ConstZod v~4.0.0 changed how z.function() worked, which made it unsuitable for validation. This was reverted in Zod v4.1.0.
FunctionSchema.parse(function () {}); // ✓ Valid
FunctionSchema.parse(() => {}); // ✓ Valid
FunctionSchema.parse(async () => {}); // ✓ Valid
FunctionSchema.parse(function* () {}); // ✓ Valid
FunctionSchema.parse(Math.max); // ✓ Valid
FunctionSchema.parse('not a function'); // ✗ Throws validation error
FunctionSchema.parse({}); // ✗ Throws validation error
import { createAssertion, use } from 'bupkis';
import { FunctionSchema } from 'bupkis/schema';
const fnAssertion = createAssertion(
[FunctionSchema, 'to be a function with arity 2'],
FunctionSchema.refine((subject) => subject.length === 2),
);
const { expect } = use([fnAssertion]);
function add(a: number, b: number) {
return a + b;
}
expect(add, 'to be a function with arity 2');
A Zod schema that validates any JavaScript function.
This schema accepts a function having any signature and avoids Zod's parsing overhead.