Best Way to Detect That Html5 ≪Canvas≫ Is Not Supported

Best way to detect that HTML5 <canvas> is not supported

This is the technique used in Modernizr and basically every other library that does canvas work:

function isCanvasSupported(){
  var elem = document.createElement('canvas');
  return !!(elem.getContext && elem.getContext('2d'));
}

Since your question was for detection when it's not supported, I recommend using it like so:

if (!isCanvasSupported()){ ...

Determine, the browser supports canvas (paperjs), or not

var canvasExists = document.createElement('canvas').getContext !== undefined;

Will be true in Chrome, false in IE8, etc.

You could also check for window.HTMLCanvasElement !== undefined

does not support HTML5 Canvas element?

This code is perfect always use higher level version if you use canvas tag.

var exmpl= document.getElementById('exmpl');
var context = example.getContext('2d');
context.fillStyle = 'red';
context.fillRect(30, 30, 50, 50);

how to resolve the canvas not supported error

I had checked this , I didn't get , why are you facing this problem,though i don't know about tableau much

   canvas != null && canvas.getContext

by this you are getting another error "Canvas not supported by your browser
when i am dong this this error comes after the canvas is drawn. then i just delete that alert which gave me this alert message.i think you must try this ,search for this alert and see what happen next it does not effect the program and it runs fine

How to detect browser support HTML 5 or not

Not sure about your problem without more detail.

Use below code to detect again:

function supportCanvas() {
    /* 
       maybe use code below for detail:
       var cvs = document.createElement('canvas');
       alert(typeof cvs.getContext); // if support, output "function"
    */
    return !!document.createElement('canvas').getContext;
}

Check support for Offscreen canvas

The problem with your code is that you create a canvas-test dom element, which is not the same as canvas.

Try this.

var canvasTest = document.createElement('canvas');
if(typeof canvasTest.transferControlToOffscreen === "function"){   console.log('true');}else{   console.log('false');}