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AJAX: Using the XMLHttpRequest Object and Fetch API

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AJAX: Using the XMLHttpRequest Object and Fetch API

Learn two ways of using AJAX in JavaScript to send and receive data from a server using the XMLHttpRequest object and Fetch API.

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We can use AJAX in JavaScript in two ways: using the XMLHttpRequest object and Fetch API. Fetch is a modern concept equivalent to XMLHttpRequest. It offers a lot of the same functionality as XMLHttpRequest, but is designed to be more extensible and efficient (MDN docs). In this article, I will introduce how to send and receive data from a server by using the XMLHttpRequest object and Fetch API through examples. I will also introduce errors that we may find and how to fix them.

Sending and Retrieving Data

The two most common methods for communicating between a client and server are GET and POST.

Using GET

If we are using XMLHttpRequest, the code looks like this:

<!DOCTYPE html>
<html>
<body>
<h2>The XMLHttpRequest Object</h2>
<button type="button" onclick="loadDoc()">Request data</button>
<p id="demo"></p>
<script>
function loadDoc() {
var xhttp = new XMLHttpRequest();
  xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
       // retrieve data from server
        document.getElementById("demo").innerHTML = this.responseText;
    }
  };
// send data to server
xhttp.open("GET", "AjaxExample.php?", true);
xhttp.send();
}
</script>
</body>
</html>

If we are using Fetch, the code looks like this:

<!DOCTYPE html>
<html>
<body>
<h2>The Fetch API</h2>
<button type="button" onclick="loadDoc()">Request data</button>
<p id="demo"></p>
<script>
function loadDoc() {
fetch('AjaxExample.php?', {method: 'get'})
.then(function(response) {
       // retrieve data from server
       if(response.ok){
              return response.text();}
       })
.then(function(text){
         document.getElementById("demo").innerHTML = text;
       })
}
</script>
</body>
</html>

The AjaxExample.php file looks like this:

$data = $_GET['name'];
echo $data;

Using POST

If we want to send a large amount of data to the server, we can use the POST method. POST is more robust and secure than GET. The following examples use POST with XMLHttpRequest and FetchAPI:

XMLHttpRequest

 <!DOCTYPE html>
<html>
<body>
<h2>The XMLHttpRequest Object</h2>
<button type="button" onclick="loadDoc()">Request data</button>
<p id="demo"></p>
<script>
function loadDoc() {
var json_upload = "json_name=" + JSON.stringify({name:"Ngoc Minh", time:"2:30 pm"});
var xhttp = new XMLHttpRequest();
  xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
       document.getElementById("demo").innerHTML = this.responseText;      
    }
  };
xhttp.open("POST", "AjaxExample.php");
xhttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
xhttp.send(json_upload);
}
</script>
</body>
</html>

Fetch

<!DOCTYPE html>
<html>
<body>
<h2>The Fetch API</h2>
<button type="button" onclick="loadDoc()">Request data</button>
<p id="demo"></p>
<script>
function loadDoc() {
var json_upload =  "json_name=" + JSON.stringify({name:"Ngoc Minh", time:"2:30 pm"});
fetch('AjaxExample.php', {
       method: 'post',
       headers: {
      "Content-Type":"application/x-www-form-urlencoded"
    },
       body: json_upload })
       .then(function(response) {
              if(response.ok)
                     return response.text();
       })
       .then(function(data){
              document.getElementById("demo").innerHTML = data;
       })
}
</script>
</body>
</html>

AjaxExample.php looks like this:

$json_data = $_POST['json_name'];
$json = json_decode($json_data,true);
echo "<h2>Wellcome you, ".$json['name']." and ".$json['time']."</h2>"

Error Handling

Consider the following example:

var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
       //retrieve data from server
document.getElementById("demo").innerHTML = this.responseText;
    }
  };
// send data to server
xhttp.open("GET", "AjaxExample.php?", true);
xhttp.send();

If I type AjxExample.php instead of AjaxExample.php, as in the following code:

xhttp.open("GET", "AjxExample.php?", true);
xhttp.send();

When we click the button, the following error will occur in the debug tool of the browser (I am using Chrome):

Image title

In the source code:

Image title

Now, if we want to display  messages to viewers when errors occur, the above examples can change as follows:

XMLHttpRequest

var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
    if (this.readyState == 4) {
       if (this.status == 200)
              document.getElementById("demo").innerHTML = this.responseText;
       else
              {
                if(this.status==404)
                   document.getElementById("demo").innerHTML = "Not Found.";
                  else
                      if(this.status==403)
                        document.getElementById("demo").innerHTML = "Forbidden";
                   else
                      document.getElementById("demo").innerHTML = "Something wrong.";
              }     
    }

  };

xhttp.open("GET", "AjaxExample.php?", true);
xhttp.send();

Fetch:

fetch('AjaxExample.php?', {
       method: 'get'})
       .then(function(response) {
              if(response.ok){
                  return response.text();
              }
              else{
                    if(response.status==404)
                       return Promise.reject("Not Found.");
                     else
                           if(response.status==403)
                             return Promise.reject("Forbidden.");
                     else
                           return Promise.reject("Something wrong.");
              }     
       })
       .then(function(text){
            document.getElementById("demo").innerHTML = text;
       })
       .catch(function(err) {
              document.getElementById("demo").innerHTML = err;
       })

If we are using the Fetch API, when working with JSON data, the following error usually occurs:

Image title

We can solve this problem as follows:

let contentType = response.headers.get('content-type')
if (contentType.includes('application/json')) {
       return response.json();
}
else if (contentType.includes('text/html')) {
      return response.text();
}

Conclusion

There are a lot of helpful articles and documents about XMLHttpRequest and Fetch API, but hopefully, with this article, I have contributed another helpful article about them.

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Topics:
javascript ,ajax ,integration

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