[javascript] How to clear text area with a button in html using javascript?

I have button in html

<input type="button" value="Clear"> 
<textarea id='output' rows=20 cols=90></textarea>

If I have an external javascript (.js) function, what should I write?

You can simply use the ID attribute to the form and attach the <textarea> tag to the form like this:

<form name="commentform" action="#" method="post" target="_blank" id="1321">
    <textarea name="forcom" cols="40" rows="5" form="1321" maxlength="188">
        Enter your comment here...
    </textarea>
    <input type="submit" value="OK">
    <input type="reset" value="Clear">
</form>

Your Html

<input type="button" value="Clear" onclick="clearContent()"> 
<textarea id='output' rows=20 cols=90></textarea>

Your Javascript

function clearContent()
{
    document.getElementById("output").value='';
}

<input type="button" value="Clear" onclick="javascript: functionName();" >

you just need to set the onclick event, call your desired function on this onclick event.

function functionName()
{
    $("#output").val("");
}

Above function will set the value of text area to empty string.

You need to attach a click event handler and clear the contents of the textarea from that handler.

HTML

<input type="button" value="Clear" id="clear"> 
<textarea id='output' rows=20 cols=90></textarea>

JS

var input = document.querySelector('#clear');
var textarea = document.querySelector('#output');

input.addEventListener('click', function () {
    textarea.value = '';
}, false);

and here's the working demo.