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grahamgr3

Validating Form Elements In Javascript

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The following code is from chapter 5 of this book. 

 

if (!comments || !comments.value || (comments.value.indexOf('<') != -1) ){
okay = false;
alert ('Please enter your comments, without any HTML!');
 
My question is why do we have to both put !comments and also !comments.value. Aren't they the same?? Why check comments if it doesn't have a value. I just learned php with Larry's book, and am new at javascript. 

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Basically, you never want to assume anything in programming.

In this case, you're ensuring that comments is not null (i.e., it's a valid DOM element) and that it has the value property (i.e., it's a valid form input element).

 

If you assume both of those things without checking, then your code would through an error on the comments.value.indexOf part of the if statement whenever comments was not set to a DOM input element.

 

That make sense?

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Just a general comment that there is also a powerful (and free) jQuery form validation plug-in that can be used.

 

Cheers from Oz, Necuima.

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