Call PHP function and return value to Javascript

First thing first. If you are working on wordpress, you should call ajax in wordpress way

See : https://codex.wordpress.org/AJAX_in_Plugins

Your javascript should be

onsubmit: function( e ) {
    var str = '';                                                                    
    if(e.data.friend_cb ) {                                                                            
        str += 'I have [value - X] points. <br><br/>';
    }

    editor.insertContent(str);

    jQuery.ajax({
        url: ajaxurl,
        type: 'POST',
        data: {
            action:'my_ajax_function',
            functionname: 'getX', 
            condition_code: condition_code
        },
        error:function(data){
            alert("failed");
            console.log(data);
        },
        success: function(data) {
            alert("success");                                                                                
            console.log(data); // Inspect this in your console
        }
    });
}

Your PHP code should be

add_action( 'wp_ajax_my_ajax_function', 'my_ajax_function' );
add_action( 'wp_ajax_nopriv_my_ajax_function', 'my_ajax_function' );

function my_ajax_function(){
    if( !isset($_POST['condition_code']) ) {
          $error .= 'No function no_friend!';          
          $condition_code = $_POST['condition_code'];
    }

    $functionName  = $_POST['functionname'];
          // $functionName = 'add_bonus_point';
    switch($functionName) {
        case 'set_no_friend':  


             //Check did it pass the functionName
            if( !isset($_POST['functionname']))
                  $error .= 'No function name!'; 
            else
                 $errorBool = false;
            break;
        case 'try_insert':
            getX();
            break;
    }
}

function getX(){
     // access global variable $wpdb database connection object
     global $wpdb; 
     $x = 0;
     //Connect to database, get X value.
     return $x;
}

You can put json array for get data. Please write below code.

function getX()
 {
     $x = 0;
     //Connect to database, get X value.
     $data = array(
            "x"     => 0
          );
      echo json_encode($data);

 }

And Javascript code like below.

jQuery.ajax({
    url: 'http://localhost:8080/wordpress/wp-content/plugins/databaseConnection.php',
    type: 'POST',
    data: {functionname: 'getX', condition_code: condition_code},
        error:function(data)
        {
            alert("failed");
            console.log(data);
        },
        success: function(data) 
        {
            var obj = jQuery.parseJSON(data);
            alert( obj.x );

            console.log(obj); // Inspect this in your console
        }
});

for example

<script type="text/javascript">
  var phpVariable = "<?php
  $x = getX();
  echo $x;
  ?>";
</script>

store the php value into a javascript variable 1st, then use it in your javascript. Just make sure that this variable is declared and assigned value to before the script that uses it.