Kata: Fractions in JavaScript ES5 using the Function as Object pattern

One of the katas I use to start practicing TDD with my mentees is the Fractions kata. This kata is great to practice writing a good list of examples and doing small baby steps. I first learn about it while doing the J. B. Rainsberger wonderful TDD course.

We usually do it in Java, but last week Antonio and I did it in JavaScript ES5 using the Function as Object pattern. This is a very interesting pattern to create objects that you can use in any language with first-class functions and closures. In Codesai we use and teach this pattern when working in projects using ES5.

We had a great time doing this kata in several short sessions that are also helping me in my recovery process.

These are the tests we wrote:

	'use strict';
	describe('Fractions', function () {
	describe('addition', function () {
	it('when one operand is zero the result is the other operand', function () {
	var result = Fraction(1, 1).add(Fraction(0, 1));
	expect(result.numerator()).toBe(1);
	expect(result.denominator()).toBe(1);
	});
	it('when both operands have same denominator', function () {
	var result = Fraction(1, 2).add(Fraction(2, 2));
	expect(result.numerator()).toBe(3);
	expect(result.denominator()).toBe(2);
	});
	it('when denominators are different', function () {
	var result = Fraction(1, 2).add(Fraction(1, 3));
	expect(result.numerator()).toBe(5);
	expect(result.denominator()).toBe(6);
	});
	it('simplifies result after operation', function () {
	var result = Fraction(1, 2).add(Fraction(1, 2));
	expect(result.numerator()).toBe(1);
	expect(result.denominator()).toBe(1);
	});
	});
	describe('subtraction', function () {
	it('is a special case of addition', function () {
	var subtraction = Fraction(1, 2).subtract(Fraction(1, 2));
	var addition = Fraction(1, 2).add(Fraction(-1, 2));
	expect(subtraction.numerator()).toBe(addition.numerator());
	expect(subtraction.denominator()).toBe(addition.denominator());
	});
	});
	});

view raw fractionsSpec.js hosted with ❤ by GitHub

and this is the resulting code:

	'use strict';
	function Fraction(numerator, denominator) {
	return {
	add: function (other) {
	var num = numerator * other.denominator()
	+ other.numerator() * denominator;
	var dem = denominator * other.denominator();
	var gcd = Integers.gcd(num, dem);
	return Fraction(num / gcd, dem / gcd);
	},
	subtract: function (other) {
	return Fraction(numerator, denominator)
	.add(Fraction(-other.numerator(), other.denominator()))
	},
	numerator: function () {
	return numerator;
	},
	denominator: function () {
	return denominator;
	}
	};
	}

view raw fractions.js hosted with ❤ by GitHub

See all the commits here if you want to follow the process. You can find all the code on this GitHub repo.