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Join Date: Oct 2007
Posts: 34
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Rep Power: 2
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Light Poster
hi, i'm wondering why is it javascript doesn't seem to accept null value as a real null? In order to know if its a null field later on, it's as if i have to check it against a string 'null'.
for example,
i have a txtName hidden field on my html.
I use javascript to document.myForm.txtName.value = null;
later on, when i try to detect it just doesn't work :
if(document.myForm.txtName.value == null){
alert('value is null');
}else if(document.myForm.txtName.value == 'null'){
alert('value is a string null'); //only this works!
}
for example,
i have a txtName hidden field on my html.
I use javascript to document.myForm.txtName.value = null;
later on, when i try to detect it just doesn't work :
if(document.myForm.txtName.value == null){
alert('value is null');
}else if(document.myForm.txtName.value == 'null'){
alert('value is a string null'); //only this works!
}
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Join Date: Jun 2007
Location: Valley Center, Kansas
Posts: 636
Reputation:
Rep Power: 3
Solved Threads: 71
Practically a Master Poster
use a variable.
var test = document.myForm.txtName.value;
test = null;
if(test == null){
alert('value is null');
}else if(test == 'null'){
alert('value is a string null'); //only this works!
}
i think this should work, but i didn't test it.
var test = document.myForm.txtName.value;
test = null;
if(test == null){
alert('value is null');
}else if(test == 'null'){
alert('value is a string null'); //only this works!
}
i think this should work, but i didn't test it.
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Join Date: Oct 2007
Posts: 34
Reputation:
Rep Power: 2
Solved Threads: 0
Light Poster
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Originally Posted by kkeith29 
use a variable.
var test = document.myForm.txtName.value;
test = null;
if(test == null){
alert('value is null');
}else if(test == 'null'){
alert('value is a string null'); //only this works!
}
i think this should work, but i didn't test it.
nope, don't think it'll work. I actually rewrote the sample when i started the thread, i'm actually assigning it to a variable already.
this should be something common, yet no one's talking about it??
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Join Date: Jun 2006
Location: India
Posts: 7,009
Reputation: 
Rep Power: 25
Solved Threads: 368
As long an HTML input element exists, it's value can never be null. It can be a blank string (string of length zero), but not null. And if that element doesn't exist, it gives you an 'undefined' instead but never null.
So, to check whether an element exists or not:
Also keep in mind that the DOM function getElementById() returns a 'null' when an element is not found in the DOM tree.
<html> <body id="bdy"> <form action="#" name="frm"> <input id="txt" name="txt" /> </form> <script> alert(document.forms['frm'].elements['txt']); alert(document.forms['frm'].elements['ok']); </script> </body> </html>
So, to check whether an element exists or not:
if(typeof(document.forms[0].elements['elementName']) === 'undefined')
{
// an element by the name of 'elementName' doesn't exist
}Also keep in mind that the DOM function getElementById() returns a 'null' when an element is not found in the DOM tree.
var e = document.getElementById('elementId');
if(e == null)
{
// an element by the id of 'elementId' doesn't exist
} Last edited by ~s.o.s~ : Nov 24th, 2007 at 1:22 am.
I don't accept change. I don't deserve to live.
Happiness corrupts people.
Failing to value the lives of others cheapens your own.
Happiness corrupts people.
Failing to value the lives of others cheapens your own.
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Join Date: Jan 2007
Posts: 2,604
Reputation:
Rep Power: 7
Solved Threads: 119
Null is a pointer that doesn't point to anything.
Daylight-saving time uses more gasoline
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Join Date: Jun 2006
Location: India
Posts: 7,009
Reputation:
Rep Power: 25
Solved Threads: 368
Every primitive value in Ecmascript is one of the following types: Null, Undefined, Number, Boolean and String. Null is a type in Ecmascript having only one value, null. The details are implementation specific. Calling it a pointer would be wrong.
I don't accept change. I don't deserve to live.
Happiness corrupts people.
Failing to value the lives of others cheapens your own.
Happiness corrupts people.
Failing to value the lives of others cheapens your own.
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