Pairs (length 2 tuples) in JavaScript

pair.js

/**
 * Pair
 * @todo Implement algebraic structures: Setoid, Functor
 */
var Pair = function(fst, snd) {
  if (this instanceof Pair) {
    if (Array.isArray(fst) && fst.length === 2 && typeof snd == 'undefined') {
      this[0] = fst[0];
      this[1] = fst[1];
    } else {
      this[0] = fst;
      this[1] = snd;
    }
    this.length = 2;
  } else {
    return new Pair(fst, snd);
  }
};
Pair.prototype.fst = function() {
  return this[0];
};
Pair.prototype.snd = function() {
  return this[1];
};
Pair.of = Pair.prototype.of = function(a1, a2) {
  return new Pair(a1, a2);
};

/**
 * Create fst() and snd() functions
 */
var fst = function(pair) {
  if (pair instanceof Pair) {
    return pair.fst();
  } else if (Array.isArray(pair) && pair.length === 2) {
    return pair[0];
  }
  throw 'Not a pair: ' + pair;
};
var snd = function(pair) {
  if (pair instanceof Pair) {
    return pair.snd();
  } else if (Array.isArray(pair) && pair.length === 2) {
    return pair[1];
  }
  throw 'Not a pair: ' + pair;
};

usage.js

/**
 * Example usage 1: Given an array of length 2
 */
var pair1 = Pair(['one', 2]);
fst(pair1); //=> 'one'
snd(pair1); //=> 2

/**
 * Example usage 2: Given 2 arguments
 */
var pair2 = Pair('one', 2);
fst(pair2); //=> 'one'
snd(pair2); //=> 2

/**
 * Example usage 3: Given an array, using Ramda
 */
var text = ['one', 'two', 'three'], numbers = [1, 2, 3];
var list = R.zip(text, numbers);
R.map(snd, list); //=> [1, 2, 3]

/**
 * Example usage 4: Creating pairs with Pair.of(), using Ramda
 */
var text = ['one', 'two', 'three'], numbers = [1, 2, 3];
var list = R.zip(text, numbers);
R.map(Pair.of, list); //=> [Pair, Pair, Pair]