function download(src) {
    var $a = document.createElement('a');
    $a.setAttribute("href", src);
    $a.setAttribute("download", "");


    var evObj = document.createEvent('MouseEvents');
    evObj.initMouseEvent( 'click', true, true, window, 0, 0, 0, 0, 0, false, false, true, false, 0, null);
    $a.dispatchEvent(evObj);
};
 

<button type="button" class="btn btn-primary" onclick=" download(src)" >下載</button>

src為圖片地址，src='http://localhost:8080/aaa/bbb/ccc/20170517111819.png';