feat: add isPromise type guard

README.md

@@ -38,6 +38,10 @@ Provides additional types and type adjusted utilities for `typescript`
 
 - `literalArray(...entries)`: return an array those items are restricted to the provided literals.
 
+### Promise function
+
+- `isPromise<R>(subject: any)`: `isPromise()` type guard.
+
 ### Type manipulation
 
 - `ExcludePropType<T, U>`: excludes type `U` from properties in `T`.

src/index.ts

@@ -2,6 +2,7 @@ export * from './filterKey';
 export * from './findKey';
 export * from './forEachKey';
 export * from './Id';
+export * from './isPromise';
 export * from './KeyTypes';
 export * from './literalArray';
 export * from './mapKey';

src/isPromise.spec.ts

@@ -0,0 +1,26 @@
+import t from 'assert';
+import { isPromise } from '.';
+
+test('false if subject is falsy value or non-object', () => {
+  t.strictEqual(isPromise(undefined), false)
+  t.strictEqual(isPromise(null), false)
+  t.strictEqual(isPromise(0), false)
+  t.strictEqual(isPromise(true), false)
+  t.strictEqual(isPromise('a'), false)
+  t.strictEqual(isPromise([]), false)
+})
+
+test('false if subject does not have a then function', () => {
+  t.strictEqual(isPromise({}), false)
+})
+
+test('false if subject.then is not a function', () => {
+  t.strictEqual(isPromise({ then: true }), false)
+})
+
+test('type guard as promise', () => {
+  const subject = { then() { return true } }
+  if (isPromise(subject)) {
+    t.strictEqual(subject.then(), true)
+  }
+})

src/isPromise.ts

@@ -0,0 +1,3 @@
+export function isPromise<R = any>(subject: any): subject is Promise<R> {
+  return !!subject && typeof subject.then === 'function'
+}

src/test-util/index.ts

@@ -9,3 +9,7 @@ export function isNever(_: never) {
 export function isNumber(_: number) {
   return true
 }
+
+export function isVoid(_: void) {
+  return true
+}