JavaScript

Generator Already Closed: What It Means and How to Fix It

Error reference

Generator Already Closed

Language

JavaScript

Severity

Low

When it happens

Once a generator function returns (or its body is exhausted), calling .next() on it returns { value: undefined, done: true } silently rather than throwing an error.

Potential fixes

Always check the done property from .next() results. Use for...of loops which handle the done signal automatically. If you need a restartable source of values, recreate the generator rather than reusing a closed one.

Related errors

RuntimeError StopIteration

Deep dive

Once a generator function returns (or its body is exhausted), calling .next() on it returns { value: undefined, done: true } silently rather than throwing an error. This can cause subtle bugs where a loop or consumer silently stops receiving values without any indication of why.

Why it happens

•Calling .next() after the generator has already returned or thrown.
•An early return statement exits the generator before all values are yielded.
•Calling gen.return() explicitly closes the generator.
•An exception thrown inside the generator closes it.

Potential fixes

Always check the done property from .next() results. Use for...of loops which handle the done signal automatically. If you need a restartable source of values, recreate the generator rather than reusing a closed one.

Examples

JavaScript

Code that triggers the error

function* counter() {
  yield 1;
  yield 2;
}

const gen = counter();
console.log(gen.next()); // { value: 1, done: false }
console.log(gen.next()); // { value: 2, done: false }
console.log(gen.next()); // { value: undefined, done: true }
console.log(gen.next()); // { value: undefined, done: true } -- silently no-ops

Error output

{ value: 1, done: false }
{ value: 2, done: false }
{ value: undefined, done: true }
{ value: undefined, done: true }

Fixed code

function* counter() {
  yield 1;
  yield 2;
}

const gen = counter();
let result = gen.next();
while (!result.done) {
  console.log(result.value);
  result = gen.next();
}

Practice in English

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"I hit an error...

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