javascript - XMLHttpRequest to Post HTML Form

Question

Current Setup

I have an HTML form like so.

<form id="demo-form" action="post-handler.php" method="POST">
   <input type="text" name="name" value="previousValue"/>
   <button type="submit" name="action" value="dosomething">Update</button>
</form>

I may have many of these forms on a page.

My Question

How do I submit this form asynchronously and not get redirected or refresh the page? I know how to use XMLHttpRequest. The issue I have is retrieving the data from the HTML in javascript to then put into a post request string. Here is the method I'm currently using for my zXMLHttpRequest`'s.

function getHttpRequest() {
    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");
    }
    
    return xmlhttp;
}

function demoRequest() {
       var request = getHttpRequest();
       request.onreadystatechange=function() {
             if (request.readyState == 4 && request.status == 200) {
                   console.log("Response Received");
             }
       }
       request.open("POST","post-handler.php",true);
       request.setRequestHeader("Content-type","application/x-www-form-urlencoded");
       request.send("action=dosomething");
}

So for example, say the javascript method demoRequest() was called when the form's submit button was clicked, how do I access the form's values from this method to then add it to the XMLHttpRequest?

EDIT

Trying to implement a solution from an answer below I have modified my form like so.

<form id="demo-form">
       <input type="text" name="name" value="previousValue"/>
       <button type="submit" name="action" value="dosomething" onClick="demoRequest()">Update</button>
</form>

However, on clicking the button, it's still trying to redirect me (to where I'm unsure) and my method isn't called?

Button Event Listener

document.getElementById('updateBtn').addEventListener('click', function (evt) {
                                evt.preventDefault();
                                
                                // Do something
                                updateProperties();
                                
                                return false;
                            });

Answer 1

The POST string format is the following:

name=value&name2=value2&name3=value3

So you have to grab all names, their values and put them into that format. You can either iterate all input elements or get specific ones by calling document.getElementById().

Warning: You have to use encodeURIComponent() for all names and especially for the values so that possible & contained in the strings do not break the format.

Example:

var input = document.getElementById("my-input-id");
var inputData = encodeURIComponent(input.value);

request.send("action=dosomething&" + input.name + "=" + inputData);

---

Another far simpler option would be to use FormData objects. Such an object can hold name and value pairs.

Luckily, we can construct a FormData object from an existing form and we can send it it directly to XMLHttpRequest's method send():

var formData = new FormData( document.getElementById("my-form-id") );
xhr.send(formData);