Make Ajax Requests in JavaScript
Tags: javascript, webdev, tutorial
Published: 2022-04-19 15:24:37

Ajax is a set of web development techniques on the client side to exchange data with a server asynchronously. It allows updating parts of a web page dynamically without the need of reloading the whole page. In this post, we show you how to make Ajax requests by using jQuery and XMLHttpRequest object.

## jQuery

The [jQuery](https://jquery.com/) JavaScript library allows developers to use Ajax easily.
The jQuery ajax() method is used on the client side to make an Ajax request asynchronously.
To use the jQuery library, we need to reference it with the HTML `<script>` tag inside the `<head>` section.
```html
<head>
    <!-- other codes ... -->
    <script src="https://code.jquery.com/jquery-3.5.1.min.js" 
    integrity="sha256-9/aliU8dGd2tb6OSsuzixeV4y/faTqgFtohetphbbj0=" 
    crossorigin="anonymous"></script>
    <!-- other codes ... -->
</head>
```

By including the jQuery library file from a CDN (Content Delivery Network) as shown above, we don't need to download and host the library file. The `integrity` and `crossorigin` attributes are used to verify the resource by using the [Subresource Integrity (SRI) checking](https://www.w3.org/TR/SRI/).

We can add a button on the web page for calling the backend PHP.
```html
<button id="demo">Call PHP</button>
```

The following JavaScript code block is for the button click event.
When the button is clicked, the jQuery ajax() method will perform an Ajax request to the backend PHP.
```javascript
<script>
    $(document).ready(function() {
        $("#demo").click(function(){
            var student = {"name":"Tom", "sex":"M", "age":"11", "class":"6Y"};
            $.ajax({
                type: 'POST',
                url: 'my-backend.php',
                data: JSON.stringify(student),
                success: function(data, textStatus, xhr) {
                    alert(data);
                },
                error: function(xhr, textStatus, errorThrown) {
                    alert('Error Msg: ' + errorThrown);
                }
            });        
        });
    });
</script>
```

The JavaScript variable `student` defines an object that stores a student's personal data. In the `ajax()` method, we set the request `type` to POST. In `url`, we set the PHP file that will receive the request. The JavaScript function `JSON.stringify()` is used to convert the object `student` to a JSON string before it is sent to the server. The functions in `success` and `error` will be called respectively when the request succeeds or fails. Here, an alert box with the data returned from the server will display if the request succeeds. If it fails, an alert box will display the textual portion of the HTTP status `errorThrown`.

In the backend PHP file my-backend.php, we can do like this:
```php
<?php
    $json = file_get_contents('php://input');
    $data = json_decode($json);
    echo $data->name;
?>
```

The function `file_get_contents()` is used to get the data sent to the server. The function `json_decode()` is then called to convert the JSON encoded string `$json` into a PHP variable `$data`. Finally, the student's name `$data->name` is returned to the client side.

## XMLHttpRequest object

XMLHttpRequest (XHR) is the other option for developers in Ajax programming. Not just XML, it can be used to fetch any type of data from a server asynchronously. It allows updating parts of a web page without reloading the whole page. To try how it works, just add a button in a web page to trigger a user event to make an Ajax request to the server.
```html
<button id="demo" onclick="makeAjaxCall()">Call PHP</button>
```

In JavaScript, we make the function `makeAjaxCall()` to handle the button click event and make an Ajax request by using XMLHttpRequest.
```javascript
function makeAjaxCall(){
        var request = new XMLHttpRequest();
        request.open('POST', 'my-backend.php');

        var student = {"name":"Tom", "sex":"M", "age":"11", "class":"6Y"};
        request.send(JSON.stringify(student));

        request.onload = function() {
            if (request.readyState == 4) {
                //the operation is done
                if (request.status == 200) {
                    alert(request.response);
                } else if (request.status >= 400){
                    //client or server errors
                    alert('Error Code: ' + request.status + ' Error Msg: ' + request.statusText);
                }

            }

        }

    }
```

First, the object XMLHttpRequest is initialized.
```javascript
var request = new XMLHttpRequest();
```

Then, its method `open()` is called to create a new request.
The first parameter defines the request method `POST`. The second parameter is for the URL to send the request to. Here, we set it as the backend PHP file `my-backend.php`. We define a data object `student` for storing a student's personal data. The XMLHttpRequest method `send()` is called to send the request to the server with the data `student`. The JavaScript function `JSON.stringify()` is used to convert the object `student` to a JSON string before it is sent to the server. The XMLHttpRequest `onload` property is set as a callback function that will be called when the request transaction completes successfully. 

After the request is sent, the XMLHttpRequest client will be in the following states:
* 0 UNSENT
* 1 OPENED
* 2 HEADERS_RECEIVED
* 3 LOADING
* 4 DONE
 
In XMLHttpRequest's `onload` callback function, we can check
the `request.readyState` property for its state. If it is equal to `4`, it means the operation is done. Then, we continue to check the HTTP status code of the XMLHttpRequest's response `request.status`. If it is equal to `200`, it means that the request has succeeded, and we can display the data returned from the server.
```javascript
alert(request.response);
```

The HTTP response status codes are grouped in following classes:
* Informational responses (100–199)
* Successful responses (200–299)
* Redirects (300–399)
* Client errors (400–499)
* Server errors (500–599)

In the JavaScript code snippet above, if the XMLHttpRequest's response status `request.status` is equal to `400` or over, we will display the error message.
```javascript
alert('Error Code: ' + request.status + ' Error Msg: ' + request.statusText);
```

The backend PHP file my-backend.php gets the JSON data posted to the server, decodes it and outputs the student's name `$data->name` to the client side.
```php
<?php

$json = file_get_contents('php://input');
$data = json_decode($json);
echo $data->name;

?>
```

## References
[HTTP response status codes](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status)

[XMLHttpRequest.readyState](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/readyState)


Thanks for reading!
To find more programming tutorials, please visit: [CodeBilby.com](https://www.codebilby.com)