Add option to reset interval on success (#18)

* Add build directory to gitignore

* Fix ts-node test compilation error

* Replace compilers flag with require

The mocha `--compilers` flag is deprecated in favor of `--require`:
https://github.com/mochajs/mocha/wiki/compilers-deprecation

* Add resetOnSuccess option to retryBackoff operator

* Add tests related to internal state

* Fix referential transparency issue

* fixup! Add resetOnSuccess option to retryBackoff operator

* Format retryBackoff with prettier

.gitignore

@@ -1,2 +1,3 @@
 node_modules
-dist
+dist
+build

spec/mocha.opts

@@ -1,2 +1,2 @@
---compilers ts:ts-node/register
-spec/**/*-spec.ts
+--require ts-node/register
+spec/**/*-spec.ts

spec/retryBackoff-spec.ts

@@ -1,8 +1,8 @@
 import { expect } from 'chai';
-import { retryBackoff } from '../src/index';
-import { of, Observable, Observer, throwError, Subject } from 'rxjs';
-import { map, mergeMap, concat, multicast, refCount } from 'rxjs/operators';
+import { Observable, Observer, of, Subject, throwError } from 'rxjs';
+import { concat, map, mergeMap, multicast, refCount } from 'rxjs/operators';
 import { TestScheduler } from 'rxjs/testing';
+import { retryBackoff } from '../src/index';
 
 describe('retryBackoff operator', () => {
   let testScheduler: TestScheduler;
@@ -253,7 +253,7 @@ describe('retryBackoff operator', () => {
     of(1, 2, 3)
       .pipe(
         concat(throwError('bad!')),
-        multicast(() => new Subject()),
+        multicast(() => new Subject<number>()),
         refCount(),
         retryBackoff({ initialInterval: 1, maxRetries: 4 })
       )
@@ -350,4 +350,101 @@ describe('retryBackoff operator', () => {
       expectSubscriptions(source.subscriptions).toBe(subs);
     });
   });
+
+  it('should be referentially transparent', () => {
+    testScheduler.run(({ expectObservable, cold, expectSubscriptions }) => {
+      const source1 = cold('--#');
+      const source2 = cold('--#');
+      const unsub = '      ---------!';
+      const subs = [
+        '                  ^-!       ',
+        '                  ---^-!    ',
+        '                  -------^-!',
+      ];
+      const expected = '   ----------';
+
+      const op = retryBackoff({
+        initialInterval: 1,
+      });
+
+      expectObservable(source1.pipe(op), unsub).toBe(expected);
+      expectSubscriptions(source1.subscriptions).toBe(subs);
+
+      expectObservable(source2.pipe(op), unsub).toBe(expected);
+      expectSubscriptions(source2.subscriptions).toBe(subs);
+    });
+  });
+
+  it('should ensure interval state is per-subscription', () => {
+    testScheduler.run(({ expectObservable, cold, expectSubscriptions }) => {
+      const source = cold('--#');
+      const sub1 = '      ^--------!';
+      const sub2 = '      ----------^--------!';
+      const subs = [
+        '                  ^-!       ',
+        '                  ---^-!    ',
+        '                  -------^-!',
+        '                  ----------^-!       ',
+        '                  -------------^-!    ',
+        '                  -----------------^-!',
+      ];
+      const expected = '   ----------';
+
+      const result = source.pipe(retryBackoff({
+        initialInterval: 1,
+      }));
+
+      expectObservable(result, sub1).toBe(expected);
+      expectObservable(result, sub2).toBe(expected);
+      expectSubscriptions(source.subscriptions).toBe(subs);
+    });
+  });
+
+  it('should reset the delay when resetOnSuccess is true', () => {
+    testScheduler.run(({ expectObservable, cold, expectSubscriptions }) => {
+      const source = cold('--1-2-3-#');
+      const subs = [
+        '                  ^-------!',
+        '                  ---------^-------!',
+        '                  ------------------^-------!',
+        '                  ---------------------------^-------!'
+        //                 interval always reset to 1 ^
+      ];
+      const unsub = '      -----------------------------------!';
+      const expected = '   --1-2-3----1-2-3----1-2-3----1-2-3--';
+
+      expectObservable(
+        source.pipe(
+          retryBackoff({
+            initialInterval: 1,
+            resetOnSuccess: true
+          })
+        ),
+        unsub
+      ).toBe(expected);
+      expectSubscriptions(source.subscriptions).toBe(subs);
+    });
+  });
+
+  it('should not reset the delay on consecutive errors when resetOnSuccess is true', () => {
+    testScheduler.run(({ expectObservable, cold, expectSubscriptions }) => {
+      const source = cold('--------#');
+      const unsub = '      -------------------------------------!';
+      const subs = [
+        '                  ^-------!                             ',
+        '                  ---------^-------!                    ',
+        '                  -------------------^-------!          ',
+        '                  -------------------------------^-----!'
+      ];
+      const expected = '   --------------------------------------';
+
+      const result = source.pipe(retryBackoff({
+        initialInterval: 1,
+        resetOnSuccess: true
+      }));
+
+      expectObservable(result, unsub).toBe(expected);
+      expectSubscriptions(source.subscriptions).toBe(subs);
+    });
+  });
 });

src/operators/retryBackoff.ts

@@ -1,7 +1,6 @@
-import { iif, Observable, throwError, timer } from 'rxjs';
-import { concatMap, retryWhen } from 'rxjs/operators';
-
-import { getDelay, exponentialBackoffDelay } from '../utils';
+import { defer, iif, Observable, throwError, timer } from 'rxjs';
+import { concatMap, retryWhen, tap } from 'rxjs/operators';
+import { exponentialBackoffDelay, getDelay } from '../utils';
 
 export interface RetryBackoffConfig {
   // Initial interval. It will eventually go as high as maxInterval.
@@ -10,6 +9,9 @@ export interface RetryBackoffConfig {
   maxRetries?: number;
   // Maximum delay between retries.
   maxInterval?: number;
+  // When set to `true` every successful emission will reset the delay and the
+  // error count.
+  resetOnSuccess?: boolean;
   // Conditional retry.
   shouldRetry?: (error: any) => boolean;
   backoffDelay?: (iteration: number, initialInterval: number) => number;
@@ -31,20 +33,32 @@ export function retryBackoff(
     maxRetries = Infinity,
     maxInterval = Infinity,
     shouldRetry = () => true,
-    backoffDelay = exponentialBackoffDelay
+    resetOnSuccess = false,
+    backoffDelay = exponentialBackoffDelay,
   } = typeof config === 'number' ? { initialInterval: config } : config;
   return <T>(source: Observable<T>) =>
-    source.pipe(
-      retryWhen<T>(errors =>
-        errors.pipe(
-          concatMap((error, i) =>
-            iif(
-              () => i < maxRetries && shouldRetry(error),
-              timer(getDelay(backoffDelay(i, initialInterval), maxInterval)),
-              throwError(error)
-            )
+    defer(() => {
+      let index = 0;
+      return source.pipe(
+        retryWhen<T>(errors =>
+          errors.pipe(
+            concatMap(error => {
+              const attempt = index++;
+              return iif(
+                () => attempt < maxRetries && shouldRetry(error),
+                timer(
+                  getDelay(backoffDelay(attempt, initialInterval), maxInterval)
+                ),
+                throwError(error)
+              );
+            })
           )
-        )
-      )
-    );
+        ),
+        tap(() => {
+          if (resetOnSuccess) {
+            index = 0;
+          }
+        })
+      );
+    });
 }