const elements = document
    .getElementsByTagName("canvas");

for (let element of elements) {
    if (!element.id) {
        continue;
    }

    const image = element
        .toDataURL("image/png")
        .replace("image/png", "image/octet-stream");

    const a = document
        .createElement("a"); 

    a.href = image;
    a.download = `${element.id}.png`;
    a.click();
}