Example 5-3. Functions that return promises should not throw errors

function badfunc(url) {
	var image;
	image.src = url; // Error: image is undefined
	return new Promise(function (resolve, reject) {
		image.onload = resolve;
		image.onerror = reject;
	});
}

function goodfunc(url) {
	return new Promise(function (resolve, reject) {
		var image;
		image.src = url; // Error: image is undefined
		image.onload = resolve;
		image.onload = reject;
	});
}