first.js

  var FirstObservable = (function (__super__) {
    inherits(FirstObservable, __super__);
    function FirstObservable(source, obj) {
      this.source = source;
      this._obj = obj;
      __super__.call(this);
    }

    FirstObservable.prototype.subscribeCore = function (o) {
      return this.source.subscribe(new FirstObserver(o, this._obj, this.source));
    };

    return FirstObservable;
  }(ObservableBase));

  var FirstObserver = (function(__super__) {
    inherits(FirstObserver, __super__);
    function FirstObserver(o, obj, s) {
      this._o = o;
      this._obj = obj;
      this._s = s;
      this._i = 0;
      __super__.call(this);
    }

    FirstObserver.prototype.next = function (x) {
      if (this._obj.predicate) {
        var res = tryCatch(this._obj.predicate)(x, this._i++, this._s);
        if (res === errorObj) { return this._o.onError(res.e); }
        if (Boolean(res)) {
          this._o.onNext(x);
          this._o.onCompleted();
        }
      } else if (!this._obj.predicate) {
        this._o.onNext(x);
        this._o.onCompleted();
      }
    };
    FirstObserver.prototype.error = function (e) { this._o.onError(e); };
    FirstObserver.prototype.completed = function () {
      if (this._obj.defaultValue === undefined) {
        this._o.onError(new EmptyError());
      } else {
        this._o.onNext(this._obj.defaultValue);
        this._o.onCompleted();
      }
    };

    return FirstObserver;
  }(AbstractObserver));

  /**
   * Returns the first element of an observable sequence that satisfies the condition in the predicate if present else the first item in the sequence.
   * @returns {Observable} Sequence containing the first element in the observable sequence that satisfies the condition in the predicate if provided, else the first item in the sequence.
   */
  observableProto.first = function () {
    var obj = {}, source = this;
    if (typeof arguments[0] === 'object') {
      obj = arguments[0];
    } else {
      obj = {
        predicate: arguments[0],
        thisArg: arguments[1],
        defaultValue: arguments[2]
      };
    }
    if (isFunction (obj.predicate)) {
      var fn = obj.predicate;
      obj.predicate = bindCallback(fn, obj.thisArg, 3);
    }
    return new FirstObservable(this, obj);
  };