https://gist.github.com/4362563 EventProxy版本，勉强挤进50行

eventproxy.api.js

var callA = function (callback) {
  setTimeout(function () {
    callback(null, {name: "a", data: "I am result A"});
  }, Math.round(Math.random() * 300));
};

var callB = function (callback) {
  setTimeout(function () {
    callback(null, {name: "b", data: Math.round(Math.random() * 300)) % 2 === 0});
  }, Math.round(Math.random() * 300));
};

var callC = function (callback) {
  setTimeout(function () {
    callback(null, {name: "c", data: "I am result C"});
  }, Math.round(Math.random() * 300));
};

var api = function (callback) {
  var result = {}, start = new Date().getTime();
  var emitter = new EventProxy();
  var done = function () {
    emitter.removeAllListeners();
    callback(result);
  };
  emitter.on('all', function (event, ret) {
    result[ret.name] = ret.data;
  });
  callA(function (a) {
    emitter.emit('a', a);
    // 200ms后，不管b了
    if (new Date().getTime() - start > 200) {
      done();
    }
  });
  callB(function (err, b) {
    // 检查b的返回值
    if (b.hasOwnProperty("b") && b.data) {
      callC(function (err, c) {
        emitter.emit('c', c);
        emitter.emit('b', b);
      });
    } else {
      emitter.emit('b', b);
    }
  });
  // a, b在200ms前完成的case
  emitter.all('a', 'b', done);
};

俺也来个不依赖任何模块的版本，大概也是50行。只是代码难看了点

function sleep (s, callback) {
  setTimeout(callback, s * 1000);
}

function call_a (callback) {
  sleep(.1, function () {
    callback('a');
  });
}

function call_b (callback) {
  sleep(Math.random() * .3, function () {
    callback('b');
  });
}

// 调用此函数，按要求返回a,b的结果
function call (callback) {
  var retA = null, retB = null, isReturn = false;

  function checkDone (isNotTimeout) {
    if (isReturn) return;
    if (isNotTimeout) {
      if (retA !== null) {
        isReturn = true;
        callback(retA, retB);
      }
    } else {
      isReturn = true;
      callback(retA, retB);
    }
  }

  call_a(function (ret) {
    retA = ret;
    checkDone(true);
  });

  call_b(function (ret) {
    retB = ret;
    checkDone(true);
  });

  sleep(5, checkDone);
};

// 开始测试
var start = Date.now();
call(function (a, b) {
  var end = Date.now();
  console.log((end - start) + 'ms', a, b);
});