有條件地退出承諾鏈而不會拋出錯誤

Question

在我的節點js應用程序中，我試圖創建一個需要幾個異步步驟的註冊路徑。在早期階段，有一種情況是我想停下來，然後回覆一條消息，但我對混合條件融入承諾感到困惑。

我的具體問題是在下面的僞代碼註釋：

router.post('/register', function(req, res, next) { 
    var identity = req.body.identity; 
    var password = req.body.password; 

    // can't register without a reservation 
    hasReservation(identity).then(function(hasReservation) { 
     if (hasReservation) { return register(identity, password); } 
     else { 
      // stuck here: I want to say: 
      res.json({ registered: false, reason: "missing reservation" }); 
      // and then I want to stop further promises 
      // but this isn't an error, I don't want to throw an error here 
     } 
    }).then(function(user) { 
     // in my app, registering an existing user is okay 
     // I just want to reset a few things about the user and save 
     // but I don't want to be here if there's no reservation 
     if (user) { 
      user.foo = 'bar'; 
      return user.save().then(function() { return user; }); 
     } else { 
      return register(identity, password); 
     } 
    }).then(function(user) { 
     // I want to do more asynch stuff here 
    }).then(function(result) { 
     res.json(result); 
    }).catch(function(error) { 
     res.status(500).json(error); 
    }); 
}); 

我怎麼能有條件地「挽救」是第一個承諾完成後，未拋出錯誤？

Answer 1

如果寄存器（身份，密碼）返回一個承諾，你可以按如下重新組織代碼：

router.post('/register', function(req, res, next) { 
    var identity = req.body.identity; 
    var password = req.body.password; 

    // can't register without a reservation 
    hasReservation(identity) 
    .then(function(hasReservation) { 
     if (hasReservation) { 
      return register(identity, password) 
      .then(function(user) { 
       // in my app, registering an existing user is okay 
       // I just want to reset a few things about the user and save 
       // but I don't want to be here if there's no reservation 
       if (user) { 
        user.foo = 'bar'; 
        return user.save().then(function() { return user; }); 
       } else { 
        return register(identity, password); 
       } 
      }) 
      .then(function(user) { 
       // I want to do more asynch stuff here 
      }) 
      .then(function(result) { 
       res.json(result); 
      }) 
     } else { 
      // stuck here: I want to say: 
      res.json({ registered: false, reason: "missing reservation" }); 
      // and then I want to stop further promises 
      // but this isn't an error, I don't want to throw an error here 
     } 
    }) 
    .catch(function(error) { 
     res.status(500).json(error); 
    }); 
}); 

否則，如果寄存器（..）只是返回一個值，包寄存器的第一個實例（......）在Promise.resolve

 return Promise.resolve(register(identity, password))