Promise.isPromise

Static method that checks whether a value is a thenable — that is, any object with a .then method.

Type	TC39	CoreJS	Babel	Civet	JS+
Polyfill	✅ Stage 2 View	❌	❌	❌	✅ via Custom

Promise.isPromise(arg: any): arg is PromiseLike<any>

Baseline

Returns true if arg is a thenable. Because it follows Promises/A+, it returns true for any object or function with a .then method, not strictly native Promise instances.

The return type is a TypeScript type predicate, so the type of arg is automatically narrowed to PromiseLike<unknown> inside a truthy branch.

Promise.isPromise(Promise.resolve(1)); // → true
Promise.isPromise(fetch('/api/data')); // → true
Promise.isPromise({ then: () => {} }); // → true  (thenable)

Promise.isPromise(null); // → false
Promise.isPromise(undefined); // → false
Promise.isPromise(42); // → false
Promise.isPromise({ then: 'not a fn' }); // → false