A bit of history
In the early years, there was no way to upload files using AJAX/JavaScript that would work in the most recent browsers without having to write different code for specific browsers.
One of the most common ways to simulate upload using AJAX was to submit a form to another page which would make some validations and then save the file in the right folder or in database. Another option was to use something that worked as an intermediary between the browser and the server; some plugins (e.g. jQuery Uploadify) used to use Flash back then.
HTML5, new APIs and new possibilities
With the arrival of HTML5 and its new JavaScript APIs, uploading file using AJAX (e.g., images, documents, PDF) became much simpler. With just a few lines of JavaScript, it makes it possible to upload files using AJAX without any third-party plugin as intermediary or workaround! The API that provides this functionality is the FormData API.
FormData
Basically, the FormData API is an interface that provides us an easy way to create sets of data, each with its own key, just like a form element with fields and values, including INPUT[FILE] that allows file upload.
It’s possible to create an empty FormData instance or pass to its constructor a form element which will have its fields and values copied to the new object:
// This way formData is empty
var formData = new FormData();
// This way formData is filled in with the fields/values from #formElement
var formData = new FormData(document.getElementById("formElement"));
It’s also possible to add new values to formData later and for such you can use the append method:
// Add more values to formData
formData.append("email", "[email protected]");
formData.append("age", "20");
FormData has many other methods you can use; you can check them in the FormData documentation.
Upload using AJAX
You can check this demo of uploading file using AJAX. Below you can see its source code which uses jQuery and the FormData API. This implementation is very simple and basic but complete, so you can easily adapt it to you page.
Notes about this code:
- It uses the jQuery library to make the AJAX request. You can use whatever library you prefer or even make AJAX request using plain JavaScript.
- In case the page that receives the AJAX request is a PHP page, the form fields are accessible through the $_POST variable and the file through the $_FILES variable.
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-type" content="text/html; charset=utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>File upload using AJAX (+jQuery) / Demo</title>
</head>
<body>
<form id="form-demo" onsubmit="return false">
<input type="file" id="image" name="image"><br><br>
<button id="button-send">Send</button>
</form>
<script src="http://code.jquery.com/jquery-3.0.0.min.js"></script>
<script>
/*
* For demonstration porpuse, all JavaScript code was incorporated in
* the HTML file. But when developing your application, your JavaScript code
* should be in a separated file. Check this page for more information:
* https://developer.yahoo.com/performance/rules.html#external
*/
$("#button-send").click(sendFormData);
function sendFormData(){
var formData = new FormData($("#form-demo").get(0));
var ajaxUrl = "process-upload.php";
$.ajax({
url : ajaxUrl,
type : "POST",
data : formData,
// both 'contentType' and 'processData' parameters are
// required so that all data are correctly transferred
contentType : false,
processData : false
}).done(function(response){
// In this callback you get the AJAX response to check
// if everything is right...
}).fail(function(){
// Here you should treat the http errors (e.g., 403, 404)
}).always(function(){
alert("AJAX request finished!");
});
}
</script>
</body>
</html>
Browser support
FormData is supported in the most recent browsers and you can check the compatibility table in its documentation page.
Extra
If you are implementing upload of images, you might want to check out the previous article covering Image preview before upload.
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