grains.js

var bigInt = require('big-integer');

/**
 * @class Grains
 *
 * Computes the number of grains on the squares of a
 * chess board, starting with one grain on the first
 * square, and doubling with each successive square.
 */
function Grains() {
  // no op!
}

/**
 * Gets the number of grains on the nth square.
 */
Grains.prototype.square = function(num) {
  return bigInt(2).pow(num - 1);
};

/**
 * Gets the total number of grains on all squares.
 */
Grains.prototype.total = function () {
  var total = bigInt(0);

  for (var squareNum = 1; squareNum <= 64; ++squareNum) {
    total = total.add(this.square(squareNum));
  }

  return total;
};

module.exports = Grains;

grains_test.spec.js

var Grains = require('./grains');

describe("Grains", function () {
  var grains = new Grains();

  it("square 1", function () {
    expect(grains.square(1).toString()).toEqual('1');
  });

  it("square 2", function () {
    expect(grains.square(2).toString()).toEqual('2');
  });

  it("square 3", function () {
    expect(grains.square(3).toString()).toEqual('4');
  });

  it("square 4", function () {
    expect(grains.square(4).toString()).toEqual('8');
  });

  it("square 16", function () {
    expect(grains.square(16).toString()).toEqual('32768');
  });

  it("square 32", function () {
    expect(grains.square(32).toString()).toEqual('2147483648');
  });

  it("square 64", function () {
    expect(grains.square(64).toString()).toEqual('9223372036854775808');
  });

  it("total", function () {
    expect(grains.total().toString()).toEqual('18446744073709551615');
  });
});