Use more normal callback definition.

lib/utils.ts

@@ -1,11 +1,8 @@
-/** A callback function that expects an error to be passed. */
-export type ErrorCallback = (error: Error, result?: never) => void
-
-/** A callback function that expects a successful result. */
-type SuccessCallback<T> = (error: null, result: T) => void
-
 /** A callback function that accepts an error or a result. */
-export type Callback<T> = SuccessCallback<T> & ErrorCallback
+export interface Callback<T> {
+  (error: Error, result?: never): void
+  (error: null, result: T): void
+}
 
 /** Safely converts any value to string, using the value's own `toString` when available. */
 export const safeToString = (val: unknown) => {

lib/validators.ts

@@ -26,7 +26,7 @@ SOFTWARE.
 
 ************************************************************************************ */
 
-import { ErrorCallback, safeToString } from './utils'
+import { Callback, safeToString } from './utils'
 
 /* Validation functions copied from check-types package - https://www.npmjs.com/package/check-types */
 
@@ -68,7 +68,7 @@ export function isInteger(data: unknown): boolean {
  */
 export function validate(
   bool: boolean,
-  cbOrMessage?: ErrorCallback | string,
+  cbOrMessage?: Callback<never> | string,
   message?: string,
 ): void {
   if (bool) return // Validation passes