[js] 实现异步串行的方法

#example

flow([getUser, getRegions, getClients], done);

function getUser (next) {
  http.get('/userByEmail', { email: input.email }, next);
}

function getRegions (res, next) {
  http.get('/regions', { regionId: res.id }, next);
}

function getClients (res, next) {
  http.get('/clients', { regions: res.regions }, next);
}

function done (err, res) {
  if (err) {
    throw err;
  }
  console.log(res.clients);
}

#method

            function flow(steps, done) {
                function next() {
                    var used;
                    return function() {
                        if (used) {
                            return;
                        }
                        used = true;
                        var step = steps.shift();
                        if (step) {
                            var args = [].prototype.slice.call(arguments);
                            var err = args.shift();
                            if (err) {
                                done(err);
                                return;
                            }
                            args.push(next());
                            step.apply(null, args);
                        } else {
                            done.apply(null, arguments);
                        }
                    };
                }
                var start = next();
                start();
            }