Converts tests to TypeScript (#300)

packages/util/src/assert.ts

@@ -7,9 +7,10 @@ import isFunction from './is/function';
 
 type MessageFn = () => string;
 
+type Falsy = null | undefined | false | 0 | ''; // No NaN type
+
 /**
  * @name assert
- * @signature assert (test: any, message: string | () => string, code: number = ExtError.CODES.ASSERT, data: any): void
  * @summary Checks for a valid test, if not ExtError is thrown.
  * @description
  * Checks that `test` is a truthy value. If value is falsy (`null`, `undefined`, `false`, ...), it throws an ExtError with the supplied `message` and an optional `code` and `data`. When `test` passes, `true` is returned.
@@ -24,7 +25,7 @@ type MessageFn = () => string;
  * assert(false, () => 'message'); // ExtError with 'message'
  * ```
  */
-export default function assert (test: any, message: string | MessageFn, code: number = ExtError.CODES.ASSERT, data?: any): boolean {
+export default function assert<T> (test: Falsy | T, message: string | MessageFn, code: number = ExtError.CODES.ASSERT, data?: any): test is T {
   if (test) {
     return true;
   }