How to display what I write inside of a canvas?

What I'm trying to accomplish:

First, an image:

There is a textbox underneath the 'code' tab - what I want is for all the canvas-related code written inside the HTML to be displayed on the canvas on the right.

My problem:

I'm struggling with doing so, I tried doing this:

run.addEventListener('click', function() {
  canvas.innerHTML = '<script>' + jsarea.value + '</script>';
})

But it didn't work. I'm also getting a 'Cannot read property 'getContext' of null' error on:

var ctx = canvas.getContext('2d');

I think this might be because the canvas is in an iframe.

How could I achieve what I'm trying to accomplish?