Set up a scenario that makes use of ambient modules

package.json

@@ -40,6 +40,7 @@
     "eslint-plugin-jsdoc": "^36.1.0",
     "eslint-plugin-node": "^11.1.0",
     "eslint-plugin-prettier": "^3.3.1",
+    "is-callable": "^1.2.4",
     "jest": "^26.4.2",
     "prettier": "^2.2.1",
     "prettier-plugin-packagejson": "^2.2.11",

src/callMeMaybe.test.ts

@@ -0,0 +1,21 @@
+import { mocked } from 'ts-jest/utils';
+import isCallable from 'is-callable';
+import callMeMaybe from './callMeMaybe';
+
+jest.mock('is-callable');
+
+const mockedIsCallable = mocked(isCallable, true);
+
+describe('callMeMaybe', () => {
+  it('returns the result of the callable when isCallable returns true', () => {
+    mockedIsCallable.mockReturnValue(true);
+    const fn = () => 42;
+    expect(callMeMaybe(fn)).toBe(42);
+  });
+
+  it('returns null when isCallable returns false', () => {
+    mockedIsCallable.mockReturnValue(false);
+    const fn = () => 42;
+    expect(callMeMaybe(fn)).toBeNull();
+  });
+});

src/callMeMaybe.ts

@@ -0,0 +1,16 @@
+import isCallable from 'is-callable';
+
+/**
+ * Returns whether the given argument is callable or not.
+ *
+ * @param callable - A function.
+ * @returns Whether the given argument is callable or not.
+ */
+function maybeCall(callable: any): any {
+  if (isCallable(callable)) {
+    return callable();
+  }
+  return null;
+}
+
+export default maybeCall;

src/dependencies.d.ts

@@ -0,0 +1,5 @@
+declare module 'is-callable' {
+  export default function isCallable<T>(
+    value: any,
+  ): value is (...args: any[]) => T;
+}

yarn.lock

@@ -2553,6 +2553,11 @@ is-callable@^1.1.4, is-callable@^1.2.3:
   resolved "https://registry.yarnpkg.com/is-callable/-/is-callable-1.2.3.tgz#8b1e0500b73a1d76c70487636f368e519de8db8e"
   integrity sha512-J1DcMe8UYTBSrKezuIUTUwjXsho29693unXM2YhJUTR2txK/eG47bvNa/wipPFmZFgr/N6f1GA66dv0mEyTIyQ==
 
+is-callable@^1.2.4:
+  version "1.2.4"
+  resolved "https://registry.yarnpkg.com/is-callable/-/is-callable-1.2.4.tgz#47301d58dd0259407865547853df6d61fe471945"
+  integrity sha512-nsuwtxZfMX67Oryl9LCQ+upnC0Z0BgpwntpS89m1H/TLF0zNfzfLMV/9Wa/6MZsj0acpEjAO0KF1xT6ZdLl95w==
+
 is-ci@^2.0.0:
   version "2.0.0"
   resolved "https://registry.yarnpkg.com/is-ci/-/is-ci-2.0.0.tgz#6bc6334181810e04b5c22b3d589fdca55026404c"