const Canvas = document.getElementById(Id); // Id here

return Canvas.toDataURL();

<canvas id="viewport"></canvas>

​

<style>

canvas#viewport { border: 1px solid white; width: 900px; }

</style>

<script>

​

var canvas = document.getElementById('viewport'),

context = canvas.getContext('2d');

​

make_base();

​

function make_base()

{

  base_image = new Image();

  base_image.src = 'img/base.png';

  context.drawImage(base_image, 100, 100);

}

</script>

function canvasURL(id) {

    let canvas = document.getElementById(id);

    return canvas.toDataURL(1.0);

}