Return consistent promise object for repeated calls with the same key

index.ts

@@ -27,15 +27,15 @@ export default function memoize<A extends unknown[], R extends unknown, T extend
   return function (this: T, ...args: A) {
     const id = hash.apply(this, args)
     if (cache.has(id)) return cache.get(id)
-    const result = fn.apply(this, args)
-    cache.set(id, result)
+    let result = fn.apply(this, args)
     if (result instanceof Promise) {
       // eslint-disable-next-line github/no-then
-      return result.catch(error => {
+      result = result.catch(error => {
         cache.delete(id)
         throw error
-      })
+      }) as R
     }
+    cache.set(id, result)
     return result
   }
 }

test/index.ts

@@ -42,6 +42,39 @@ describe('memoize', () => {
     expect(fn).to.have.been.called.with.exactly(noop)
   })
 
+  it('returns the same Promise when called multiple times', async () => {
+    const memoized = memoize(a => Promise.resolve(a))
+    const p1 = memoized('1')
+    const p2 = memoized('1')
+    const p3 = memoized('1')
+    expect(p2).to.equal(p1)
+    expect(p3).to.equal(p1)
+  })
+
+  it('does not catch promises as a side-effect', async () => {
+    let failed = false
+    function setFailed() {
+      failed = true
+    }
+    process.on('unhandledRejection', setFailed)
+    const error = new Error('Rejected promise')
+    const memoized = memoize(() => Promise.reject(error))
+    let rejected = false
+    try {
+      await memoized()
+    } catch (e) {
+      if (e === error) {
+        rejected = true
+      } else {
+        throw e
+      }
+    }
+    expect(rejected).to.equal(true, 'Promise should reject when memoized')
+    await new Promise(setImmediate)
+    expect(failed).to.equal(false, 'Promise should not reject as a side effect')
+    process.off('unhandledRejection', setFailed)
+  })
+
   describe('hash', () => {
     it('calls hash to get key for cache store', () => {
       let key = '1'