fromnodecallback.js

function createNodeObservable(fn, ctx, selector, args) {
  var o = new AsyncSubject();

  args.push(createNodeHandler(o, ctx, selector));
  fn.apply(ctx, args);

  return o.asObservable();
}

function createNodeHandler(o, ctx, selector) {
  return function handler () {
    var err = arguments[0];
    if (err) { return o.onError(err); }

    var len = arguments.length, results = [];
    for(var i = 1; i < len; i++) { results[i - 1] = arguments[i]; }

    if (isFunction(selector)) {
      var results = tryCatch(selector).apply(ctx, results);
      if (results === errorObj) { return o.onError(results.e); }
      o.onNext(results);
    } else {
      if (results.length <= 1) {
        o.onNext(results[0]);
      } else {
        o.onNext(results);
      }
    }

    o.onCompleted();
  };
}

/**
 * Converts a Node.js callback style function to an observable sequence.  This must be in function (err, ...) format.
 * @param {Function} fn The function to call
 * @param {Mixed} [ctx] The context for the func parameter to be executed.  If not specified, defaults to undefined.
 * @param {Function} [selector] A selector which takes the arguments from the callback minus the error to produce a single item to yield on next.
 * @returns {Function} An async function which when applied, returns an observable sequence with the callback arguments as an array.
 */
Observable.fromNodeCallback = function (fn, ctx, selector) {
  return function () {
    typeof ctx === 'undefined' && (ctx = this); 
    var len = arguments.length, args = new Array(len);
    for(var i = 0; i < len; i++) { args[i] = arguments[i]; }
    return createNodeObservable(fn, ctx, selector, args);
  };
};