yield是否需返回值？关于Bluebird Promise.coroutine的技术疑问

Hey there! Let's break down what's going on with your code and answer your core question clearly.

First: Why that "undefined yielded" error? #

The error Unhandled rejection TypeError: A value undefined was yielded that could not be treated as a promise happens because Bluebird doesn't know how to handle an undefined value you're yielding.

When using Promise.coroutine, the values you yield need to be one of these:

• A standard Promise object
• A "thenable" object (something with a .then() method)
• Any non-undefined plain value (Bluebird automatically wraps these in a resolved Promise)

If you yield undefined, Bluebird can't tell if this is intentional or an accident (like a function that forgot to return something), so it throws that error to alert you.

Your core question: Do yielded functions need to be coroutines? #

Nope! You don't need to turn every function you yield into a coroutine. The only requirement is that the value you yield (usually the return value of the function) is one of the valid types I listed above.

Coroutines are just a way to wrap generators so you can write sequential-looking async code. The async operations you yield can be regular functions that return Promises, native async functions, or even plain values—they don't need to be coroutines themselves.

Fixing your code example #

Let's assume your problematic code looks something like this (based on your linked example):

const Promise = Bluebird;

// Problem: This function doesn't return a Promise (or any value!)
function getSomeData() {
  setTimeout(() => {
    console.log("Data fetched");
    // No return statement here, so function returns undefined
  }, 1000);
}

const myCoroutine = Promise.coroutine(function*() {
  console.log("Starting...");
  // Yielding undefined here causes the error
  const data = yield getSomeData();
  console.log("Got data:", data);
});

myCoroutine();

To fix this, just make sure the function you're yielding returns a Promise:

const Promise = Bluebird;

// Fixed: Returns a Promise that resolves with the data
function getSomeData() {
  return new Promise((resolve) => {
    setTimeout(() => {
      console.log("Data fetched");
      resolve("Sample data");
    }, 1000);
  });
}

const myCoroutine = Promise.coroutine(function*() {
  console.log("Starting...");
  // Now we're yielding a valid Promise—no error!
  const data = yield getSomeData();
  console.log("Got data:", data);
});

myCoroutine();

If you really need to yield an undefined value (e.g., a function that intentionally has no return), you can wrap it explicitly to avoid the error:

yield Promise.resolve(undefined);

Or if you're calling a function that might return undefined, you can handle it like this:

const result = yield (maybeReturnsUndefined() || Promise.resolve());

That should get your coroutine working as expected!

内容的提问来源于stack exchange，提问作者pourmesomecode