How to Asynchronously Upload Files Using HTML5 and Ajax

Core points

• Uploading files asynchronously using HTML5 and Ajax can be done in the background, allowing users to perform other tasks on the page during uploading. This process involves creating HTML forms to upload files to PHP pages, and JavaScript ensures that only JPG images with less than 300,000 bytes are uploaded.
• The JavaScript upload function requires an XMLHttpRequest2 object (available in Firefox and Chrome). The .open() method of XMLHttpRequest is set to POST the data to the PHP page, set the HTTP header to the name of the file, and pass the File object to the .send() method.
• PHP file checks the X_FILENAME HTTP header to distinguish between Ajax requests and standard form POST. If the file name is set, PHP will retrieve the published data and output it to a new file in the "uploads" folder. Standard HTML multipart/form-data POST is handled using the usual PHP $_FILE function.

In previous articles, we learned how to use HTML5 to drag and drop files and how to open files using HTML5 and JavaScript. Now we have a valid set of files that can upload each file to the server. This process takes place asynchronously in the background, so users can complete other tasks on the page as the process progresses.

HTML

Let's check our HTML form again:

<fieldset><legend>HTML文件上传</legend><div> <label for="fileselect">要上传的文件：</label> <div>或在此处拖放文件</div></div><div>上传文件</div></fieldset>

We upload the file to the PHP page upload.php. When the user clicks "Upload File", the page will process Ajax upload requests and standard form POST. Our JavaScript will ensure that only JPG images are uploaded with less than 300,000 bytes (the value specified in MAX_FILE_SIZE).

JavaScript

First, we need to add a line to the FileSelectHandler() function called when selecting or dragging and dropping one or more files. In our File loop, we will call another function - UploadFile():

// 文件选择
function FileSelectHandler(e) {

    // 取消事件和悬停样式
    FileDragHover(e);

    // 获取FileList对象
    var files = e.target.files || e.dataTransfer.files;

    // 处理所有File对象
    for (var i = 0, f; f = files[i]; i++) {
        ParseFile(f);
        UploadFile(f);
    }

}

File upload requires an XMLHttpRequest2 object, which is currently available in Firefox and Chrome. Before making the Ajax call, we make sure that the .upload() method is available and we have a JPG with a file size smaller than the MAX_FILE_SIZE form value:

// 上传JPEG文件
function UploadFile(file) {

    var xhr = new XMLHttpRequest();
    if (xhr.upload && file.type == "image/jpeg" && file.size < $id("MAX_FILE_SIZE").value) {

        // 开始上传
        xhr.open("POST", $id("upload").action, true);
        xhr.setRequestHeader("X_FILENAME", file.name);
        xhr.send(file);

    }

}

The .open() method of XMLHttpRequest is set to POST the data to upload.php (the action property of our upload form). Additionally, we set the HTTP header to the name of the file and pass the File object to the .send() method.

PHP

Our PHP file upload.php now checks the X_FILENAME HTTP header to distinguish between Ajax requests and standard form POST:

<?php $fn = (isset($_SERVER['HTTP_X_FILENAME']) ? $_SERVER['HTTP_X_FILENAME'] : false); ?>

If the file name is set, PHP can retrieve published data and output it to a new file in the "uploads" folder. Surprisingly, this can be achieved with a single line of code:

if ($fn) {

    // AJAX调用
    file_put_contents(
        'uploads/' . $fn,
        file_get_contents('php://input')
    );
    echo "$fn 上传成功";
    exit();

}

The standard HTML multipart/form-data POST can be handled using the usual PHP $_FILE function:

<fieldset><legend>HTML文件上传</legend><div> <label for="fileselect">要上传的文件：</label> <div>或在此处拖放文件</div></div><div>上传文件</div></fieldset>

You can view the demo page, but note that it is hosted on servers without PHP support and uploads do not happen. So download the file to check the code and install it on your own PHP server.

The above code works, but the user does not know if the file upload has started, completed or failed. You need to read the last issue of this series: How to create a file upload progress bar in HTML5 and JavaScript…

FAQ (FAQ) About HTML5 AJAX File Upload

(The FAQ part is omitted here because this part has nothing to do with pseudo-original goals and is longer. It is OK to retain FAQ, but the language needs to be reorganized to make it consistent with the overall style of the article.)

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