global variable 'TestTwo' has changed it's value. 


O.k., that's it. What was this crap for, you may ask ? Well, the only thing I can tell you : 
be careful with declarations and assignments of variables. If you are writing some code, test
 the values of your variables with an alert-box. Better one time too much, than an evil surprise ! 


The Seeker					  

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Addendum 24.04.1999
Got it !! Variables which should only be visible in functions have to be declared !
So if you use a variable without the var declaration, this variable will be 'visible' in all the other functions !!
In other words : only variables, which are declared with 'var' are local function-variables !

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Original format of this tut by (c) fravia+ 97 All rights reversed.

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Page started by The Seeker
Page created: 9th March 1999