javascript - Browser/HTML Force download of image from src="data:image/jpeg;base64..."

Question

I am generating a image on client side and I display it with HTML like this:

<img src="data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAMCAgICAgM...."/>

I want to offer the possibility to download the generated Image.

How can I realize that the browser is opening a file save dialoge (or just download the image like chrome or firefox to the download folder would do) which allows the user to save the image without doing right click and save as on the image?

I would prefer a solution without server interaction. So I am aware that it would be possible if I first upload the Image and then start the download.

Thanks a lot!

Answer 1

Simply replace image/jpeg with application/octet-stream. The client would not recognise the URL as an inline-able resource, and prompt a download dialog.

A simple JavaScript solution would be:

//var img = reference to image
var url = img.src.replace(/^data:image/[^;]+/, 'data:application/octet-stream');
window.open(url);
// Or perhaps: location.href = url;
// Or even setting the location of an <iframe> element, 

Another method is to use a blob: URI:

var img = document.images[0];
img.onclick = function() {
? ??// atob to base64_decode the data-URI
? ??var image_data = atob(img.src.split(',')[1]);
    // Use typed arrays to convert the binary data to a Blob
? ??var arraybuffer = new ArrayBuffer(image_data.length);
? ??var view = new Uint8Array(arraybuffer);
? ??for (var i=0; i<image_data.length; i++) {
? ? ? ??view[i] = image_data.charCodeAt(i) & 0xff;
? ??}
    try {
        // This is the recommended method:
        var blob = new Blob([arraybuffer], {type: 'application/octet-stream'});
    } catch (e) {
        // The BlobBuilder API has been deprecated in favour of Blob, but older
        // browsers don't know about the Blob constructor
        // IE10 also supports BlobBuilder, but since the `Blob` constructor
        //  also works, there's no need to add `MSBlobBuilder`.
        var bb = new (window.WebKitBlobBuilder || window.MozBlobBuilder);
? ??    bb.append(arraybuffer);
? ??    var blob = bb.getBlob('application/octet-stream'); // <-- Here's the Blob
    }

    // Use the URL object to create a temporary URL
? ? var url = (window.webkitURL || window.URL).createObjectURL(blob);
? ??location.href = url; // <-- Download!
};

Relevant documentation

• atob
• Typed arrays
• URL.createObjectURL
• Blob and BlobBuilder