AJAX

AJAXRead data from a web server - after the page has loadedUpdate a web page without reloading the pageSend data to a web server - in the background

XMLHttpRequest Object

All modern browsers (Chrome, Firefox, IE7+, Edge, Safari, Opera) have a built-in XMLHttpRequest object.

Syntax for creating an XMLHttpRequest

var xhttp = new XMLHttpRequest();

request

$.ajax({
  url: "http://localhost:8080/login",
  type: 'GET',
  // Fetch the stored token from localStorage and set in the header
  headers: {"Authorization": localStorage.getItem('token')}
});

$.ajax({
  url: "http://localhost:8080/login",
  type: "POST",
  headers: { Authorization: $`Bearer ${localStorage.getItem("token")}` },
  data: formData,
  error: function(err) {
    switch (err.status) {
      case "400":
        // bad request
        break;
      case "401":
        // unauthorized
        break;
      case "403":
        // forbidden
        break;
      default:
        //Something bad happened
        break;
    }
  },
  success: function(data) {
    console.log("Success!");
  }
});

XMLHttpRequest Object Methods

Method	Description
new XMLHttpRequest()	Creates a new XMLHttpRequest object
abort()	Cancels the current request
getAllResponseHeaders()	Returns header information
getResponseHeader()	Returns specific header information
open(method, url, async, user, psw)	Specifies the request method: the request type GET or POST url: the file location async: true (asynchronous) or false (synchronous) user: optional user name psw: optional password
send()	Sends the request to the server Used for GET requests
send(string)	Sends the request to the server. Used for POST requests
setRequestHeader()	Adds a label/value pair to the header to be sent

XMLHttpRequest Object Properties

Property	Description
onreadystatechange	Defines a function to be called when the readyState property changes
readyState	Holds the status of the XMLHttpRequest. 0: request not initialized 1: server connection established 2: request received 3: processing request 4: request finished and response is ready
responseText	Returns the response data as a string
responseXML	Returns the response data as XML data
status	Returns the status-number of a request 200: "OK" 403: "Forbidden" 404: "Not Found" For a complete list go to the Http Messages Reference
statusText	Returns the status-text (e.g. "OK" or "Not Found")

Send a Request To a Server

To send a request to a server, we use the open() and send() methods of the XMLHttpRequest object:xhttp.open("GET", "ajax_info.txt", true); xhttp.send();

Method	Description
open(method, url, async)	Specifies the type of request method: the type of request: GET or POST url: the server (file) location async: true (asynchronous) or false (synchronous)
send()	Sends the request to the server (used for GET)
send(string)	Sends the request to the server (used for POST)

A simple GET request:

xhttp.open("GET", "demo_get.asp", true); xhttp.send();

xhttp.open("GET", "demo_get2.asp?fname=Henry&lname=Ford", true); xhttp.send();

fetch('http://example.com/movies.json')
  .then(function(response) {
    return response.json();
  })
  .then(function(myJson) {
    console.log(myJson);
  });