pattern-matching.js

function when(x) {
  return function () {
    for (var i in arguments) {
      var result = arguments[i](x);
      if (result !== false) {
        return result;
      }
    }
    throw "No patterns matched when(" + x + ")";
  };
}

function match(pattern) {
  return function (then) {
    return function (x) {
      var match = pattern === "*" ? true : pattern(x);
      return match !== false ? then(match) : false;
    }
  }
}

function isZero(n) { return n === 0; }
function nonZero(n) { var i = parseInt(n); return i > 0 ? i : false; }

var fact = function (n) {
  return when(n)(
    match(isZero)(function () { return 1; }),
    match(nonZero)(function (n) { return n * fact(n - 1); })
  );
}

console.log(fact(10));