Stackoverflow HTML5 examples Animation and Responsive full screen canvas

This is an example of code from the stackoverflow HTML5-canvas Documentation See code for more info.

JavaScript

/* This contains two examples from stackoverflow documents. */
var exampleRunning = false;
/* From stackoverflow documents 
   html5-canvas 
   Introduction to html5-canvas 
       Creating a responsive full page canvas
       http://stackoverflow.com/documentation/html5-canvas/1892/introduction-to-html5-canvas/7516/creating-a-responsive-full-page-canvas#t=20160809212935818717 
*/
var canvas;
var ctx;
function createCanvas(){ // creates a canvas
    var canvas = document.createElement("canvas"); 
    canvas.style.position = "absolute"; // set the style 
    canvas.style.left     = "0px";      // position in top left
    canvas.style.top      = "0px";
    canvas.style.zIndex   = 1;        
    document.body.appendChild(canvas);  // add to document
    return canvas;
}

function sizeCanvas(){                  // resizes canvas. Will create a canvas if it does not exist
    if(canvas === undefined){           // if there is no canvas create it
        canvas = createCanvas();
        ctx = canvas.getContext("2d");  // get the 2D context
    }
    canvas.width  = window.innerWidth;  // set the resolution to fill the page
    canvas.height = window.innerHeight; 
    if(exampleRunning){  
        reset();  // reset anmation 
    }
}
// removes the canvas
function removeCanvas(){
    if(canvas !== undefined){                // make sure there is something to remove
        window.removeEventListener("resize",sizeCanvas); // remove resize event
        document.body.removeChild(canvas);   // remove the canvas from the DOM
        ctx = undefined;                     // dereference the context
        canvas = undefined;                  // dereference the canvas
     }
}

// the resize listener
window.addEventListener("resize", sizeCanvas); 
// call sizeCanvas to create and set the canvas resolution
sizeCanvas();
/* End of stackoverflow example  Creating a responsive full page canvas*/


    /* From stackoverflow documents 
       html5-canvas 
       Introduction to...