document.getElementById() is always returning null

问题

I'm trying to get the value of a hidden input from a form in my HTML, using javascript. But after trying several different ways to get the value, it always says in console(firebug) that the variable is null.. heres the javascript:

var mcq_test = form.mcq_test.value;
console.log("mcqid=" + mcq_test)
var mcq_num_questions = form.mcq_num_questions.value;
console.log("totalquestions=" + mcq_num_questions)
var x = 1;
var send = [];
send.push("mcq_test=");
send.push(mcq_test);

console.log("Send: " + send);

// have commented out the bits below...just go through them carefully looking at the string functions, put them in the send += (SRING 1 + STRING 2 + ...) format

for (var x = 1; x <= mcq_num_questions; x++) {
var questionidd = "mcq_question_id";
console.log("1 = " + questionidd);
var questionid = questionidd.concat(x); // mcq_question_id$ctr the question numer
console.log("2 = " + questionid);
var mcqid = form.questionid.value; // the questions id on db
console.log("3 = " + mcqid);
var answerr = "mcq_question";
var answer = answerr.concat(x); // mcq_question$ctr the questions chosen answer
var chosenanswer = form.answer.value; // the answers value
console.log("4 = " + chosenanswer);
var amp = "&";
var equal = "=";
var questionide = questionid.concat(equal); // "mcq_question_id$ctr="
var questionida = amp.concat(questionide); // "&mcq_question_id$ctr="
var answere = amp.concat(answer,equal); // "&mcq_question$ctr="
if (x = 1) { send.push(questionide, mcqid, answere, chosenanswer); }
else {
send.push(questionida, mcqid, answere, chosenanswer);
}
}

Console:

[04:08:00.328] TypeError: questionID is null 
[04:08:00.327] My new function
[04:08:00.327] mcqid=566
[04:08:00.327] totalquestions=3
[04:08:00.327] Send: mcq_test=,566
[04:08:00.328] 1 = mcq_question_id
[04:08:00.328] 2 = mcq_question_id1

Form:

echo"<input type=\"hidden\" name=\"mcq_question_id$ctr\" id=\"mcq_question_id$ctr\" value=\"$questionID\" form=\"SubmitTest\" />";

-- Update

Answer found! Turns out, you can't use the syntax document.getElementByID() or form.variable.value; and put a variable in there, it has to be a string or written like this:

form[variable].value;

I didn't find an equivalent for getElementById but the form method works for me.

回答1:

You need to make sure that the DOM has loaded before you try accessing it. Put the Javascript after the HTML or setup an event to trigger the variable definition.

回答2:

when using this

echo"<input type=\"hidden\" name=\"mcq_question_id$ctr\" id=\"mcq_question_id$ctr\" value=\"$questionID\" form=\"SubmitTest\" />";

here before going to create input type make sure your '$questionID' is not null

use this then test

echo"<input type=\"hidden\" name=\"mcq_question_id$ctr\" id=\"mcq_question_id$ctr\" value=\"questions\" form=\"SubmitTest\" />";

来源：https://stackoverflow.com/questions/14078655/document-getelementbyid-is-always-returning-null

标签

javascript

getelementbyid