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Array to Newline String to Array again through HTML

I have an array that comes in from from my API that I would like to arrange in a way that is better for the user (namely, in a column as opposed to the typical comma separated printed array).
This is my JS Fiddle to give a clearer picture: https://jsfiddle.net/2z89owas/
My question is, how can I get output3 to display just like output (and maintain its status as an iterable array like it was as dates)?

First you should not be using value for an html element. You can use .value for extracting value from inputs. Change your line to:
var val = document.getElementById('output2').innerHTML;
Afterwards, you have to split the same way you did join.
var dates3 = val.split('<br>');
document.getElementById('output3').innerHTML = dates3;

You can directly use join, something like:
document.getElementById('output3').innerHTML = dates.join(',');

You can try mapping over the contents of dates instead, as so:
let datesElem = dates.map(date =>`<p>${date}</p>`);
// test: console.log(datesElem)
document.getElementById('output3').innerHTML = datesElem

Dynamic string cutting

Okay, so I have a filepath with a variable prefix...
C:\Users\susan ivey\Documents\VKS Projects\secc-electron\src\views\main.jade
... now this path will be different for whatever computer I'm working on...
is there a way to traverse the string up to say 'secc-electron\', and drop it and everything before it while preserving the rest of it? I'm familiar with converting strings to arrays to manipulate elements contained within delimiters, but this is a problem that I have yet to come up with an answer to... would there be some sort of regex solution instead? I'm not that great with regex so I wouldn't know where to begin...

What you probably want is to do a split (with regex or not):
Here's an example:
var paragraph = 'C:\\Users\\susan ivey\\Documents\\VKS Projects\\secc-electron\\src\\views\\main.jade';
var splittedString = paragraph.split("secc-electron"); // returns an array of 2 element containing "C:\\Users\\susan ivey\\Documents\\VKS Projects\\" as the first element and "\\src\\views\\main.jade" as the 2nd element
console.log(splittedString[1]);
You can have a look at this https://www.w3schools.com/jsref/jsref_split.asp to learn more about this function.

With Regex you can do:
var myPath = 'C:\Users\susan ivey\Documents\VKS Projects\secc-electron\src\views\main.jade'
var relativePath = myPath.replace(/.*(?=secc-electron)/, '');
The Regex is:
.*(?=secc-electron)
It matches any characters up to 'secc-electron'. When calling replace it will return the last part of the path.

You can split the string at a certain point, then return the second part of the resulting array:
var string = "C:\Users\susan ivey\Documents\VKS Projects\secc-electron\src\views\main.jade"
console.log('string is: ', string)
var newArray = string.split("secc-electron")
console.log('newArray is: ', newArray)
console.log('newArray[1] is: ', newArray[1])

Alternatively you could use path.parse(path); https://nodejs.org/api/path.html#path_path_parse_path and retrieve the parts that you are interested in from the object that gets returned.

Get data from URL address

I have a website like below:
localhost:3000/D129/1
D129 is a document name which changes and 1 is section within a document.
Those two values change depends on what user selects.
How do I just extract D129 part from the URL using javascript?

window.location.pathname.match(/\/([a-zA-Z\d]*)/)[1]
^ that should get you the 1st string after the slash
var path = "localhost:3000/D129/1";
alert(path.match(/\/([a-zA-Z\d]*)/)[1])

You can use .split() and [1]:
a = "localhost:3000/D129/1";
a = a.split("/");
alert(a[1]);
This works if your URLs always have the same format. Better to use RegEx. Wanted to answer in simple code. And if you have it with http:// or something, then:
a = "http://localhost:3000/D129/1";
a = a.split("/");
alert(a[3]);
ps: For the RegEx version, see Tuvia's answer.

Javascript Object not working with Native javascript methods like match(), replace etc

Here is the issue:
I go this Code:
var str = {"Acc":10 , "adm_data":"Denied"};
When I do something like:
console.log(str.Acc.match(/[0-9]+/g)) // To Get the Integer Value from the "Acc" key
Firebug Screams:
TypeError: str.Acc.match is not a function
console.log(str.Acc.match(/[0-9]+/g));
See Image:
I always do something like:
var str = "Hello _10";
console.log(str.match(/[0-9]+/g)) // This Works
Why is the Object thingi not working?
PLEASE NOTE:
As mentioned by #Fabrício Matté. The issue was that I was trying to
pass an integer Value to the .match method which does not belong
to integers. The solution was to do what #kundan Karn Suggested. Something like:
str.Acc.toString().match(/[0-9]+/g)// Converting it first to string then match. It worked!

match function works with string. So convert it to string first
str.Acc.toString().match(/[0-9]+/g)

It works just fine: http://jsfiddle.net/nKHLy/
but in order to get rid of the error you might want to try:
var str = {"Acc":"Hello_10" , "adm_data":"Denied"};
console.log(String(str.Acc).match(/[0-9]+/g));
or
var str = {"Acc":"Hello_10" , "adm_data":"Denied"};
console.log(str.Acc.toString().match(/[0-9]+/g));
To know the difference between the 2 options, check: What's the difference between String(value) vs value.toString()

Why is my .data() function returning [ instead of the first value of my array?

I want to pass an array into a jQuery data attribute on the server side and then retrieve it like so:
var stuff = $('div').data('stuff');
alert(stuff[0]);
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.6.1/jquery.min.js"></script>
<div data-stuff="['a','b','c']"></div>
Why does this appear to alert '[' and not 'a' (see JSFiddle link)
JSFiddle Link: http://jsfiddle.net/ktw4v/3/

It's treating your variable as a string, the zeroth element of which is [.
This is happening because your string is not valid JSON, which should use double-quotes as a string delimiter instead of single quotes. You'll then have to use single-quotes to delimit the entire attribute value.
If you fix your quotation marks your original code works (see http://jsfiddle.net/ktw4v/12/)
<div data-stuff='["a","b","c"]'> </div>
var stuff = $('div').data('stuff');
When jQuery sees valid JSON in a data attribute it will automatically unpack it for you.

Declaring it as an attribute means that it is a string.
So stuff[0] would be equivalent to: var myString = "['a','b','c']"; alert(myString[0]);
You need to make it look like this:
<div data-stuff="a,b,c"></div>
var stuff = $('div').data('stuff').split(',');
alert(stuff[0]);
Retraction: jQuery's parsing fails because it didn't meet the rules of parseJSON.
However, I will stand behind my solution. There are aspects of the others that are less than ideal, just as this solution is less than ideal in some ways. All depends on what your paradigms are.

As others have identified the value is treated as string so it is returning "[". Please try this (aaa is the name of the div and I took out the data-stuff):
$(function(){
$.data($("#aaa")[0],"stuff",{"aa":['a','b','c']});
var stuff = $.data($("#aaa")[0],"stuff").aa;
alert(stuff[0]); //returns "a"
});

A different approach is posted at jsfiddle; var stuff = $('div').data('stuff'); stuff is a string with 0th character as '['
Well, var stuff = eval($('div').data('stuff')); should get you an array