Generator.prototype.throw()

exception

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

If the thrown exception is caught by a try...catch and the generator resumes to yield more values, it will return 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.

TypeError

Thrown if the generator is already running.

If the exception is not caught by a try...catch within the generator function, it is also thrown to the caller of throw().

The throw() method, when called, can be seen as if a throw exception; statement is inserted in the generator's body at the current suspended position, where exception is the exception passed to the throw() method. Therefore, in a typical flow, calling throw(exception) will cause the generator to throw. However, if the yield expression is wrapped in a try...catch block, the error may be caught and control flow can either resume after error handling, or exit gracefully.

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.

function* gen() {
  while (true) {
    try {
      yield 42;
    } catch (e) {
      console.log("Error caught!");
    }
  }
}

const g = gen();
g.next();
// { value: 42, done: false }
g.throw(new Error("Something went wrong"));
// "Error caught!"
// { value: 42, done: false }

• function*