Chart.js/test/context.js

// Code from https://stackoverflow.com/questions/4406864/html-canvas-unit-testing
var Context = function() {
	this._calls = []; // names/args of recorded calls
	this._initMethods();

	this._fillStyle = null;
	this._lineCap = null;
	this._lineDashOffset = null;
	this._lineJoin = null;
	this._lineWidth = null;
	this._strokeStyle = null;

	// Define properties here so that we can record each time they are set
	Object.defineProperties(this, {
		fillStyle: {
			get: function() {
				return this._fillStyle;
			},
			set: function(style) {
				this._fillStyle = style;
				this.record('setFillStyle', [style]);
			}
		},
		lineCap: {
			get: function() {
				return this._lineCap;
			},
			set: function(cap) {
				this._lineCap = cap;
				this.record('setLineCap', [cap]);
			}
		},
		lineDashOffset: {
			get: function() {
				return this._lineDashOffset;
			},
			set: function(offset) {
				this._lineDashOffset = offset;
				this.record('setLineDashOffset', [offset]);
			}
		},
		lineJoin: {
			get: function() {
				return this._lineJoin;
			},
			set: function(join) {
				this._lineJoin = join;
				this.record('setLineJoin', [join]);
			}
		},
		lineWidth: {
			get: function() {
				return this._lineWidth;
			},
			set: function(width) {
				this._lineWidth = width;
				this.record('setLineWidth', [width]);
			}
		},
		strokeStyle: {
			get: function() {
				return this._strokeStyle;
			},
			set: function(style) {
				this._strokeStyle = style;
				this.record('setStrokeStyle', [style]);
			}
		},
	});
};

Context.prototype._initMethods = function() {
	// define methods to test here
	// no way to introspect so we have to do some extra work :(
	var me = this;
	var methods = {
		arc: function() {},
		arcTo: function() {},
		beginPath: function() {},
		bezierCurveTo: function() {},
		clearRect: function() {},
		clip: function() {},
		closePath: function() {},
		fill: function() {},
		fillRect: function() {},
		fillText: function() {},
		lineTo: function() {},
		measureText: function(text) {
			// return the number of characters * fixed size
			return text ? {width: text.length * 10} : {width: 0};
		},
		moveTo: function() {},
		quadraticCurveTo: function() {},
		rect: function() {},
		restore: function() {},
		rotate: function() {},
		save: function() {},
		setLineDash: function() {},
		stroke: function() {},
		strokeRect: function() {},
		setTransform: function() {},
		translate: function() {},
	};

	Object.keys(methods).forEach(function(name) {
		me[name] = function() {
			me.record(name, arguments);
			return methods[name].apply(me, arguments);
		};
	});
};

Context.prototype.record = function(methodName, args) {
	this._calls.push({
		name: methodName,
		args: Array.prototype.slice.call(args)
	});
};

Context.prototype.getCalls = function() {
	return this._calls;
};

Context.prototype.resetCalls = function() {
	this._calls = [];
};

module.exports = Context;