CanvasRenderingContext2D is not defined when testing in mocha / jsdom

[BUG] When running tests in node (mocha + jsdom), CanvasRenderingContext2D is not globally defined (see tmpvar/jsdom#1672). This causes problems in acquireContext:

	if (item instanceof HTMLCanvasElement) {
		// To prevent canvas fingerprinting, some add-ons undefine the getContext
		// method, for example: https://github.com/kkapsner/CanvasBlocker
		// https://github.com/chartjs/Chart.js/issues/2807
		var context = item.getContext && item.getContext('2d');
		if (context instanceof CanvasRenderingContext2D) {
			initCanvas(item, config);
			return context;
		}
	}

Expected Behavior

test should run in node.js as they would in the browser

Current Behavior

exception thrown

Possible Solution

Not really sure how to solve this. On the one hand, it doesn’t seem like this code achieves much that’s useful, given that in the cases where canvas.getContext() === undefined, no exceptions are raised (so I’m guessing that there will just be no graphs). On the other hand, this might be more jsdom’s problem than chart.js’s.

Context

• unit tests are good, they should be runnable in node.js

Environment

• Chart.js version: 2.4
• Browser name and version: node.js 6.x

[gor181]

I’m using following to render the chart in tests (mocha):

setup/config.js

require('babel-register')();

const canvas = require('canvas');

const jsdom = require('jsdom');
const document = jsdom.jsdom();
const window = document.defaultView;

const canvasMethods = ['HTMLCanvasElement'];

Object.keys(window).forEach(property => {
  if (typeof global[property] === 'undefined') {
    global[property] = window[property];
  }
});

canvasMethods.forEach(method =>
  global[method] = window[method]
);

global['CanvasRenderingContext2D'] = canvas.Context2d;

global.navigator = {
 userAgent: 'node.js'
};

Ignore babel require if you do not need it.

The test script in package.json is:

"test": "mocha test/config/setup.js test/__tests__/**/*"

You would need canvas dependency

[jacktuck]

I think #5214 is potentially related