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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>

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.

Add JavaScript onClick to dynamically generated asp button

Background My page creates a list of objects based on rows of an SQL Database. For each object, a DIV is dynamically generated that contains a few items including a LinkButton and a further child DIV that is initially hidden. I want the link button to toggle the child DIV's hidden property. The JavaScript is not dynamically generated and is included in the ASPX page.
Problem I don't know how to make this generated LinkButton fire JavaScript that is included in the ASPX page and pass in the correct DIV's ID.
I'm guessing I need to add an attribute to the button like so:
myButton.Attributes.Add(reference to JS function + parameter of DIV's ID)
Maybe like:
myButton.Attributes.Add("onclick", "Show_Hide_Display('"<%="' +idString+ '".ClientID%>"')");
Where the button is given an attribute of a JS onClick handler pointing to the function "Show_Hide_Display" and a parameter of a DIV's ID that is calculated as the rendered ID. This syntax is incorrect though.
How do I write this so it calls 'Show_Hide_Display' and passes the ID of the current child DIV? All of the DIVs have the same ID apart from a number that references their row number, for example '"myDiv_" + counter.ToString'
The JavaScript I am trying to add a call to on the button:
function Show_Hide_Display(divID) {
var div = document.getElementById(divID);
var style = document.defaultView.getComputedStyle(div);
var display = style.getPropertyValue('display');
if (display == '' || display == 'block') {
div.style.display = 'none';
} else {
div.style.display = 'block';
}
}

Use the following syntax ...
myButton.Attributes.Add("onclick", "Show_Hide_Display(this.id);");
the above syntax allows to call the function with id as its parameter.
suggestion:
Try to write a common function which does not depend on generated ids of controls.
If this is not useful for your requirement, please post your code which might gives me a better idea.

If you are using jQuery, you could you jQuery delegate method.
$(document).on("click", "div.parent", function(){
var subDivId = getSubDivByParent(this);
Show_Hide_Display(subDivId);
};
You need to implement getSubDivByParent according your DOM structure.
If you are not using jQuery, you need to attach event yourself. For each dynamically generated element. You need to manually add following script in your server code to register event.
... your html code ...
<script>
var elem = document.getElementById('new-created-element');
elem.addEventListener("click", function(){
var subDivId = getSubDivByParent(this);
Show_Hide_Display(subDivId);
};)
</script>

My suggestion is use jquery to achieve the functionality.
My solution works if you want to toggle the immediate div for the link.just call onclientclick method to toggle the div.
in linkbutton onclientclick="Show_Hide_Display(this)"
function Show_Hide_Display(id) {
$(id).next('div').toggle();
}
I hope this helps you .. Thanks

Change value of textarea when hovering link

I would like to change the value of a textarea when hovering over a link. I am not very proficient at javascript and do not quite understand the intricacies of 'this.' and 'document.' etc..
Currently I have a textarea 'info' that on page load is unpopulated and two links that should change its value. I can not seem to get it to work..
<textarea name="info"></textarea>
Foo.com
Bar.com
I'm sure there is a way to accomplish what I need to do but I can't find it.
Thanks in advance.

Create a function, that accepts the string you want, and sets the textarea:
// Select the textarea by its ID (that you need to give it)
var textarea = document.getElementById('info');
// Define the function that sets the value passed
function changeTextarea( str ) {
textarea.value = str;
}
Assign an ID to the textarea, and call the function in the onmouseover, passing the string you want to set:
<textarea name="info" id='info'></textarea>
Foo.com
Bar.com
Example: http://jsfiddle.net/nmZb9/

How can I write this as a JavaScript function?

I have the following code snippet embedded into some of my divs so when those divs get clicked a certain radio button gets checked.
onclick="document.g1.city[0].checked=true;"
However I would like to convert the above call to a function call like below:
onclick="checkRadioButton(city[0]);"
And the function will be something like this
function checkRadioButton(input){
document.g1.input.checked=true;
}
Is there a way I can accomplish this?

You can write any Javascript code inside the onclick attribute. Remember that city[0] is not defined anywhere. To access it, you must specify the full document.g1.city[0]. So the onclick becomes:
onclick="checkRadioButton(document.g1.city[0]);"
Inside your function, you are already receiving the element, and don't have to retrieve it from the document again. You could directly set it's checked property:
function checkRadioButton(input) {
input.checked = true;
}

onclick="checkRadioButton(document.g1.city[0]);"
checkRadioButton(var input){
input.checked=true;
}
You can also reduce the need for supplying document.g1.city[0] if, for example, every DIV that you mark thusly has an attribute "radioID", and the value of the attribute must match the ID given to the radio button:
onclick="checkRadioButton(this);"
checkRadioButton(var div){
document.getElementById(div.radioID).checked=true;
}