How to read the post request parameters using JavaScript

If you're working with a Java / REST API, a workaround is easy. In the JSP page you can do the following:

    <%
    String action = request.getParameter("action");
    String postData = request.getParameter("dataInput");
    %>
    <script>
        var doAction = "<% out.print(action); %>";
        var postData = "<% out.print(postData); %>";
        window.alert(doAction + " " + postData);
    </script>

I have a simple code to make it:

In your index.php :

<input id="first_post_data" type="hidden" value="<?= $_POST['first_param']; ?>"/>

In your main.js :

let my_first_post_param = $("#first_post_data").val();

So when you will include main.js in index.php (<script type="text/javascript" src="./main.js"></script>) you could get the value of your hidden input which contains your post data.

<script>
<?php
if($_POST) { // Check to make sure params have been sent via POST
  foreach($_POST as $field => $value) { // Go through each POST param and output as JavaScript variable
    $val = json_encode($value); // Escape value
    $vars .= "var $field = $val;\n";
  }
  echo "<script>\n$vars</script>\n";
}
?>
</script>

Or use it to put them in an dictionary that a function could retrieve:

<script>
<?php
if($_POST) {
  $vars = array();
  foreach($_POST as $field => $value) {
    array_push($vars,"$field:".json_encode($value)); // Push to $vars array so we can just implode() it, escape value
  }
  echo "<script>var post = {".implode(", ",$vars)."}</script>\n"; // Implode array, javascript will interpret as dictionary
}
?>
</script>

Then in JavaScript:

var myText = post['text'];

// Or use a function instead if you want to do stuff to it first
function Post(variable) {
  // do stuff to variable before returning...
  var thisVar = post[variable];
  return thisVar;
}

This is just an example and shouldn't be used for any sensitive data like a password, etc. The POST method exists for a reason; to send data securely to the backend, so that would defeat the purpose.

But if you just need a bunch of non-sensitive form data to go to your next page without /page?blah=value&bleh=value&blahbleh=value in your url, this would make for a cleaner url and your JavaScript can immediately interact with your POST data.

function getParameterByName(name, url) {
            if (!url) url = window.location.href;
            name = name.replace(/[\[\]]/g, "\\$&");
            var regex = new RegExp("[?&]" + name + "(=([^&#]*)|&|#|$)"),
                results = regex.exec(url);
            if (!results) return null;
            if (!results[2]) return '';
            return decodeURIComponent(results[2].replace(/\+/g, " "));
        }
var formObj = document.getElementById("pageID");
formObj.response_order_id.value = getParameterByName("name");

It depends of what you define as JavaScript. Nowdays we actually have JS at server side programs such as NodeJS. It is exacly the same JavaScript that you code in your browser, exept as a server language. So you can do something like this: (Code by Casey Chu: https://stackoverflow.com/a/4310087/5698805)

var qs = require('querystring');

function (request, response) {
    if (request.method == 'POST') {
        var body = '';

        request.on('data', function (data) {
            body += data;

            // Too much POST data, kill the connection!
            // 1e6 === 1 * Math.pow(10, 6) === 1 * 1000000 ~~~ 1MB
            if (body.length > 1e6)
                request.connection.destroy();
        });

        request.on('end', function () {
            var post = qs.parse(body);
            // use post['blah'], etc.
        });
    }
}

And therefrom use post['key'] = newVal; etc...

POST variables are only available to the browser if that same browser sent them in the first place. If another website form submits via POST to another URL, the browser will not see the POST data come in.

SITE A: has a form submit to an external URL (site B) using POST SITE B: will receive the visitor but with only GET variables