The Undefined Defined Variable
Article Index
The Undefined Defined Variable
Solution

Solution

The answer to the problem is "hoisting".

When you declare a variable within a function the declaration is automatically moved to the start of the function - i.e. it is hoisted to the top.

You can write Javascript for years without ever running into problems with hoisting or even being aware that it is going on - but when you blunder into it as in our example it can seem very strange.

Notice that hoisting only moves the declaration of the variable and not any initialisation. With hoisting taken into acount our function reads more like:

function test()
{
 var i;
alert(i);
for(i=0;i<10;i++)
{
alert(i);     
}
}

Now it is perfectly clear why i is undefined.

The local variable i has been declared at the start of the function as the result of hoisting so hiding the global variable i which isn't initialised until the for loop starts.

Now it is perfectly obvious why i is reported as undefined.

Notice that the declaring of the variable in the for loop is quite irrelevant to the the problem which would have happened no matter how i was declared. This is all a matter of where i is declared not how.


Banner

Pattern

The pattern that you need to follow to make sure that hoisting never causes you any problem is simple:

always declare, and if appropriate initialize, all of the variables that a function uses at the start of the function.

Following this pattern makes hoisting explicit.

 

 

Banner

More Puzzles

Python
Python Puzzle - Where Did The Time Go

A Python programming puzzle to get you up to speed. This one is all about time keeping, or is it? There are some strange things that go on in Python when you aren't paying attention. 


Sharpen Your Coding Skills
Merkles and Social Engineering

Joe Celko has come up with a math puzzle based on one of the current political hot topics - but let the team from International Storm Door & Software explain the problem of faced by planners who w [ ... ]


Sharpen Your Coding Skills
The Post Production Problem

Joe Celko has posed another puzzle that requires you to think like a programmer. This one is all about Post tag machines, which have absolutely nothing to do with mail of any type but a lot to do with [ ... ]


Other Articles

<ASIN:059680279X>

<ASIN:0470526912>

<ASIN:1590597273>

<ASIN:0596806752>



 
 

   
RSS feed of all content
I Programmer - full contents
Copyright © 2015 i-programmer.info. All Rights Reserved.
Joomla! is Free Software released under the GNU/GPL License.