[javascript] Set Value of Input Using Javascript Function

I'm currently using a YUI gadget. I also do have a Javascript function to validate the output that comes from the div that YUI draws for me:

Event.on("addGadgetUrl", "click", function(){
    var url = Dom.get("gadget_url").value;  /* line x ------>*/
    if (url == "") {
        error.innerHTML = "<p> error" /></p>";
    } else {
        /* line y ---> */ 
        /*  I need to add some code in here to set the value of "gadget_url" by "" */
    }
}, null, true);

Here is my div:

<div>
<p>URL</p>
<input type="text" name="gadget_url" id="gadget_url" style="width: 350px;" class="input"/>
<input type="button" id="addGadgetUrl" value="add gadget"/>
<br>
<span id="error"></span>
</div>

As you can see my question is, how can I set the value of gadget_url to be ""?

document.getElementById('gadget_url').value = '';

Depending on the usecase it makes a difference whether you use javascript (element.value = x) or jQuery $(element).val(x);

When x is undefined jQuery results in an empty String whereas javascript results in "undefined" as a String.

I'm not using YUI, but in case it helps anyone else - my issue was that I had duplicate ID's on the page (was working inside a dialog and forgot about the page underneath).

Changing the ID so it was unique allowed me to use the methods listed in Sangeet's answer.

Try

gadget_url.value=''

addGadgetUrl.addEventListener('click', () => {_x000D_
   gadget_url.value = '';_x000D_
});

<div>_x000D_
  <p>URL</p>_x000D_
  <input type="text" name="gadget_url" id="gadget_url" style="width: 350px;" class="input" value="some value" />_x000D_
  <input type="button" id="addGadgetUrl" value="add gadget" />_x000D_
  <br>_x000D_
  <span id="error"></span>_x000D_
</div>

Update

I don't know why so many downovotes (and no comments) - however (for future readers) don't think that this solution not work - It works with html provided in OP question and this is SHORTEST working solution - you can try it by yourself HERE

The following works in MVC5:

document.getElementById('theID').value = 'new value';

document.getElementById('gadget_url').value = 'your value';