Note: this is different from the placeholder attribute which is like a fake version of value. The value of the input is still an empty string, and as soon as you start typing the placeholder value will disapppear.
<input type="text" placeholder="enter your name">
and here is the "correct way" to clear an entire form:
<form id="form"> <input id="input"> </form>
//document.getElementById('input').value = null; // sinlge input document.getElementById('form').reset(); //all inputs
again this has the added action of removing the focus from the form (prevents double submissions)