如果使用async和await重写逻辑，事情会变得更容易：

createNewMember = async (req, res, next) => {
    try {
        const members = await Member.findOne({email}).exec();
        if (members?.length) return res.status(409).json({message: "Member already present"});
        // create newMember object
        // ...
        await newMember.save();
        res.status(201).json({message: "Member created"});
    } catch(err) {
        // handle error and return error response
        // ...
    }
}

除了抛出自己的Error并跳到catch块之外，没有其他方法可以打破promise链。但是，你不想像那样使用Error's，因为这种情况本身不是Error，它只是某种逻辑环境（成员已经存在）。我建议您将.then()看作是蛋糕层，具有单独的关注点，如下所示：

Member.findOne({email: email}).exec()
    // DB layer
    .then((members) => {
         // If member does not exist, begin saving and return promise
         if(members == null || members.length==0) {
              // create newMember object
              return newMember.save()   // 1 --> promise returned
         } 
         // If member exists, return undefined
         else {
             return;
         }
     })
     // Response Layer
     .then((newMember) => {
         let statusCode;
         let message;
         // On .save() success, the promise resolves to the new member
         if(newMember) {
           statusCode = 201;
           message = "member created";
         }
         // If we returned undefined in the first then(), we'd land here
         else {
           statusCode = 409;
           message = "Member already present";
         }
         return res.status(statusCode).json({message});
     })
     .catch(err => {
          // handle error and return error response
     })