partializer.js

// A function decorator that returns a curryable version of the original function with an arbitrary number of arguments

// This is an introspective helper function that allows us get the parameter names of our original function and return it as an array
function getParamNames(func) {
  var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
  var ARGUMENT_NAMES = /([^\s,]+)/g;
  var fnStr = func.toString().replace(STRIP_COMMENTS, '');
  var result = fnStr.slice(fnStr.indexOf('(') + 1, fnStr.indexOf(')')).match(ARGUMENT_NAMES);
  if (result === null)
    result = [];
  return result;
}

// another small helper in created the closing brackets for the string we pass to the function constructor
function repeater(thing, times) {
  var result = '';
  for (var i = 0; i < times; i++) {
    result += thing
  }
  return result
}

// Main Function/Decorator: takes a function and returns a version that is curryable 
function partialized(fn) {
  var fnString = '';
  var fnArgs = '';
  var fnParameters = getParamNames(fn)
  
  // for loop builds a string version of our final function that will be passed to the Function constructure
  for (var i = 0; i < fnParameters.length; i++) {
    fnString += 'return function (' + fnParameters[i] + ') {';
    if (i < fnParameters.length - 1) {
      fnArgs += fnParameters[i] + ', '
    } else {
      fnArgs += fnParameters[i]
    }
  }
  // the final curry has to call the function with the arguments passed to it
  var called = 'fn(' + fnArgs + ')'
  // add all the closing brackets
  fnString += 'return ' + called + repeater('}', fnParameters.length)
  // the function constructor does not create a closure. 
  // So we have to explicitly pass in the original function 
  // in order for it's environment to be available inside of the newly created function
  // if we don't do this, it will complain that 'fn' is undefined. this is kind of a JavaScript gotcha IMO.
  return new Function('fn', fnString)(fn);
}

// original function that adds together five items
function addBunch(a, b, c, d, e) {
  return a + b + c + d + e;
}

var partialAdd = partialized(addBunch);

// now we can partially apply each argument
var v = partialAdd(5);
var w = v(5);
var x = w(3);
var y = x(2);
var z = y(30);
console.log(z)

// or . . . .
var result = partialAdd(5)(4)(3)(2)(6);
console.log(result);