fix: Allow retrying with non-Promise thenables

src/retrier.js

@@ -191,13 +191,13 @@ export class Retrier {
         }
 
         // if the result is not a promise then reject an error
-        if (!(result instanceof Promise)) {
+        if (!result || typeof result.then !== "function") {
             return Promise.reject(new Error("Result is not a promise."));
         }
 
         // call the original function and catch any ENFILE or EMFILE errors
         // @ts-ignore because we know it's any
-        return result.catch(error => {
+        return Promise.resolve(result).catch(error => {
             if (!this.#check(error)) {
                 throw error;
             }
@@ -240,7 +240,9 @@ export class Retrier {
 
         // otherwise, try again
         task.lastAttempt = Date.now();
-        task.fn()
+
+        // Promise.resolve needed in case it's a thenable but not a Promise
+        Promise.resolve(task.fn())
             // @ts-ignore because we know it's any
             .then(result => task.resolve(result))
 

tests/retrier.test.js

@@ -46,6 +46,33 @@ describe("Retrier", () => {
             assert.equal(result, 2);
         });
 
+        it("should retry a function that rejects an error using a non-Promise thenable", async () => {
+
+            let count = 0;
+            const retrier = new Retrier(error => error.message === "foo");
+            const result = await retrier.retry(() => {
+                count++;
+
+                if (count === 1) {
+                    return {
+                        then() {
+                            throw new Error("foo");
+                        }
+                    };
+                }
+
+                return {
+                    then(fn) {
+                        fn(count);
+                    }
+                };
+            });
+
+            assert.equal(result, 2);
+        });
+
+
+
         it("should retry a function that rejects an error multiple times", async () => {
 
             let count = 0;