通过使用解决或拒绝或Promisify（蓝鸟）进行的承诺

Question

Hi I have a method in which I use the Promise class provided by bluebird to either resolve or reject based on the result of a third party function. My concern is that the third party function is synchronous and hence could throw an error. Below is my code:

 Authenticator.prototype.createJWTResponse = function(user, secret) { if (user && user.ID) { var expires = moment().add(7, 'days').valueOf(); //encode is third party and synchronous that can throw error var token = jwt.encode({ iss: user.ID, exp: expires }, secret); //throw new Error("thrown by a promise method"); return Promise.resolve({ token: token, expires: expires, user: {} }); } else { return Promise.reject(new Error('Authenticator:createJWTResponse: user object missing or doesnt have ID')); } }; 

One option is that I do not use reject and response in the method and make it a normal callback method which is then promisified by Promise.promisify. However my question is; if there is a way to use resolve and reject and create a promise in case the third party method throws an exception. Another question that I have is; it is not stated in bluebird documentation; that if one is using resolve and reject in a method; can such a method be promisified? I tried to promisify the above method but it fails to return for some reason & doesnt throw an error; which makes me suspect that if one is using resolve, reject in a method then such a method cannot be promisifed.

Can someone clarify this for me? Thanks in advance.

(Update)

If I wrap the method in Promise.method; bluebird throws a very clear error as follows:

Possibly unhandled Error: Synchronous Error in otherwise promise returning method

Returning a Promise.try from inside this method also throws the same error as in case of Promise.method.

Wrapping the code in the method in a try catch and rejecting on error also throws:

Possibly unhandled Error:

(Solution)

As suggested by Benjamin; taking away all resolve and reject calls from within the method and wrapping it in Promise.method gives the expected result. The final code is as follows:

 Authenticator.prototype.createJWTResponse = Promise.method(function(user, secret) { if (user && user.ID) { var expires = moment().add(7, 'days').valueOf(); var token = jwt.encode({ iss: user.ID, exp: expires }, secret); //Note: This is caught by the catch block on this method //throw new Error('sync error thrown'); return { token: token, expires: expires, user: {} } } else { throw new Error('Authenticator:createJWTResponse: user object missing or doesnt have id'); } }); 

Even a callback in not required in this case.

Answer 1

First of all .promisify already converts throws to rejections.

As for normal promisification - the promise constructor is throw safe, you can use Promise.method which is throw safe.

The reason sync throws are converted to rejects is in order to create a consistent API and so you won't have to .catch and catch all the time.