validateFunction

Checks whether a value is a callable function. This validator uses isFunction internally.

Accepts arrow functions, traditional functions, and class constructors. Never throws and never mutates input.

Signature

function validateFunction<F extends (...args: unknown[]) => unknown>(
  value: unknown,
  field: string
): ValidationResult<F>

Parameters

Name	Data type	Description
value	unknown	The value to validate.
field	string	The name of the field being validated, used in error reporting.

Returns

One of:

• { ok: true, value: F }: If the value is a callable function matching the generic signature.
• { ok: false, field: string, message: string }: If the value is not a function.

Behavior

• Accepts arrow functions, traditional functions, and class constructors.
• Rejects primitives, objects, arrays, and non-function values.
• Performs no coercion.
• Never throws or mutates input.
• Fully generic: args and return type are typed as unknown, ensuring strict TypeScript compliance without any.

Examples

// Arrow function
validateFunction(() => 42, "fn")
// { ok: true, value: [Function] }

// Traditional function
validateFunction(function hello() {}, "fn")
// { ok: true, value: [Function: hello] }

// Class constructor
class MyClass {}
validateFunction(MyClass, "fn")
// { ok: true, value: [class MyClass] }

// Generic signature with unknown args and return
const result = validateFunction<(...args: unknown[]) => unknown>(
  (x) => x,
  "callback"
);
// { ok: true, value: [Function] }

// Invalid value
validateFunction(123, "fn")
// { ok: false, field: "fn", message: "Value must be a function" }

Notes

• Using generics with unknown[] avoids ESLint and TypeScript warnings.
• This validator is suitable for validating callbacks, hooks, or configuration entries.
• Fully compatible with Jane’s pure, zero side-effect philosophy.