Knexjs和es6是否尝试/捕获导致未处理的承诺拒绝警告？

Question

I've been working on a new application that uses PostgreSQL and Knexjs as a query builder, but have been running into an issue I'm not sure how to handle.

I have a route handler that looks like so:

export const getSingleUser = async(req, res) => {
  let respObj = {
    status: 'fail',
    message: 'User does not exist'
  };

  try {
    const user = await knex('users').where('id', req.params.id).first();
    if (!user) {
      res.send(respObj);
    }
    respObj = {
      status: 'success',
      data: user
    };
    res.send(respObj);
  } catch(e) {
    res.send(respObj);
  }
};

It works great, until I throw a non-existent user ID into the mix. I assumed the catch statement would handle the error if no user is found for the query, but that doesn't seem to work, it just spits out the respObj in the try block. So I added an if statement to check if the user object doesn't exist, and thats when I received the warning below:

(node:25711) UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at validateHeader (_http_outgoing.js:503:11)
    at ServerResponse.setHeader (_http_outgoing.js:510:3)
    at ServerResponse.header (/Users/munsterberg/Sites/fullstack_workspace/esports-manager/services/project/node_modules/express/lib/response.js:767:10)
    at ServerResponse.send (/Users/munsterberg/Sites/fullstack_workspace/esports-manager/services/project/node_modules/express/lib/response.js:170:12)
    at ServerResponse.json (/Users/munsterberg/Sites/fullstack_workspace/esports-manager/services/project/node_modules/express/lib/response.js:267:15)
    at ServerResponse.send (/Users/munsterberg/Sites/fullstack_workspace/esports-manager/services/project/node_modules/express/lib/response.js:158:21)
    at _callee3$ (/Users/munsterberg/Sites/fullstack_workspace/esports-manager/services/project/src/controllers/userController.js:45:7)
    at tryCatch (/Users/munsterberg/Sites/fullstack_workspace/esports-manager/services/project/node_modules/regenerator-runtime/runtime.js:65:40)
    at Generator.invoke [as _invoke] (/Users/munsterberg/Sites/fullstack_workspace/esports-manager/services/project/node_modules/regenerator-runtime/runtime.js:303:22)
    at Generator.prototype.(anonymous function) [as next] (/Users/munsterberg/Sites/fullstack_workspace/esports-manager/services/project/node_modules/regenerator-runtime/runtime.js:117:21)
    at step (/Users/munsterberg/Sites/fullstack_workspace/esports-manager/services/project/src/controllers/userController.js:14:191)
    at /Users/munsterberg/Sites/fullstack_workspace/esports-manager/services/project/src/controllers/userController.js:14:361
    at <anonymous>
(node:25711) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:25711) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Can anyone provide more info on why this is happening, and whats the fix?

Answer 1

Work around

A response is sent twice in the try block if the user does not exist. Presumably this indirectly accounts for the error raised:

"Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client".

Redirecting the "user does not exist" case to the catch block, by throwing an error, could be a work around to avoid the issue in the first place. A cut-down example:

export const getSingleUser = async(req, res) => {
  try  {
    const user = await knex('users').where('id', req.params.id).first();
    if (!user) {
      throw new Error(" user does not exist");
    }
    res.send( {
      status: 'success',
      data: user
    });
  } catch(e) {
    // console.log(e);  // debug if needed
    res.send( {
      status: 'fail',
      message: 'User does not exist'
    });
  }
};

No reason for why the try/catch block generates an uncaught promise rejection error stands out to me. Perhaps res.send has been coded to do something unusual in case of errors, say by raising a rejected promise asynchronously, but unfortunately that is only conjecture on my part and the true reason may be something else entirely.