/********************************************************

       T E S T I N G :  M E S S Y  V S  C L E A N

********************************************************/

​

//////////////////////// MESSY //////////////////////////

// If we want to test a messy function like daysThisMonth

// we have to know how it is implemented and ovveride or

// "mock" the responses of its environment.

​

var realDate = Date;

var Date = function() {

  if(arguments.length === 0) {

    return new realDate(1929, 10-1, 24);

  } else {

    return new realDate(

      arguments[0], arguments[1], arguments[2]);

  }

};

​

assertEqual( daysThisMonth(), 31 );

Date = realDate;

​

//////////////////////// CLEAN //////////////////////////

// Gross, isn't it? We want to test a function's

// interface, not its implementation. It is much easier

// to test daysInMonth(). Pick a month and write the

// test. (Thirty days hath September, April, June and

// November...)

​

assertEqual( daysInMonth(1929, 10), 31 );

​

​

console.log("All tests pass.");

​

/********************************************************

              B A C K G R O U N D  C O D E

********************************************************/

​

function daysThisMonth() {

  var date  = new Date(),

    y     = date.getFullYear(),

    m     = date.getMonth(),

    start = new Date(y, m, 1),

    end   = new Date(y, m + 1, 1);

  return (end - start) / (1000 * 60 * 60 * 24);

}

​

function daysInMonth(y, m) {

  var start = new Date(y, m - 1, 1),

    end   = new Date(y, m, 1);

  return (end - start) / (1000 * 60 * 60 * 24);

}

​

function assertEqual(x,y, msg) {

  if(x != y) throw("expected "+x+" to equal "+y);

}