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Getting array out of data attribute without jQuery

I want to work with an array to pass through using data-attribute.
In my HTML-tag I've this attribute:
data-toshow='["tblp"]'
I can access and use it with jQuery when using
$().data('toshow')
But when using dataset.toshow I don't get the same result. I actually don't get an array.
Can someone explain this to me? And give me the answer how to do the same without the use of jQuery?

jQuery's .data() method automatically tries to convert the string in your custom data attribute to whatever type it appears to be (in this case an array). JavaScript just treats it as a string, so you need to parse the string to get the same array output you get with jQuery. For example:
// jQuery approach
const jqtest = $('div').data('toshow');
console.log(jqtest);
// Plain JavaScript approach
const jstest = JSON.parse(document.querySelector('div').dataset.toshow);
console.log(jstest);
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div data-toshow='["tblp"]'></div>

//Use dataset to get a string of all data-* props
const stringVal = document.querySelector('#divA').dataset['toshow'];
//Parse the value of "data-toshow" to get your array
const array = JSON.parse(stringVal);
console.log(array);
<div id="divA" data-toshow='["tblp"]'></div>

Assuming you have HTML similar to:
<div id="theThing" data-toshow='["tblp"]'></div>
or
<div id="theThing" data-toshow='["tblp","arrItem2","arrItem3"]'></div>
//jQuery
var container_jq = $("#theThing");
var container_jq_dataArr = decodeURIComponent(container_jq.data('toshow')).split(",");
//vanilla
var container_vanilla = document.getElementById("theThing");
var container_vanilla_dataArr = JSON.parse(decodeURIComponent(container_vanilla.dataset.toshow));
console.info({jQuery: container_jq_dataArr,vanilla: container_vanilla_dataArr});
jsfiddle in action

Every HTMLElement has dataset property, this property could be null if there is no data attribute in the element, but if there is any data attribute, the dataset property is an array containing all the data values declared on the element.
Given an html like <div data-name='Usher' data-max-number='5'> There are two ways you can get this data attribute using javascript,
One way is to call the element.getAttribute('data-name') or element.getAttribute('data-max-number') of that element.
The second way is through the dataset property of the element. which you would use element.dataset.name to obtain the name attribute or element.dataset.maxNumber
NOTE: How max-number becomes maxNumber. This is how you access hyphen seperated data-set attribute, using camelCase

loop in JSON with undefined elements

I am trying to figure out how to retrieve the values of a json of which i do not know the number of elements.
Example:
my json can be something like
var json = ["fixelement1":"value1","fixelement2":"value2","fixelement3":"value3","variableelement4":"value4","variableelement5":"value5"]
or
var json =["fixelement1":"value1","fixelement2":"value2","fixelement3":"value3","variableelement7":"value7","variableelement8":"value8", "variableelementN":"valueN"]
the only thing that I know is that the first 3 elements are always the same.
I use .indexOf() to search a value in fixelement3. What I would like to do is, if I find the element, I would like to retrieve the name of all the following elements (which number is variable and that are unknown) and their values.
javascript or jquery would work for me, but I have no idea..
thank you in advance!

var json ={
"fixelement1":"value1",
"fixelement2":"value2",
"fixelement3":"value3",
"variableelement7":"value7",
"variableelement8":"value8",
"variableelementN":"valueN"
}
for(prop in json){
console.log('key ======> value', prop, '=====>', json[prop]);
}

How to write HTML5 Data Attribute as JavaScript Array Elements

I have the following code:
HTML:
<span data-test="15, 17">
JavaScript / JQuery:
$value = [ $(object).data('test') ]
The $value variable now contains an array with one element. This element has the value "15, 17". I want to change the value of data-test in HTML in a way that $value will contain an array with two elements "15" and "17". In my environment I can only change the data-test attribute of the span. I can not change the JavaScript.
jsFiddle how the result currently looks like: ["15, 17"]
But I need it to be: ["15", "17"]
Is there any way?
Edit 1: I want to highlight the following part again: I can not change the JavaScript, only the HTML
Edit 2: I guess from the answers and comments below that with these requirements it is not possible to achieve the result I aimed for.

You need to store the value as a serialised array, in other words like this:
<span data-test="[15, 17]">
When jQuery stores it in the data cache object it will be deserialised to an array:
var value = $('span').data('test'); // == Array
console.log(value.length); // = 2
Updated fiddle

The right answer: is impossible what you want, if you can't change the Javascript.
However, using javascript, an way to do is:
var array = $('#demo').data('test').split(',');
Example: https://jsfiddle.net/3xfez/65/ (Don't forget to open the console.)

You need to store the array in JSON format.
See this fiddle: https://jsfiddle.net/3xfez/62/
The correct data value should be: [15, 17].
<span id="demo" data-test="[15, 17]"></div>

How to point to an JS Array by Name in the Form of String?

I have a JS array.
//This is Dynamic, I maynot know the name in the beginning
var myArray = ["apple","ball","cat"];
var NameOfArray = "myArray";
How can I access myArray if I know only it's name in the form on String?
In PHP, I would use $$. How Do I do that in JS?

Depending on where the array is defined, you can access it directly i.e. if it is defined globally as in your example:
console.log(window[NameOfArray][1]); // Outputs "ball"

Use:
eval("myArray")
It is an array which has the contents of myArray.

If you're able to control the name of the variable NameOfArray, i.e. control the code, you could just set it as a variable on the window in that line using eval
var myArray = [1,2,3];
window.theArrayIWant = eval('myArray');
then using theArrayIWant moving forward

How to retrieve value from object in JavaScript

Hi I am using a Java script variable
var parameter = $(this).find('#[id$=hfUrl]').val();
This value return to parameter now
"{'objType':'100','objID':'226','prevVoting':'" // THIS VALUE RETURN BY
$(this).find('[$id=hfurl]').val();
I want to store objType value in new:
var OBJECTTYPE = //WHAT SHOULD I WRITE so OBJECTTYPE contain 400
I am trying
OBJECTTYPE = parameter.objType; // but it's not working...
What should I do?

Try using parameter['objType'].
Just a note: your code snippet doesn't look right, but I guess you just posted it wrong.

Ok, not sure if I am correct but lets see:
You say you are storing {'objType':'100','objID':'226','prevVoting':' as string in a hidden field. The string is not a correct JSON string. It should look like this:
{"objType":100,"objID":226,"prevVoting":""}
You have to use double-quotes for strings inside a JSON object. For more information, see http://json.org/
Now, I think with $(this).find('[$id=hfurl]'); you want to retrieve that value. It looks like you are trying to find an element with ID hfurl,but $id is not a valid HTML attribute. This seems like very wrong jQuery to me. Try this instead:
var parameter = $('#hfurl').val();
parameter will contain a JSON string, so you have to parse it before you can access the values:
parameter = $.parseJSON(parameter);
Then you should be able to access the data with parameter.objType.
Update:
I would not store "broken" JSON in the field. Store the string similar to the one I shoed above and if you want to add values you can do it after parsing like so:
parameter.vote = vote;
parameter.myvote = vote;
It is less error prone.