How do I download image from img tag with base64 string in JavaScript?

Use something:

var a = document.createElement("a"); //Create <a>
a.href = "data:image/png;base64," + ImageBase64; //Image Base64 Goes here
a.download = "Image.png"; //File name Here
a.click(); //Downloaded file

Try following method:

function downloadImage() {
            var base64String = $("#imageid").attr('src');
            var imageType = base64String.match(/[^:/]\w+(?=;|,)/)[0];
            var a = document.createElement("a");
            a.href = base64String;
            a.download = "Image." + imageType;
            a.click();
        }