Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<body>

<canvas id="myCanvas" width="200" height="200" style="border:1px solid">
Your browser does not support the HTML5 canvas tag.
</canvas>
<br>

<script>
var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
ctx.fillStyle="#92B901";
ctx.fillRect(50,50,100,100);
function clearCanvas()
{
c.height=300;
}
</script>

<button onclick="clearCanvas()">Clear canvas</button>

</body>
</html>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.