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How to toggle css visibility with Javascript

I am trying to create a FAQ page much like the one here: https://www.harrys.com/help
I want to create the effect where clicking a question will display an answer.
My code can be seen here: http://jsfiddle.net/8UVAf/1/
Can anybody tell me why my javascript is not working? I realized I combined jQuery and Javascript, but I read somewhere that it should compile fine.
HTML:
<div class="questions-answer-block">
<p class="question">This is a Question?</p>
<p id="answer" class="hideinit">Here is the Answer</p>
</div>
<div class="questions-answer-block">
<p class="question">This is a Question?</p>
<p id="answer" class="hideinit">Here is the Answerdadawdawdawdawdawdawdawdwadawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawdawd</p>
</div>
JS:
$(".question").click(function (argument) {
if(document.getElementById("answer").className.match(/(?:^|\s)hideinit(?!\S)/)) {
document.getElementByID("answer").className = "display";
}
});

Basically your Javascript could be shortened to:
$(".question").click(function(argument) {
$(this).parent().find(".answer").removeClass("hideinit").addClass("display");
});
In order to make this work the only other thing you need to do is to make question a class rather than as an id. That looks like:
<p class="answer hideinit">the answer</p>
See the fiddle here
Edit: Add Hide / Show
To get this to hide and show as expected you'll want to update the code to check the current class before hiding and showing. That looks like:
$(".question").click(function(argument) {
var el = $(this).parent().find(".answer");
if (el.hasClass("display")) {
el.removeClass("display").addClass("hideinit");
} else {
el.removeClass("hideinit").addClass("display");
}
});
See the fiddle here

Well, for one thing, in your JSFiddle you were not including the jQuery library. I've adjusted your code, I think this is what you were going for:
$(".question").click(function() {
$(this).siblings().toggle();
});
Here's an updated JSFiddle.

Please watch your includes in your JSFiddle as the version you linked was not including the jQuery library. You should also clean up your multiple id references (as this is invalid HTML and will cause some issues down the road).
Those issues aside, you can use jQuery's .next() method to help you with this particular problem:
$(".question").click(function (argument) {
$(this).next(".hideinit").removeClass("hideinit").addClass("display");
});
JSFiddle

$(".question").on('click',function() {
$(this).next().toggle();
});

Get onClick element to return to the main element?

I'm awful with javascript and I'm having a problem with this one.
I'm using this code
<script>
function changeNavigation(id){
document.getElementById('members')
.innerHTML=document.getElementById(id).innerHTML
}
</script>
and HTML
`<span onClick="changeNavigation('members')" >MEMBERS</span>
<span onClick="changeNavigation('help')" >HELP</span>`
<div id="members>...</div>
<div id="help" style="display: none;>...</div>
But I can't get <span onClick="changeNavigation('members')" >MEMBERS</span> to actually go to an element "members" without duplicating everything inside of it in another id.
Is there a way to do this?

This can be done using only standard javascript, but personally I'd recommend going ahead and getting used to using jQuery. Here's an example jsfiddle using jQuery: http://jsfiddle.net/JnvCR/2/
Don't forget to include jQuery in your website:
<script src="http://code.jquery.com/jquery.js"></script>

You need to correct your syntax errors. Use onclick instead of onClick (pedantic). Make sure you close your attributes properly, you are missing a few closing " marks.
updated html
<span onclick="changeNavigation('members')" >MEMBERS</span>
<span onclick="changeNavigation('help')" >HELP</span>`
<div id="members">...</div>
<div id="help" style="display: none;">...</div>
There is also an error with your logic as you are simply replacing the contents of div#members with itself.
Updated JS without syntax errors, but still with dodgy logic
function changeNavigation(id){
document.getElementById('members').innerHTML=document.getElementById(id).innerHTML;
}
Demo fiddle
http://jsfiddle.net/ADGCV/
As far as your actual question goes, can you explain what you would like to happen a bit better??
Here's a possible solution http://jsfiddle.net/ADGCV/1/

Can't fire click event on the elements with same id

Could you help me to understand - where I made the mistake. I have the following html code:
<div id="container">
Info mail.ru
</div>
<div id="container">
Info mail.com
</div>
<div id="container">
Info mail.net
</div>
and the following js code (using jQuery):
$('#getInfo').click(function(){
alert('test!');
});
example here
"Click" event fired only on first link element. But not on others.
I know that each ID in html page should be used only one time (but CLASS can be used a lot of times) - but it only should (not must) as I know. Is it the root of my problem?
TIA!
upd: Big thx to all for explanation!:)

Use a class for this (and return false in your handler, not inline):
<div id="container">
Info mail.ru
</div>
<div id="container">
Info mail.com
</div>
<div id="container">
Info mail.net
</div>
$('.getInfo').click(function(){
alert('test!');
return false;
});
http://jsfiddle.net/Xde7K/2/
The reason you're having this problem is that elements are retrieved by ID using document.getElementById(), which can only return one element. So you only get one, whichever the browser decides to give you.

While you must, according to the W3 specifications, have only one element with a given id within any document, you can bypass this rule, and the issues arising from the consequences if document.getElementById(), if you're using jQuery, by using:
$('a[id="getInfo"]').click(function() {
alert('test!');
return false;
});
JS Fiddle demo.
But, please, don't. Respect the specs, they make everybody's life easier when they're followed. The above is a possibility, but using html correctly is much, much better for us all. And reduces the impact of any future changes within the browser engines, jQuery or JavaScript itself.

It must only be used once or it will be invalid so use a class instead, return false can also be added to your jQuery code as so: -
$('.getInfo').click(function(){
alert('test!');
return false;
});
<a href="#info-mail.net" **class**="getInfo" ....

First id's are for one element only, you should have same id for several divs.
you can make it class instead.
your example changed:
<div class="container">
<a href="#info-mail.ru" class="getInfo" >Info mail.ru</a>
</div>
<div class="container">
<a href="#info-mail.com" class="getInfo" >Info mail.com</a>
</div>
<div class="container">
<a href="#info-mail.net" class="getInfo" >Info mail.net</a>
</div>
$('.getInfo').click(function(ev){
ev.preventDefault(); //this is for canceling your code : onClick="return false;"
alert('test!');
});

You can use the same id for several element (although the page won't validate), but then you can't use the id to find the elements.
The document.getElementById method only returns a single element for the given id, so if you would want to find the other elements you would have to loop through all elements and check their id.
The Sizzle engine that jQuery uses to find the elements for a selector uses the getElementById method to find the element when given a selector like #getInfo.

I know this is an old question and as everyone suggested, there should not be elements with duplicate IDs. But sometimes it cannot be helped as someone else may have written the HTML code.
For those cases, you can just expand the selector used to force jQuery to use querySelectorAll internally instead of getElementById. Here is a sample code to do so:
$('body #getInfo').click(function(){
alert('test!');
});
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
</head>
<body>
<div id="container">
Info mail.ru
</div>
<div id="container">
Info mail.com
</div>
<div id="container">
Info mail.net
</div>
</body>
However as David Thomas said in his answer
But, please, don't. Respect the specs, they make everybody's life easier when they're followed. The above is a possibility, but using html correctly is much, much better for us all. And reduces the impact of any future changes within the browser engines, jQuery or JavaScript itself.

How to select an object in JQuery?

I'm beginner in JQuery, how could I select an object using JQuery ?
This is the code:
<script type="text/javascript" language="javascript">
function Hide(senderID) {
$("#" + senderID).hide(200);
// this exception is thrown // Microsoft JScript runtime error: Object expected
}
</script>
<div id="div1" onclick="javascript:Hide(this.id)"
Any help!

Don't:
get an id from an element
pass that id to a function
use the id to get the element.
Do: Just pass the element.
Don't stick javascript: at the front of an intrinsic event attribute, it doesn't mean what you think it means.
Don't use intrinsic event attributes for that matter (although I didn't fix this in this example). Use unobtrusive JS.
Avoid triggering events based on clicks on a div. This can't be targeted with a focus based navigation device (such as using the tab key on the keyboard and numerous devices used by people with disabilities) without using new features introduced in HTML 5 that don't see widespread support yet. Use an element that is designed as an interaction control (such as a button). (Also not fixed in the example below)
Example:
function Hide(sender) {
$(sender).hide(200);
}
<div id="div1" onclick="Hide(this)"

Code is exactly the same as yours, I added the correct tags, and the call to include the jquery library:
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js"></script>
<script>
function Hide(senderID) {
$("#" + senderID).hide();
}
</script>
<div id="div1" onclick="javascript:Hide(this.id)">Click Me</div>

function Hide(sender) {
$(sender).hide(200);
}
<div id="div1" onclick="javascript:Hide(this)"></div>
hope it helps

I can't resist. Why not use jQuery's full power?
HTML:
<div class="hideable-div">Click me and get rid of me.</div>
jQuery:
$('.hideable-div').click(function () {
$(this).hide(200);
});

you misplaced those "" in
<div id="div1" class=""hideable-div>Click me and get rid of me.</div>
Should be like
<div id="div1" class="hideable-div">Click me and get rid of me.</div>

Simpler DIV/JavaScript update?

This is a newbie question: Can the following HTML/JavaScript code be further simplified by just keeping the DIV to be updated + the INPUT button?
<div id="main_section" name="main_section">
<div id="update_div">Old stuff</div>
<input type="button" value="Update" id="update_button"/>
</div>
<script type="text/javascript" src="/jquery.js"></script>
<script type='text/javascript'>
$("#update_button").click(function() {
$("#update_div").html("New stuff");
})
</script>
Thank you.

You can even inline JavaScript code in your HTML but that is a horrible practice unless you know exactly what you're doing. Reads as:
<div id="update_div">Old stuff</div>
<input type="button" value="Update" onclick="$('#update_div').html('...')" />
If you want to encode the knowledge of what gets updated with that on click, then you can encode that knowledge in the HTML elements itself.
<div id='target'>Old</div>
<input type='button' value='Update' data-target='#target' date-value='New' />
In jQuery's onload, define this for all such buttons:
Since the data seems to be static here, a better global approach might be to define the data on the elements itself, and setup all handlers in one global sweep of the DOM.
$(function() {
$(':button').click(function() {
var dest = $(this).attr('data-target');
var value = $(this).attr('data-value');
$(dest).html(value);
});
});
The above code still requires external JavaScript but only need it once for all such button and div elements on the page.