Edit in JSFiddle

describe('odd', function() {
  it('returns true for all odd numbers', function() {
    expect(function(n) { return odd(n) === true; }).forAll(oddNumbers);
  });
  it('returns false for all even numbers', function() {
    expect(function(n) { return odd(n) === false; }).forAll(evenNumbers);
  });
});

function evenNumbers(size) {
  var num = qc.int(size);
  return num + num;
}

function oddNumbers(size) {
  return evenNumbers(size) + 1;
}

function odd(n) {
  return n % 2 === 1;
}