<canvas id="canvas"></canvas>
<a id="download">download</a>

<script>
    const filename = "test.png";
    document.getElementById("download").onclick = (event) => {
        let canvas = document.getElementById("canvas");
        let link = document.createElement("a");
        link.href = canvas.toDataURL("image/png");
        link.download = filename;
        link.click();
    }
</script>