Javascript: How can I use a variable name when updating a form value?

Question

Here is part of the function:

function updateResponse(question, answer) {
    questionId = 'q'+question;
    document.forms[0].questionId.value = answer;

Calling the function with something like:

<td onClick='javascript:updateResponse(1, 1);'>

The form element I want it to update looks something like:

<input type='hidden' name='q1' value='' />

Using Firefox web console I get:

TypeError: document.forms[0].questionId is undefined

Thanks!

UPDATE: Found this after much googling and it works. Still seems like there may be a more graceful way to achieve this though?

eval(\"document.forms[0].\"+questionId+\".value = answer;\");

You do not need the javascript: prefix there..

techfoobar
– techfoobar

2013-03-28 05:52:35 +00:00
Commented Mar 28, 2013 at 5:52 — techfoobar
– techfoobar, Commented Mar 28, 2013 at 5:52
removed that, thanks. still the same error message.

Matt
– Matt

2013-03-28 05:55:24 +00:00
Commented Mar 28, 2013 at 5:55 — Matt
– Matt, Commented Mar 28, 2013 at 5:55

James Chen · Accepted Answer · 2013-03-28 05:57:32Z

2

If you're sure there's an input in the form that has the name questionId:

document.forms[0][questionId].value

And you should use var to avoid global variable:

var questionId = 'q' + question;

edited Mar 28, 2013 at 5:57

answered Mar 28, 2013 at 5:49

James Chen

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6 Comments

Matt Over a year ago

questionId is the name of the variable. It's value should be q1 or similar.

Matt Over a year ago

Thanks. Added var but still getting the same error. To be clear, I want javascript to substitute the word questionId with it's value 'q1'. It looks to me like it is trying to find a form element called 'questionId'. It needs to find 'q1', 'q2', 'q3' etc depending on the value of variable 'questionId'.

James Chen Over a year ago

Have you even tried the first JavaScript line I answered, e.g.: document.forms[0][questionId].value = answer;? That would satisfy your need whatever value the questionId holds.

Matt Over a year ago

Yes, sorry I should have posted the result. When using document.forms[0].[questionId].value = answer; I get: SyntaxError: missing name after . operator

James Chen Over a year ago

You've added an extra '.'.

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