Adds jQuery style getters and setters to a given constructor function.

magicMethod.js

Function.prototype.method = function (name, func) {
    this.prototype[name] = func;
    return this;
}

var getParamNames = function (func) {
  var funStr = func.toString()
  return funStr.slice(funStr.indexOf('(')+1, funStr.indexOf(')')).match(/([^\s,]+)/g)
}

function addMagicMethod (Constructor) {
  var params = getParamNames(Constructor)
  for (var j = 0; j < params.length; j++) {
    eval('\
      var magicMethod = function (v) {\
        if (v) {\
          this._' + params[j] + ' = v;\
          return this;\
        } else {\
          return this._' + params[j] + ';
        } 
      }')
    Constructor.method(params[j], magicMethod)
  }
}

// example
Function Foo (a, b, c) {}
addMagicMethod(Foo)
var f = new Foo()
f.a("Magic")
f.a() // returns "Magic"
f.b("Something Else")
f.b() // returns "Something Else"
f.c("More")
f.c() // returns "More"

Seems like you have a couple typos there. Function Foo should be function Foo (lowercase function), also missing 2 line ending escapes in the eval string.

Interesting code nonetheless.