Reactive Expressions

ex.js

var EX = function (expr) {
  var result
  ,   operands = {}
  ,   operVals = Array.prototype.slice.call(arguments, 1)
  ,   operSetFuncs = {}
  ;
  
  function genResult () {
    result = expr.apply(null, Object.keys(operands).map(function(v) { 
      return operands[v]; 
    })); 
  }
  
  expr.toString().replace(/^[^(]*\(/g, '').replace(/\).*/g, '').split(/\s*,\s*/).forEach(function(v, i) { 
    var curr = v;
    operands[curr] = operVals[i];
    operSetFuncs[curr] = function (val) {
      if (val !== undefined) {
        operands[curr] = val;
        genResult();
      }
      return operands[curr];
    }
  });

  genResult();

  return [
    function () { 
      return result; 
    }, operSetFuncs
  ];
};