generator function

@nick/is/generator-function

isGeneratorFunction

Signature

function isGeneratorFunction(it: unknown): it is GeneratorFunction;

Checks if a given value is a generator function, which is a function that uses a distinct syntax and returns a generator object when called.

To check if a value is a generator object (the actual iterable iterator that is typically used within for..of loops), use isGenerator instead.

Parameters

Name	Info
it	The value to check.

Returns

true if the value is a generator function, false otherwise.

Category

Generators

Examples

import { isGeneratorFunction } from "jsr:@nick/is/generator-function";

function* gen() {
  yield 1;
}
const iter = gen();

console.log(isGeneratorFunction(gen)); // true
console.log(isGeneratorFunction(iter)); // false

Another example, highlighting the differences between a generator function and a generator object, using a custom iterable class:

import { isGenerator, isGeneratorFunction, isIterable } from "jsr:@nick/is";

class Foo {
  *delegate() {
    return yield* this; // <- thanks to the [Symbol.iterator] method below
  }

  *[Symbol.iterator](): IterableIterator<number> {
    for (let i = 0; i < 10; i++) yield i;
  }
}

const foo = new Foo();

// Since Foo.prototype has a [Symbol.iterator] method, it is iterable:
if (isIterable(foo)) {
  // This will log the numbers 1 through 10
  for (const value of foo) console.log(value);
}

// However, it is _not_ a generator not generator function:
isGenerator(foo); // false
isGeneratorFunction(foo); // false

// It's delegate and [Symbol.iterator] methods _are_ generator functions:
isGeneratorFunction(foo.delegate); // true
isGeneratorFunction(foo[Symbol.iterator]); // true

// And they _return_ generator objects...:
const iter1 = foo.delegate();
const iter2 = foo[Symbol.iterator]();
isGenerator(iter1); // true
isGenerator(iter2); // true

// ...which are also iterable:
isIterable(iter1); // true
isIterable(iter2); // true

Notes

This function will not return true for async generator functions, as they are a distinctly different type of function that returns a different type of object. Use isAsyncGeneratorFunction to check for an async generator function instead.