Flag Enumerations in JavaScript

flags.js

// Flag Enumerations in JavaScript
// ===============================

// Flag enums
// ----------
// Values must increment by powers of 2

var SEASONS = {
  Spring : 1,
  Summer : 2,
  Fall   : 4,
  Winter : 8
};

var DAYS = {
  None      : 0x0,  // 0 *
  Sunday    : 0x1,  // 1
  Monday    : 0x2,  // 2
  Tuesday   : 0x4,  // 4
  Wednesday : 0x8,  // 8
  Thursday  : 0x10, // 16
  Friday    : 0x20, // 32
  Saturday  : 0x40  // 64
};

// * Cannot use 0 in a bitwise `&` operation to test for a flag b/c it will
// always result in zero. However, it can be used for logical comparisons.



// Usage examples (contrived)
// --------------------------

var getSeasonsSelected = function( seasons ) {
  var selected = [];

  // The perens are needed around the bitwise operation due to the
  // greater operator precedence of `===`
  if ( (seasons & SEASONS.Spring) === SEASONS.Spring ) selected.push('Spring');
  if ( (seasons & SEASONS.Summer) === SEASONS.Summer ) selected.push('Summer');
  if ( (seasons & SEASONS.Fall)   === SEASONS.Fall )   selected.push('Fall');
  if ( (seasons & SEASONS.Winter) === SEASONS.Winter ) selected.push('Winter');

  return selected;
};


var s1 = getSeasonsSelected( SEASONS.Spring | SEASONS.Fall );
console.log(s1);
//=> ["Spring", "Fall"]


// ----------------


var getDaysSelected = function( days ) {
  if (days === DAYS.None) return [];

  var selected = [];

  for (var flag in DAYS) {
    if (flag === 'None') continue;

    var val = DAYS[flag];

    if ( (days & val) === val ) selected.push(flag);
  }

  return selected;
}


var myDays = DAYS.Monday | DAYS.Friday;
myDays |= DAYS.Wednesday;

var d1 = getDaysSelected( DAYS.Monday | DAYS.Sunday | DAYS.Monday | myDays | DAYS.Monday );
console.log(d1);
//=> ["Sunday", "Monday", "Wednesday", "Friday"]


var d2 = getDaysSelected( DAYS.None );
console.log(d2);
//=> []