Traits in javascript

Question

How can I implement traits in javascript ?

Answer 1

function Trait (methods) {
  this.traits = [methods];
};

Trait.prototype = {
    constructor: Trait

  , uses: function (trait) {
      this.traits = this.traits.concat (trait.traits);
      return this;
    }

  , useBy: function (obj) {
      for (var i = 0; i < this.traits.length; ++i) {
        var methods = this.traits [i];
        for (var prop in methods) {
          if (methods.hasOwnProperty (prop)) {
            obj [prop] = obj [prop] || methods [prop];
          }
        }
      }
    }
};

Trait.unimplemented = function (obj, traitName) {
  if (obj === undefined || traitName === undefined) {
    throw new Error ("Unimplemented trait property.");
  }
  throw new Error (traitName + " is not implemented for " + obj);
};

Example:

var TEq = new Trait ({
    equalTo: function (x) {
      Trait.unimplemented (this, "equalTo");
    }

  , notEqualTo: function (x) {
      return !this.equalTo (x);
    }
});

var TOrd = new Trait ({
    lessThan: function (x) {
      Trait.unimplemented (this, "lessThan");
    }

  , greaterThan: function (x) {
      return !this.lessThanOrEqualTo (x);
    }

  , lessThanOrEqualTo: function (x) {
      return this.lessThan (x) || this.equalTo (x);
    }

  , greaterThanOrEqualTo: function (x) {
      return !this.lessThan (x);
    }
}).uses (TEq);


function Rational (numerator, denominator) {
  if (denominator < 0) {
    numerator *= -1;
    denominator *= -1;
  }
  this.numerator = numerator;
  this.denominator = denominator;
}

Rational.prototype = {
    constructor: Rational

  , equalTo: function (q) {
      return this.numerator * q.numerator === this.denominator * q.denominator;
    }

  , lessThan: function (q) {
      return this.numerator * q.denominator < q.numerator * this.denominator;
    }
};

TOrd.useBy (Rational.prototype);

var x = new Rational (1, 5);
var y = new Rational (1, 2);

[x.notEqualTo (y), x.lessThan (y)]; // [true, true]