当element是表单的一部分时，document.getElementById返回null

Question

I have php file that has some javascript inside. Consider the following javascript code, which prints null to the console (line 124 in the file):

console.log(document.getElementById('d_min')); 

Below the script, I have the following form (line 140 in the file):

<FORM name="DataCollect" ACTION="save_phase1.php" method="post">
    <input type="hidden" name="d_min" id="d_min" value=""/>
</FORM>

Why is it that it could be null ? Why can't it find that element?

---

I FIXED THE PROBLEM:

I put the entire <form> as a document.write statement after the existing document.write statements I had.

Answer 1

The DOM is not fully loaded by the time the script executes. Move your script below that form and see if you get better results.

Answer 2

在window.onload函数中包装JavaScript代码。

Answer 3

I have no idea why it's not grabbing the field, but try giving the form an id foo and using

document.getElementById('foo').d_min

instead. At least it will narrow the source of the problem.