Accessors via property descriptors

accessor-descriptors.js

var C = (function() {
  // Map requires --harmony flag
  var map = new Map();

  function C( initial ) {
    // define accessor via property descriptor:
    Object.defineProperties(this, {
      square: {
        get: function() {
          return Math.pow( map.get(this).value || 0, 2 );
        }
      }
    });

    map.set(this, {
      value: initial,
      history: []
    });
  }

  C.prototype = {
    constructor: C,

    get value() {
      return map.get(this).value;
    },

    // Validate assignments and add to history
    set value( val ) {
      if ( typeof val !== "number" ) {
        throw new TypeError("val must be a number");
      }
      // Save the previous value to the history
      map.get(this).history.push( map.get(this).value );

      return (map.get(this).value = val);
    },

    history: function( filterFn ) {
      if ( !filterFn ) {
        filterFn = function() { return true };
      }
      return map.get(this).history.filter( filterFn );
    }
  };

  return C;
}());

var c = new C(10);


console.log( c.square );  // 100
console.log( c.value );   // 10
console.log( c.history() ); // []

c.value = 4;

console.log( c.history() ); // [ 10 ]

try {
  c.value = "hi!";
} catch (e) {
  console.log( e.message );
  // TypeError: val must be a number
}

c.value = 5;

console.log( c.history() ); // [ 10, 4 ]