Alert with HTML Element

Question

How can I create an Alert Writing Hello+username (Input with id="username" from HTML)

Answer 1

You need to handle some interactivity, by adding at least a button ( or a link, or something else ) to validate user entry ( and showing the alert box ), as you can't automatize the opening of the box ( where, when, and how? ):
<input id="username" type="text">
<input type="button" value="ok" onclick="alert('Hello '+document.getElementById('username').value);">

Answer 2

write some username in text field

Answer 3

var user=document.getElementById('username').value;
alert("Hello " + user);

Answer 4

okay, what should i do in the HTML document to define the .value ?