Checks if any value is a function type or the type obtained from its object class equal to 'function' and an instance of Function. It also denies it's a class by checking whether the function converted with Function.prototype.toString() to the string does not contain the word class at the beginning.

const isFunction = <Payload extends object>(
  value: any,
  callback: ResultCallback<any, Payload> = resultCallback,
  payload?: Payload
): value is Function =>
    typeof value === 'function' ||
      (typeOf(value) === 'function' && (value as any) instanceof Function)
      ? /class/.test(, 5)) ===
      : false,

Generic type variables


The Payload generic type variable constrained by object indicates the type of optional parameter payload of the supplied callback function and payload optional parameter of the isFunction() function from which it captures its value.


value: any

The value of any type to check.

callback: ResultCallback<any, Payload>

A callback function of ResultCallback type with parameters, the value that has been checked, the result of this check, and payload of generic type variable Payload with optional properties from the provided payload, to handle them before the result return. By default, it uses resultCallback() function.

payload?: Payload

An optional object of the generic type variable Payload is assigned to the payload of the given callback function.

Return type

value is Function

The return type is a boolean as the result of its statement indicating the value is a Function.


The return value is a boolean indicating whether the provided value is a function.

Example usage

// Example usage.
import { isFunction } from '@angular-package/type';

class Class { x = 5; }
const FUNC = (x: number): any => x + 5;

isFunction(Class); // Returns `false` as `value is typeof Class`, it must not be a `class`.
isFunction(FUNC); // Returns `true` as `value is (x: number) => any`
isFunction(() => 5); // Returns `true` as `value is () => 5`

Parameters callback and payload

// Example usage with callback and payload.
import { isFunction } from '@angular-package/type';

isFunction(() => 5, (result, value, payload) => {
  value // Returns `() => 5`
  if (payload) {
    payload.number // Returns `true`
  return result;
}, { number: true }); // Returns `true` as `value is () => 5`

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