Javascript document.getElementById(“id”).value ret

问题:

I have a text box element whose value I am trying to access using document.getElementById("id-name").value. I find that the call is returning a null instead of empty string. The data-type of the returned value is still string. Is null a string value?

<input type="text" value="" id="mytext"> is the textbox whose value I am trying to fetch using var mytextvalue = document.getElementById("mytext").value;

回答1:

Posting your HTML might help a bit. Instead, you can get the element first and then check if it is null or not and then ask for its value rather than just asking for the value directly without knowing if the element is visible on the HTML or not.

element1 = document.getElementById(id);

if(element1 != null)
{
    //code to set the value variable.
}

回答2:

fyi, this can happen if you are using the html type="number" attribute on your input tag. Entering a non-number will clear it before your script knows what's going on.

回答3:

Please check this fiddle and let me know if you get an alert of null value. I have copied your code there and added a couple of alerts. Just like others, I also dont see a null being returned, I get an empty string. Which browser are you using?

回答4:

This demo is returning correctly for me in Chrome 14, FF3 and FF5 (with Firebug):

var mytextvalue = document.getElementById("mytext").value;
console.log(mytextvalue == ''); // true
console.log(mytextvalue == null); // false

and changing the console.log to alert, I still get the desired output in IE6.

回答5:

try this...    
<script type="text/javascript">
    function test(){
    var av=document.getElementById("mytext").value;
    alert(av);
    }
    </script>

    <input type="text" value="" id="mytext">
    <input type="button" onclick="test()" value="go" />

回答6:

For your code

var mytextvalue = document.getElementById("mytext");

mytextvalue will contain null if you have a document.write() statement before this code. So remove the document.write statement and you should get a proper text object in the variable mytextvalue.

This is caused by document.write changing the document.

回答7:

It seems that you've omitted the value attribute in HTML markup.

Add it there as <input value="" ... >.

回答8:

I think the textbox you are trying to access is not yet loaded onto the page at the time your javascript is being executed.

ie., For the Javascript to be able to read the textbox from the DOM of the page, the textbox must be available as an element. If the javascript is getting called before the textbox is written onto the page, the textbox will not be visible and so NULL is returned.