throw method

A generator object also has a throw method to pass a value to it and trigger an exception to throw inside of the generator object. Both throw and next methods can send values to generator objects and change their behaviors. A value passed using next is treated as the result of last yield expression, but a value passed using throw is treated as replacing last yield expression with a throw statement.

In the code below, when passing hello to the generator object using throw('hello'), an uncaught error is thrown and the generator object is finished. When func.throw('hello') is invoked, the last yield expression yield x + 1 is replaced with throw 'hello'. Since the thrown object is not caught, it's propagated to the JavaScript engine.

function *sample() {
  let x = yield 1;
  let y = yield x + 1;
  yield y * 10;
}

let func = sample();
func.next();
// -> {value: 1, done: false}
func.next(1);
// -> {value: 2, done: false}
func.throw('hello');
// -> Uncaught hello
func.next();
// -> {value: undefined, done: true}

Although it's possible to pass any types of values to throw(), it's recommended to pass Error objects for better debugging, e.g. throw(new Error('boom!')).

We can use try-catch in the generator function to handle errors. In the code below, when func.throw(new Error('boom!')) is invoked, last yield expression yield 2 is replaced with throw new Error('boom!'). The thrown object is caught by try-catch. So the execution continues until the next yield expression yield 3.

function *sample() {
  yield 1;
  try {
    yield 2;
  } catch (e) {
    console.error(e);
  }
  yield 3;
  yield 4;
}

let func = sample();
func.next();
// -> {value: 1, done: false}
func.next();
// -> {value: 2, done: false}
func.throw(new Error('boom!'));
// -> Error: boom!
// -> {value: 3, done: false}
func.next();
// -> {value: 4, done: false}

If the value passed by throw() is caught and handled by the generator object, it can continue to generate all remaining values. Otherwise, it will finish with a uncaught error.