AsyncGenerator.prototype.throw()

exception

The exception to throw. For debugging purposes, it is useful to make it an instanceof Error.

If the thrown error is not caught, it will return a Promise which rejects with the exception passed in.

If the exception is caught by a try...catch and the generator resumes to yield more values, it will return a Promise which resolves with an Object with two properties:

done

A boolean value:

• true if the generator function's control flow has reached the end.
• false if the generator function is able to produce more values.

value

The value yielded from the next yield expression.

The following example shows a generator and an error that is thrown using the throw method. An error can be caught by a try...catch block as usual.

// An async task. Pretend it's doing something more useful
// in practice.
function sleep(time) {
  return new Promise((resolve, reject) => {
    setTimeout(resolve, time);
  });
}

async function* createAsyncGenerator() {
  while (true) {
    try {
      await sleep(500);
      yield 42;
    } catch (e) {
      console.error(e);
    }
  }
}

const asyncGen = createAsyncGenerator();
asyncGen.next(1).then((res) => console.log(res)); // { value: 42, done: false }
asyncGen
  .throw(new Error("Something went wrong")) // Error: Something went wrong
  .then((res) => console.log(res)); // { value: 42, done: false }

• async function*
• Iterators and generators guide