Make jitter subtract only

test/exponential-backoff.test.ts

@@ -9,8 +9,8 @@ import { ExponentialBackoffMaxRetriesHitError } from '../source/errors.ts';
 const testExponentialBackoffRunUsesDefaultOptions = async (): Promise<void> => {
     // Fake timers let us advance time without waiting real seconds between retries.
     vi.useFakeTimers();
-    // Pin Math.random so jitter does not randomize the delay we are about to measure.
-    vi.spyOn(Math, 'random').mockReturnValue(0.5);
+    // Pin Math.random to 0 so jitter does not reduce the default delay.
+    vi.spyOn(Math, 'random').mockReturnValue(0);
 
     try {
         // The wrapped function fails on its first invocation and succeeds on the second.
@@ -315,11 +315,11 @@ const testExponentialBackoffCapsDelayAtMaxDelay = async (): Promise<void> => {
 };
 
 /**
- * Tests that jitter scales the capped delay up or down by jitter * cappedDelay based on Math.random.
+ * Tests that jitter subtracts up to jitter * cappedDelay from the capped delay based on Math.random.
  */
 const testExponentialBackoffAppliesJitter = async (): Promise<void> => {
     vi.useFakeTimers();
-    // random = 1 → jitterDirection = 2*1 - 1 = +1 → full positive 10% jitter on the delay.
+    // random = 1 → full 10% reduction: 1000 - (1 * 0.1 * 1000) = 900ms.
     vi.spyOn(Math, 'random').mockReturnValue(1);
 
     try {
@@ -337,8 +337,11 @@ const testExponentialBackoffAppliesJitter = async (): Promise<void> => {
         await Promise.resolve();
         expect(fn).toHaveBeenCalledTimes(1);
 
-        // cappedDelay = 1000; with +10% jitter the retry fires after 1100ms, not 1000ms.
-        await vi.advanceTimersByTimeAsync(1_100);
+        // Advancing 899ms is one ms short of the jittered delay; 900ms triggers the retry.
+        await vi.advanceTimersByTimeAsync(899);
+        expect(fn).toHaveBeenCalledTimes(1);
+
+        await vi.advanceTimersByTimeAsync(1);
         expect(fn).toHaveBeenCalledTimes(2);
 
         await expect(promise).resolves.toBe('success');
@@ -364,7 +367,7 @@ const runTests = async (): Promise<void> => {
     test('ExponentialBackoff: retries indefinitely when maxAttempts is 0', testExponentialBackoffRetriesIndefinitelyWhenMaxAttemptsIsZero);
     test('ExponentialBackoff: increases delay exponentially between attempts', testExponentialBackoffIncreasesDelayExponentially);
     test('ExponentialBackoff: caps delay at maxDelay', testExponentialBackoffCapsDelayAtMaxDelay);
-    test('ExponentialBackoff: applies jitter around the capped delay', testExponentialBackoffAppliesJitter);
+    test('ExponentialBackoff: subtracts jitter from the capped delay', testExponentialBackoffAppliesJitter);
 };
 
 await runTests();