add retry.delay option to control delay

source/core/Ky.ts

@@ -234,8 +234,8 @@ export class Ky {
 				}
 			}
 
-			const BACKOFF_FACTOR = 0.3;
-			return Math.min(this._options.retry.backoffLimit, BACKOFF_FACTOR * (2 ** (this._retryCount - 1)) * 1000);
+			const retryDelay = this._options.retry.delay(this._retryCount);
+			return Math.min(this._options.retry.backoffLimit, retryDelay);
 		}
 
 		return 0;

source/types/options.ts

@@ -138,7 +138,7 @@ export interface Options extends Omit<RequestInit, 'headers'> { // eslint-disabl
 
 	If `maxRetryAfter` is set to `undefined`, it will use `options.timeout`. If [`Retry-After`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After) header is greater than `maxRetryAfter`, it will cancel the request.
 
-	Delays between retries is calculated with the function `0.3 * (2 ** (retry - 1)) * 1000`, where `retry` is the attempt number (starts from 1).
+	By default delays between retries are calculated with the function `0.3 * (2 ** (retry - 1)) * 1000`, where `retry` is the attempt number (starts from 1), however this can be changed by passing a `delay` function.
 
 	Retries are not triggered following a timeout.
 

source/types/retry.ts

@@ -49,4 +49,11 @@ export type RetryOptions = {
 	@default Infinity
 	*/
 	backoffLimit?: number;
+
+	/**
+	A function to calculate the delay between retries given `attemptCount` (starts from 1).
+
+	@default attemptCount => 0.3 * (2 ** (attemptCount - 1)) * 1000
+	*/
+	delay?: (attemptCount: number) => number;
 };

source/utils/normalize.ts

@@ -18,6 +18,7 @@ const defaultRetryOptions: Required<RetryOptions> = {
 	afterStatusCodes: retryAfterStatusCodes,
 	maxRetryAfter: Number.POSITIVE_INFINITY,
 	backoffLimit: Number.POSITIVE_INFINITY,
+	delay: attemptCount => 0.3 * (2 ** (attemptCount - 1)) * 1000,
 };
 
 export const normalizeRetryOptions = (retry: number | RetryOptions = {}): Required<RetryOptions> => {

test/retry.ts

@@ -8,6 +8,8 @@ const fixture = 'fixture';
 const defaultRetryCount = 2;
 const retryAfterOn413 = 2;
 const lastTried413access = Date.now();
+// We allow the tests to take more time on CI than locally, to reduce flakiness
+const allowedOffset = process.env.CI ? 1000 : 300;
 
 test('network error', async t => {
 	let requestCount = 0;
@@ -458,9 +460,6 @@ test('respect maximum backoff', async t => {
 		}
 	});
 
-	// We allow the test to take more time on CI than locally, to reduce flakiness
-	const allowedOffset = process.env.CI ? 1000 : 300;
-
 	// Register observer that asserts on duration when a measurement is performed
 	const obs = new PerformanceObserver(items => {
 		const measurements = items.getEntries();
@@ -499,3 +498,46 @@ test('respect maximum backoff', async t => {
 
 	await server.close();
 });
+
+test('respect custom retry.delay', async t => {
+	const retryCount = 5;
+	let requestCount = 0;
+
+	const server = await createHttpTestServer();
+	server.get('/', (_request, response) => {
+		requestCount++;
+
+		if (requestCount === retryCount) {
+			response.end(fixture);
+		} else {
+			response.sendStatus(500);
+		}
+	});
+
+	// Register observer that asserts on duration when a measurement is performed
+	const obs = new PerformanceObserver(items => {
+		const measurements = items.getEntries();
+
+		const duration = measurements[0].duration ?? Number.NaN;
+		const expectedDuration = 200 + 300 + 400 + 500;
+
+		t.true(Math.abs(duration - expectedDuration) < allowedOffset, `Duration of ${duration}ms is not close to expected duration ${expectedDuration}ms`);
+
+		obs.disconnect();
+	});
+	obs.observe({entryTypes: ['measure']});
+
+	// Start measuring
+	performance.mark('start');
+	t.is(await ky(server.url, {
+		retry: {
+			limit: retryCount,
+			delay: n => 100 * (n + 1),
+		},
+	}).text(), fixture);
+	performance.mark('end');
+
+	performance.measure('linear', 'start', 'end');
+
+	await server.close();
+});