Download Binary Files with Javascript

Have a look at the MDN article on XMLHttpRequest.

If you set the response of the XMLHttpRequest to ArrayBuffer you could do the following:

var xhr = new XMLHttpRequest();
xhr.open("GET", requestUrl);
xhr.responseType = "arraybuffer";

xhr.onload = function () {
    if (this.status === 200) {
        var blob = new Blob([xhr.response], {type: "application/pdf"});
        var objectUrl = URL.createObjectURL(blob);
        window.open(objectUrl);
    }
};
xhr.send();

Option 2:
You could use Blob as the response of the XMLHttpRequest. And then maybe save it in the FileSystem (FileSystem API)

It may look like:

var xhr = new XMLHttpRequest();
xhr.open("GET", requestUrl);
xhr.responseType = "blob";

xhr.onload = function () {
    onDownloaded(this);
};
xhr.send();

Option 3:
If you only want to download and “show” images you can easily do this like so:

var img = new Image();

// add the onload event before setting the src
img.onload = function() {
    onImageDownloaded(img);
}

// start the download by setting the src property
img.src = requestUrl