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jQuery ID Return is undefined although HTML ID is defined

I'm trying to retrieve the ID of one element, store it as a variable and then use that ID value to interact with other elements in that section with the same ID.
<div class="mainContent">
<div class="articleContent">
<h1>header1</h1>
<p class="articlePara" id="one">para1</p>
</div>
<div class="articleFooter" id="one" onclick="readMore()">
</div>
</div>
<div class="mainContent">
<div class="articleContent">
<h1>header2</h1>
<p class="articlePara" id="two">para2</p>
</div>
<div class="articleFooter" id="two" onclick="readMore()">
</div>
</div>
And then the JS/jQuery
function readMore() {
var subID = event.target.id;
var newTarget = document.getElementById(subID).getElementsByClassName("articlePara");
alert(newTarget.id);
}
At this point I'm only trying to display the ID of the selected element but it is returning undefined and in most cases people seem to notice that jQuery is getting confused because of the differences between DOM variables and jQuery ones.
jsfiddle: https://jsfiddle.net/dr0f2nu3/
To be completely clear, I want to be able to click on one element, retrieve the ID and then select an element in the family of that clicked element using that ID value.

just remove the getElementsByClassName("articlePara"); in end of the newTarget .already you are call the element with id alert the element of the id is same with target.id
function readMore() {
var subID = event.target.id;
var newTarget = $('[id='+subID+'][class="articlePara"]')
console.log(newTarget.attr('id'));
console.log(newTarget.length);
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="mainContent">
<div class="articleContent">
<h1>header</h1>
<p class="articlePara" id="one"></p>
</div>
<div class="articleFooter" id="one" onclick="readMore()">click
</div>
</div>

As you have read before, you should keep your id's unique, and you should avoid using onclick in html, but you could do it like this.
With querySelector you get the element and then with parentElement you can retrieve the parent of that element.
function readMore(el) {
var articleFooterId = el.id;
var articlePara = document.querySelector(".articleContent #"+articleFooterId);
var articleContent = articlePara.parentElement;
console.log('articleFooter', articleFooterId);
console.log('articlePara', articlePara);
console.log('articleContent', articleContent);
}
In your html you can return the 'this' object back to the function by doing readMore(this).
<div class="mainContent">
<div class="articleContent">
<h1>header1</h1>
<p class="articlePara" id="one">para1</p>
</div>
<div class="articleFooter" id="one" onclick="readMore(this)">footertext</div>
</div>
<div class="mainContent">
<div class="articleContent">
<h1>header2</h1>
<p class="articlePara" id="two">para2</p>
</div>
<div class="articleFooter" id="two" onclick="readMore(this)">footertext</div>
</div>
jsfiddle

if you're using Jquery:
$(function () {
$('div.articleFooter').click(function () {
var para = $(this).prev().find('p.articlePara').text();
alert('T:' + para);
});
})

$('.articleFooter').click(function() {
var b=subId; //can be any
var a="p[id="+b+"]"+"[class='articlePara']";
$(a).something;
});

You have forgotten to pass in event as parameter in your onclick= call in html.
In your javascript, you need to include event in the parenthesis as well.
window.readMore = function(event) {...}
if you write document.getElementById(subID).getElementsByClassName("articlePara"); That's saying you want to get your clicked element's CHILD elements that have class equal to articlePara . There is none. So you get undefined.
If you want to find all element with a ID one and a class articlePara, it can be done easily with jQuery:
newtarget = $("#one.articlePara");
You can insert a line: debugger; in your onclick handler function to trigger the browser's debugging tool and inspect the values of variables. Then you will know whether you are getting what you want.

Jquery get closest a href attribute

I have the below markup and I am trying to get the href but always getting undefined. Any help is appreciated.
<div class="wrapper">
<span class="mixSpanLeft" style="background-image: url(http://cdn.wallpapersafari.com/29/20/3HE5Mx.jpg)">
</span>
<div class="mixDivRight">
<p class="bottomP"><button>Select</button><p>
</div>
</div>
$container = $('.wrapper');
$container.on('click', '.bottomP', function (event) {
console.log($(this).closest('a').attr('href'));
});

Assuming that you fix the class/ID issue noted in the comments by Mohammad you could use:
$('.wrapper').on('click', '.bottomP', function (event) {
console.log($(this).closest('.wrapper').find('a').attr('href'));
});
$('.wrapper').on('click', '.bottomP', function (event) {
console.log($(this).closest('.wrapper').find('a').attr('href'));
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="wrapper">
<span class="mixSpanLeft" style="background-image: url(http://cdn.wallpapersafari.com/29/20/3HE5Mx.jpg)">
</span>
<div class="mixDivRight">
<p class="bottomP"><button>Select</button><p>
</div>
</div>

Aside from what Mohammad mentioned about needing to use .wrapper instead of #wrapper. I recommend using .find() instead of .closest(). .closest() does not work in IE, but that might not be an issue for you. you can also do something like this:
$("div.wrapper").on('click', '.bottomP', function () {
console.log($("div.wrapper a:first").attr('href'));
});
This will grab the first <a> tag inside the wrapper div.

jquery multiple show/hide buttons

I am trying to convert a lot of JavaScript to jQuery and I have a task that I believe has a simple jQuery shortcut, but I don't know what it is and I haven't been able to find an example.
I have a page with many toggled divs. They are in the form that follows where ### is a unique integer for each pair.
<button class='togglebutton' onclick="toggle('div###');">+</button>
<div id='div###'>...some text...</div>
I'd assume the shortcut is something like:
$('.togglebutton').onclick(function() {
var divid = '#div'+?????;
$(divid).toggle();
});
My theory... if I give each button an id, such as 'button###', then I can use substring to get the value after the word 'button', something like:
$(this).id().substring(6,3);
Obviously, that didn't work. So, I figured that I should ask if there is a simple shortcut in jQuery to pair a showhide button with a separate div.

You may want to do this no matter where your div locates http://jsfiddle.net/tg5op333/25/
HTML
<button class='togglebutton' data-id="1">+</button>
<div id='1' style="display:none">...some text...</div>
<button class='togglebutton' data-id="2">+</button>
<div id='2' style="display:none">...some text...</div>
<button class='togglebutton' data-id="3">+</button>
<div id='3' style="display:none">...some text...</div>
<button class='togglebutton' data-id="4">+</button>
<div id='4' style="display:none">...some text...</div>
JS:
$('.togglebutton').click(function() {
$("#"+ $(this).data('id')).toggle();
});

If the HTML is ordered like you show in your question, then use:
$('.togglebutton').click(function() {
$(this).next().toggle();
});
$('.togglebutton').click(function() {
$(this).next().toggle();
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<button class='togglebutton'>+</button>
<div id='div1'>...some text...</div>
<button class='togglebutton'>+</button>
<div id='div2'>...some text...</div>
<button class='togglebutton'>+</button>
<div id='div3'>...some text...</div>

Two ways:
1) You can use some common class attribute to all dives and select all using that.
Or
2) Or else you can use this wildcard selector :
$("[id^=div]")
It gives you all divs whose id starts with div.

Select div children

I want to select the child of a div using jquery. I try with children() but didn't work
<div class="main" id="this_456" onclick="change(456)">
<div id="title">some text</div>
<div id="body">some text as well</div>
</div>
javascript
function change(id)
{
$('#this_'+id).children("#body").fadeOut();
}

Your code works if you specify 456 as the argument rather than this_456 (see: http://jsfiddle.net/aLxTz/).
However, since <div id="body"/> is identified by ID (#body) it's redundant to look for it inside and other element - it should be unique document-wide. Use the class="" attribute if you expect to have several instances of a body <div/>, e.g. <div class="body">...</div>.
Furthermore, note that the onclick handler has the this variable set to the context element. Since this is the element in question itself, you could write
<div class="main" id="this_456"> ... </div>
$(".main").click(function() {
$(this).chlidren(".body").fadeOut();
});

<div class="main" id="this_456" onclick="change(this)">
<div id="title">some text</div>
<div id="body">some text as well</div>
</div>
function change(elm) {
$(elm).children("#body").fadeOut();
}
FIDDLE

Try this code:
function change(id) {
$('#this_'+id+ ' > div').find("#body").fadeOut();
}

Accessing a inner div value in javascript, with no ids to help

I have following structure
<div onClick="javascript:Myfunction('value');">
<div title="Mytitle"> </div>
</div>
Can I access in the javascript Myfunction, the title of the inner div.
There are no ids here.

If you do not want to change the html code then you can use this.
function MyFunction ( elem )
{
var child = jQuery(elem).find("div");
alert ( child.attr("title") );
}
<div onclick="MyFunction(this);">
<div title="Mytitle"> </div>
</div>
Otherwise try this
$(document).ready ( function () {
$('#divMain').click(function() {
var titleElem = $(this).find("div");
alert ( titleElem.attr("title") );
});
});
<div id="divMain" style="width: 100px; height: 100px;">
<div title="Mytitle">
</div>
</div>

Depends if you use any JavascriptLibraries like jQuery. If you do, you can select your objects via CSS selectors, which would be in your case
$('div[onclick] > div[title]')
You'd get your inner DIV element with it. If there are more elments that match this criteria, you could limit them even more by attribute values.

If it's the only div with the title you can use this selector:
$('div[title]')
The easiest solution would to add a class or id to the div with onClick defined (I suggest moving onlick to JS code as well)
<div class="myClass">
<div title="MyTitle"> ... </div>
</div>
And in JS:
$('.myClass').click(function() {
MyFunction('value');
});
Then you can find inner div with
$('.myClass div')