<a id="downloadLnk" download="your-drawing.png" href="javascript:void(0)"><img src="images/save.png" height="30px" width="30px" /></a>

<script language="javascript">
    function download() {
        // get the canvas 2d context
        var ctx = canvas.getContext('2d');
        // set the ctx to draw beneath your current content
        ctx.globalCompositeOperation = 'destination-over';
        // set the fill color to white
        ctx.fillStyle = 'white';
        // apply fill starting from point (0,0) to point (canvas.width,canvas.height)
        // these two points are the top left and the bottom right of the canvas
        ctx.fillRect(0, 0, canvas.width, canvas.height);
        var dt = canvas.toDataURL('image/png');
        this.href = dt;
    };

    downloadLnk.addEventListener('click', download, false);

</script>