Using AJAX in a WordPress plugin

WordPress environment

First of all, in order to achieve this task, it's recommended to register then enqueue a jQuery script that will push the request to the server. These operations will be hooked in wp_enqueue_scripts action hook. In the same hook you should put wp_localize_script that it's used to include arbitrary JavaScript. By this way there will be a JS object available in front end. This object carries on the correct url to be used by the jQuery handle.

Please take a look to:

1. wp_register_script(); function
2. wp_enqueue_scripts hook
3. wp_enqueue_script(); function
4. wp_localize_script(); function

In main plugin file, add these.

add_action( 'wp_enqueue_scripts', 'so_enqueue_scripts' );
function so_enqueue_scripts(){
  wp_register_script( 
    'ajaxHandle', 
    plugins_url('PATH TO YOUR SCRIPT FILE/jquery.ajax.js', __FILE__), 
    array(), 
    false, 
    true 
  );
  wp_enqueue_script( 'ajaxHandle' );
  wp_localize_script( 
    'ajaxHandle', 
    'ajax_object', 
    array( 'ajaxurl' => admin_url( 'admin-ajax.php' ) ) 
  );
}

File: jquery.ajax.js

This file makes the AJAX call.

jQuery(document).ready( function($){
  // Some event will trigger the ajax call, you can push whatever data to the server, 
  // simply passing it to the "data" object in ajax call
  $.ajax({
    url: ajax_object.ajaxurl, // this is the object instantiated in wp_localize_script function
    type: 'POST',
    data:{ 
      action: 'myaction', // this is the function in your functions.php that will be triggered
      name: 'John',
      age: '38'
    },
    success: function( data ){
      //Do something with the result from server
      console.log( data );
    }
  });
});

Also add below codes to plugin main file.

Finally, on your functions.php file, there should be the function triggered by your AJAX call. Remember the suffixes:

1. wp_ajax ( allow the function only for registered users or admin panel operations )
2. wp_ajax_nopriv ( allow the function for no privilege users )

These suffixes plus the action compose the name of your action:

wp_ajax_myaction or wp_ajax_nopriv_myaction

add_action( "wp_ajax_myaction", "so_wp_ajax_function" );
add_action( "wp_ajax_nopriv_myaction", "so_wp_ajax_function" );
function so_wp_ajax_function(){
  //DO whatever you want with data posted
  //To send back a response you have to echo the result!
  echo $_POST['name'];
  echo $_POST['age'];
  wp_die(); // ajax call must die to avoid trailing 0 in your response
}

You need to add an 'action' to your AJAX call.

jQuery.ajax({
  type: "POST",
  url: "/wp-admin/admin-ajax.php",
  data: newcontact,
  action: 'addcontact',
  success: function(data) {
    jQuery("#feedback").html(data);
  }
});

The value should be the same as the add_action hook to wp_ajax. e.g.

add_action( wp_action_{action_value}, 'myfunc' );

This allows WordPress to know which function to run when the AJAX call is made.

This Codex page has some useful info and this article describes how to better refine the code you have.