let's assume the original f object was supposed to be an array

makeFun.js

function makeFun() {
  var f = [];

  // closure to freeze vars in async loop
  var sum = function (n, m) {
    return function () {
      console.log("sum=" + (n + m)); 
    }
  };

  for( var i=0 ; i<3 ; i++ ) {
    f[i] = sum(i, f.length);
  }
  return f;
}

makeFun()[0]()
makeFun()[1]()
makeFun()[2]()
// Expected Results:
// makeFun()[0]() should alert “sum=0”
// makeFun()[1]() should alert “sum=2”
// makeFun()[2]() should alert “sum=4”

Object.keys is available in ES5 (works in Chrome / Node). Assuming this was the intent of the original 'f.length', which is otherwise undefined.

perhaps the original 'f' object was supposed to be an array. rev 5 adds a closure to retain values in an async loop.