Generator Methods (next, return, throw)

Generator objects (returned by generator functions) have three methods that control their execution: next(), return(), and throw(). Each method influences the generator’s state and return value.

The next() Method

The next() method resumes generator execution and returns the next value.

generator.next(value);

• value: Optional value to pass into the generator (becomes the result of the current yield)
• Returns: { value, done } object

The return() Method

The return() method finishes the generator prematurely.

generator.return(value);

• Forces the generator to complete immediately
• The provided value is returned as the value property
• done is set to true
• Any finally blocks inside the generator will execute

The throw() Method

The throw() method injects an error into the generator’s execution point.

generator.throw(error);

• Resumes the generator by throwing an error at the current yield position
• If caught inside the generator, execution continues
• If not caught, the generator terminates with done: true

Key Points

• These methods give you fine-grained control over generator execution.
• return() is useful for cleanup (like closing resources).
• throw() enables advanced error handling patterns.
• All methods can be called even after the generator is done (safe no-ops).

Example Code

function* taskGenerator() {
  try {
    yield "Task 1";
    yield "Task 2";
    yield "Task 3";
  } catch (error) {
    console.log("Caught:", error.message);
    yield "Recovery task";
  }
  yield "Cleanup";
}

const tasks = taskGenerator();

// Normal execution with next()
console.log(tasks.next().value); // 'Task 1'
console.log(tasks.next().value); // 'Task 2'

// Using throw() - injects error
console.log(tasks.throw(new Error("Something went wrong")).value);
// 'Caught: Something went wrong'
// 'Recovery task'

console.log(tasks.next().value); // 'Cleanup'
console.log(tasks.next().done); // true

// Using return() to terminate early
function* countdown(n) {
  for (let i = n; i > 0; i--) {
    yield i;
  }
  yield "Liftoff!";
}

const counter = countdown(5);
console.log(counter.next().value); // 5
console.log(counter.next().value); // 4
console.log(counter.return("Aborted").value); // 'Aborted'
console.log(counter.next().done); // true

// Using return() with cleanup (finally blocks)
function* withCleanup() {
  try {
    yield "Working";
    yield "Still working";
  } finally {
    console.log("Cleanup executed");
  }
}

const resource = withCleanup();
console.log(resource.next().value); // 'Working'
resource.return(); // 'Cleanup executed' (finally runs)