AsyncGenerator.prototype.throw()

The throw() method of an async generator acts as if a throw statement is inserted in the generator's body at the current suspended position, which informs the generator of an error condition and allows it to handle the error, or perform cleanup and close itself.

Syntax

asyncGeneratorObject.throw(exception)

Parameters

exception

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

Return value

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.

Examples

Using throw()

The following example shows a simple 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 }

Specifications

Specification
ECMAScript Language Specification
# sec-asyncgenerator-prototype-throw

Browser compatibility

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See also