document.getElementById（）始終返回null

Question

我正在嘗試使用javascript從HTML表單中獲取隱藏輸入的值。 但是在嘗試了幾種不同的方法來獲取值之后，它總是在console（firebug）中說該變量為null。

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);
}
} 

安慰：

[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

形成：

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

-更新

找到答案！ 事實證明，您不能使用語法document.getElementByID（）或form.variable.value;。 並在其中放置一個變量，它必須是字符串或這樣寫：

form[variable].value;

我沒有找到等效的getElementById，但是form方法對我有用。

Answer 1

您需要先確保已加載DOM，然后再嘗試訪問它。 將Javascript放在HTML后面，或設置一個事件以觸發變量定義。

Answer 2

當使用這個

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

在此處創建輸入類型之前，請確保您的'$ questionID'不為null

用這個然后測試

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