example.js

var f = wrapFunc(function() {
  function Fraction(nom, denom) {
    this.nominator   = nom
    this.denominator = denom
  }

  Fraction.prototype['*'] = function(other) {
    return new Fraction(this.nominator   * other.nominator,
                        this.denominator * other.denominator)
  }

  Fraction.prototype.toString = function() {
    return this.nominator + "/" + this.denominator
  }

  var one_half  = new Fraction(1, 2)
    , one_third = new Fraction(1, 3)

  return one_half * one_third
})

console.log(String(f())) // 1/6

library.js

var falafel = require('falafel')
  , assert  = require('assert')


// Preparations
Number.prototype['+'] = function(other) { return this + other }
Number.prototype['-'] = function(other) { return this - other }
Number.prototype['*'] = function(other) { return this * other }
Number.prototype['/'] = function(other) { return this / other }

String.prototype['+'] = function(other) { return this + other }


function wrapFunc(fn) {
  assert.equal(typeof fn, 'function', "Passed non-function to wrapFunc!")

  var processed = process("(" + fn + ")")
    , newParts  = explodeFunction(processed.toString())

  return Function(newParts.args, newParts.body)

  function process(body) {
    return falafel(body, function(node) {
      if (node.type == 'BinaryExpression') {
        node.update( wrap("%s",     node.left.source())
                   + wrap("['%s']", node.operator)
                   + wrap("(%s)",   node.right.source()))
      }
    })

    function wrap(format, other) {
      return format.replace(/%s/g, other)
    }
  }

  function explodeFunction(str) {
    if (str[0] == '(' && str.slice(-1) == ')') {
      // Unwrap parenthesized function: ((((x)))) --> x
      return explodeFunction(str.slice(1, -1))
    }

    var match = str.match(/^function\s*(\w*)\s*\(([^)]*)\)\s*{([\s\S]*)}$/)

    assert.ok(match, "Couldn't explode function!")

    var name  = match[1]
      , args  = match[2]
      , body  = match[3]

    return { name:name, args:args, body:body }
  }
}