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Is there a to change the value of an element using JavaScript

I'm trying to change the value of an element on a third-party web page using a JavaScript Add-on to display a hyperlink
I already have the link on the page i would like to be able to click it
I think I'm on the right track using document.getElementById although I'm not sure how to then change the id into a "a href" and then how to pass it back into the value.
Sorry, this is a bit of a tricky situation so I'll try my best to explain it. On a third-party web-page which we use for our HR related tasks, there is a section titled "File Link" although this isn't a link. When you copy and paste the address into a browser it displays the file. What i am trying to do is create a hyperlink on the "File Link" section to remove the need to copy and paste the link. Because this is a third party website. We have access to the JavaScript on the website and need to change the address into a hyperlink. I'm not entirely sure this is possible.The element id is "__C_cb_file_link" and i would like to insert the link address into the element using a variable then add the link parameters into the variable then reinsert it into the element/value.
function linkIt() {
var intoLink = document.getElementById("__C_cb_file_link");
var hLink = "<a href="+intoLink+"</a>;
intoLink.value = hLink;
}
window.onload = linkIt();
<td><div class="sui-disabled" title="">m-files://view/37FF751C-A23F-4233-BD8B-243834E67731/0-46524?object=C46A7624-D24B-45F3-A301-5117EFC1F674</div>
<input type="hidden" name="__C_cb_file_link" id="__C_cb_file_link" value="m-files://view/37FF751C-A23F-4233-BD8B-243834E67731/0-46524?object=C46A7624-D24B-45F3-A301-5117EFC1F674"/></td></tr>

In below code first we read input value with new link (however we can read this value from other html tags), then we remove this element (and button) and add to parent element (of removed input) the new link
function linkIt() {
let intoLink = __C_cb_file_link.value;
let parent = __C_cb_file_link.parentNode;
__C_cb_file_link.remove();
btn.remove();
parent.innerHTML += `${intoLink}`;
}
<input id="__C_cb_file_link" value="https://example.com">
<button id="btn" onclick="linkIt()">Link It</button>

There are a number of issues with your code:
1) The code snippet in your question doesn't run because of a missing " at the end of the second line of the linkIt() function.
2) intoLink is a hidden field so anything you add to it will not be visible in the page
3) Even if point 2 were not true, setting the value of a form field will not cause HTML to appear on the page (at best you might get some plain text in a textbox).
4) "<a href="+intoLink+"</a>" doesn't work because intoLink is a complex object which represents the entire hidden field element (not just its value property). You can't convert a whole object into a string directly. You need to extract the value of the field.
A better way to do this is by creating a new element for the hyperlink and appending it to the page in a suitable place. Also I recommend not adding your event via onload - when written using this syntax only one onload event can exist in a page at once. Since you're amending another page which isn't under your control you don't want to disable any other load events which might be defined. Use addEventListener instead, which allows multiple handlers to be specified for the same event.
Demo:
function linkIt() {
var intoLink = document.getElementById("__C_cb_file_link");
var hLink = document.createElement("a");
hLink.setAttribute("href", intoLink.value);
hLink.innerHTML = "Click here";
intoLink.insertAdjacentElement('beforebegin', hLink);
}
window.addEventListener('load', linkIt);
<td>
<div class="sui-disabled" title="">m-files://view/37FF751C-A23F-4233-BD8B-243834E67731/0-46524?object=C46A7624-D24B-45F3-A301-5117EFC1F674</div>
<input type="hidden" name="__C_cb_file_link" id="__C_cb_file_link" value="m-files://view/37FF751C-A23F-4233-BD8B-243834E67731/0-46524?object=C46A7624-D24B-45F3-A301-5117EFC1F674" /></td>
</tr>
P.S. m-files:// is not a standard protocol in most browsers, unless some kind of extension has been installed, so even when you turn it into a hyperlink it may not work for everyone.

[UPDATE] I supose that your "__C_cb_file_link" was a paragraph so I get the previous text http://mylink.com and create a link with, is it what you want, right?
function linkIt() {
let fileLink = document.getElementById("__C_cb_file_link");
let hLink = fileLink.textContent;
fileLink.innerHTML = ""+hLink+"";
}
linkIt();
<div>
<p id="__C_cb_file_link">http://myLink.com</p>
</div>

How to get html element after append with pure JavaScript?

I found some JQuery solutions, but I am limited by school task restrictions to use pure Javascript, and I need to use specific early appended element that is still not in DOM for replacing by my CKEDITOR.
Code:
function newOption(){
...
mainUL = document.getElementById("myUL");
var inputA = document.createElement("input");
inputA.type ="text";
inputA.style = "margin-right: 45px";
inputA.name = "option[]";
inputA.id = inputID;
mainUL.appendChild(inputA );
CKEDITOR.replace(inputID).setData('Type anything you want ...');
...
}
By replacing my input with CKEDITOR will JS fail, because input, commonly, is still not in DOM. I tried to use
mainUL.innerHTML += "all elements like html text";
and this is working and will immediately insert elements into DOM, but I can't to use innerHTML, because it will remove old listeners (for example checked checkboxes that JS will set from checked to unchecked, what is my main problem due to I have to try using append DOM function).

Try changing the code to wrap the call to CKEDITOR.replace in a setTimeout:
setTimeout(function() {
CKEDITOR.replace(inputID).setData('Type anything you want ...');
},0).
This will allow the browser time to insert the element before trying to replace it.
And I assume that inputID has a valid value in it...

Javascript Generating Text into Textbox?

I wrote this little bit of code but I'm not sure why it's not working? It's supposed to take in the persons name and depending on what they selected it will output a website with their name at the end of it.
JSFiddle
http://jsfiddle.net/tQyvp/135/
JavaScript
function generateDynamicSignature() {
var dynSig = "";
var user = document.getElementById("usernameInput");
var e = document.getElementById("scriptListInput");
var strUser = e.options[e.selectedIndex].text;
if (strUser == "example") {
dynSig = "http://example.com/users/";
}
document.getElementById("generateSignature").addEventListener('click', function () {
var text = document.getElementById('dynamicSignatureOutput');
text.text = (dynSig + user);
});
}
HTML
<select class="form-control" id="scriptListInput">
<option value="example">Example 1</option>
</select>

There are a few problems with your code, I'll try to list them all.
First, you never added the username input to your HTML.
Next, you seem mixed up on the way to access/set the text of an HTML input. You do this through the value field. For the username input, you forgot to access any property, so you'll need to change it to:
var user = document.getElementById("usernameInput").value;
You later used the text property of both the select element and the output. These should also both be value.
Another problem is that you've placed a listener inside a listener. Your outer function, generateDynamicSignature, listens for the onclick event of the button. This function only runs after the button is clicked. But inside this function, you attach a new listener. This inner listener will only run if someone clicks the button twice.
I've included these changes in a new fiddle:
https://jsfiddle.net/zdfnk77u/

where is usernameInput in your html?
in the if, use === instead of ==

If and when you add the missing "usernameInput" element in your HTML, all you'll have to do is...
dynSig='http://example.com/users/'+usernameInput.value;

I think part of the problem is that you want to access the value and not the text of input elements. So for text and strUser, you want to do text.value instead of text.text and such.
Also, based on the JSfiddle, you probably want to rewrite how you're using the document listener and the onclick of the html element. Every time the button is clicked it goes through the generateDynamicSignature and creates a listener to change the value, but doesn't necessarily change the value itself. If you move the logic of the generate function inside the click listener, that should fix most of your problems.

You create your generateDynamicSignature inside $(document).ready.
There are two approaches.
define function generateDynamicSignature outside
$(document).ready
or
bind your button.click to a handler inside $(document).ready
Do not mix these two.

Why are only some attributes updated from my javascript function?

I have a button script to change the buttons in a frame based on the page loaded in the main frame. The problem I'm experiencing is that while the background images, tabindex and text on the button (innerHTML) all change as expected, the onclick doesn't. It appears to completely ignore it. Here's the script I'm using:
function createbutton(btn_N, btn_I, btn_L, btn_D) // (Div Name, Tab Index, Button Text, Page To Load){
var btnN = top.frames['buttonbar'].document.getElementById(btn_N);
btnN.style.cssText = "display:block; cursor:pointer; padding-left:16px; padding-top:5px;";
btnN.onmouseover = function() {this.style.backgroundImage = "url('./osdimages/navBG_roll.png')";};
btnN.onmouseout = function() {this.style.backgroundImage = '';};
btnN.tabindex = btn_I;
btnN.innerHTML = btn_L;
btnN.onclick = btn_D;
}
The button call looks like this:
createbutton("button01", 1, "New Order/Browse", "parent.frames['content'].location.href='createorder/createorder.asp';");

There is a difference between attributes and properties.
The best example of this is as follows:
HTML: <input type="text" value="hello" id="test" />
Type something in the text box
document.getElementById('test').value is whatever you typed
document.getElementById('test').getAttribute("value") is whatever was in the HTML
Some attributes are directly mapped to properties and vice versa, but this is not always the case.
For instance, the onClick attribute takes a string that is then eval'd, but the onclick property takes a function. This is why your code isn't working.
Either pass a valid function, or use setAttribute.

You are setting onclick with a string, it needs a function to execute.
createbutton("button01", 1, "New Order/Browse", function(){ parent.frames['content'].location.href='createorder/createorder.asp'; });

How do read/write <input> values in an HTML document with JavaScript?

I have an <input> in my document. There I want the user to do some input.
var myVar = document.getElementById('userInput').value;
Then I want this value to be shown in another <input>.
document.getElementById('Preview').value = myVar;
This code somehow doesn't work.
Can anybody help?
Thanks in advance!

Update based on further information in comments before:
<button onClick="calculateThatObscenityDeleted()">"Save"</button >
Submit buttons will submit forms, thus running the JS but immediately blanking the form.
(That might still not be the actual issue, the question is still missing most of the code)
Original answer before it was revealed that the question didn't reflect the problem:
var myVar=document.getElementById('userInput').value;
Don't forget the =.
(And, obviously, you need to use that code in an event handler so it isn't executed only when the document loads and before the user has typed anything.)

Is it form?
Try something like this:
oFormObject = document.forms['myform_id'];
oFormObject.elements["element_name"].value = 'Some Value';

Besides the typo, you have to bind an event handler to the first <input>:
var input_a = document.getElementById('userInput');
var input_b = document.getElementById('Preview');
input_a.onkeyup = function(){
input_b.value = input_a.value;
}