Check if a value is a generator function

Read the internal tag of function* and async function* declarations.

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

function isGeneratorFunction(value) { return ['[object GeneratorFunction]', '[object AsyncGeneratorFunction]'].includes(Object.prototype.toString.call(value)); }

Usage

isGeneratorFunction(function* () {}); // → true
isGeneratorFunction(async function* () {}); // → true
isGeneratorFunction(function () {}); // → false

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

Know the limits

• The original missed async generator functions, as the second line shows. Symbol.toStringTag can spoof the tag.
• A plain function that returns an iterator is not detected; to consume any iterable, check the value it returns for Symbol.iterator.

The function above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Logical xor operator — True when exactly one of two values is truthy.
• Identity function — Return the argument unchanged.
• Box handler — Chain transformations on a value, then unwrap the result.