在express.js中，当我使用Promise时，是否需要在执行程序中包含reject（）？

Question

In the situation that we do not deal with reject, must we include reject in the promise executor?

for example:

 new promise ((res)=>{
   res(a);
 })

Answer 1

It's not necessary, no, but it's not a very elegant way to handle errors - if an error is thrown whilst your promise is pending, you'll want to take some other action and reject the promise, letting the user know what's wrong.

Have a look at MDN Promise Docs , they explain the concept of handling errors in Promises pretty well!

Answer 2

If you know that a promise will never be rejected, adding a rejection handler is not necessary. Eg it's obvious this will never be rejected and adding a rejection handler would be plain silly:

Promise.resolve("abc")
    .then(result => console.log(result));

In all other cases, not providing a rejection handler is pretty much the same as not wrapping an error-throwing piece of code in try - catch . It might be intentional, of course, but bear in mind that node.js will treat unhandled promise rejections as if they were uncaught errors: by terminating the node.js process. That's what it says on the console when an unhandled promise rejection occurs:

(node:7741) 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.

So, summing up:

• skip rejection handlers when you know for a fact a rejection is impossible,
• skip rejection handlers when you require the application be terminated upon rejection (in future versions of Node.js),
• write rejection handlers in all other cases.

EDIT (the question became clearer after the edits were made, hence my update to the answer):

As for including the reject() call in the promise executor, no, you do not need to add it. Again though, you have to make sure that the internal logic guarantees that resolve() will always be called.