refactor(core): add isPromise

packages/container/libraries/core/src/common/calculations/isPromise.spec.ts

@@ -0,0 +1,56 @@
+import { beforeAll, describe, expect, it } from '@jest/globals';
+
+import { isPromise } from './isPromise';
+
+describe(isPromise.name, () => {
+  describe.each<[string, unknown, boolean]>([
+    ['null', null, false],
+    ['a string', 'string-fixture', false],
+    ['a function with no "then" property', () => undefined, false],
+    ['an object with no "then" property', {}, false],
+    [
+      'a function with non function "then" property',
+      (() => {
+        const value: (() => void) & {
+          then?: unknown;
+        } = () => undefined;
+
+        value.then = 'fixture';
+
+        return value;
+      })(),
+      false,
+    ],
+    ['an object with non function "then" property', { then: 'fixture' }, false],
+    [
+      'a function with function "then" property',
+      (() => {
+        const value: (() => void) & {
+          then?: unknown;
+        } = () => undefined;
+
+        value.then = () => undefined;
+
+        return value;
+      })(),
+      true,
+    ],
+    [
+      'an object with function "then" property',
+      { then: () => undefined },
+      true,
+    ],
+  ])('having %s', (_: string, value: unknown, expectedResult: boolean) => {
+    describe('when called', () => {
+      let result: unknown;
+
+      beforeAll(() => {
+        result = isPromise(value);
+      });
+
+      it('should return expected value', () => {
+        expect(result).toBe(expectedResult);
+      });
+    });
+  });
+});

packages/container/libraries/core/src/common/calculations/isPromise.ts

@@ -0,0 +1,9 @@
+export function isPromise<T>(object: unknown): object is Promise<T> {
+  const isObjectOrFunction: boolean =
+    (typeof object === 'object' && object !== null) ||
+    typeof object === 'function';
+
+  return (
+    isObjectOrFunction && typeof (object as PromiseLike<T>).then === 'function'
+  );
+}