ajax，没有jquery的普通javascript：将要作为GET数据发送到php的文本放在哪里？

Question

In my HTML I have this line of code:

<td>Username:</td>
<td><input type="text" size="40" id="username" name="username" /></td>
<td><input type="button" value="check" onclick="checkExistenceUsername()" /></td>
<td id="result_of_checking"></td>   

How I tell to ajax that he has to take the content of the tag with id "username"?

This is javascript:

function checkExistenceUsername() 

{ var xmlhttp;

if (window.XMLHttpRequest) {
    // code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp = new XMLHttpRequest();
} else {
    // code for IE6, IE5
    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}

xmlhttp.onreadystatechange = function() {
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        document.getElementById("result_of_checking").innerHTML = xmlhttp.responseText;
    }
}

xmlhttp.open("GET", "check_existence.php", true);
xmlhttp.send();

}

Answer 1

I think you would need something like this answer

But instead of hard coded parameters you would need to add the input variable:

var params = "myvar="+encodeURIComponent(document.getElementById('username').value);