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.

If the thrown exception is not caught by a try...catch, the exception passed to throw() will be thrown out from the generator function.

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 simple 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*