feat: support retryDelay with callback function (#372)

src/fetch.ts

@@ -62,7 +62,9 @@ export function createFetch(globalOptions: CreateFetchOptions = {}): $Fetch {
           ? context.options.retryStatusCodes.includes(responseCode)
           : retryStatusCodes.has(responseCode))
       ) {
-        const retryDelay = context.options.retryDelay || 0;
+        const retryDelay = typeof context.options.retryDelay === 'function' ?
+          context.options.retryDelay(context) :
+          context.options.retryDelay || 0;
         if (retryDelay > 0) {
           await new Promise((resolve) => setTimeout(resolve, retryDelay));
         }

src/types.ts

@@ -53,7 +53,7 @@ export interface FetchOptions<R extends ResponseType = ResponseType>
 
   retry?: number | false;
   /** Delay between retries in milliseconds. */
-  retryDelay?: number;
+  retryDelay?: number | ((context: FetchContext) => number);
   /** Default is [408, 409, 425, 429, 500, 502, 503, 504] */
   retryStatusCodes?: number[];
 

test/index.test.ts

@@ -273,7 +273,7 @@ describe("ofetch", () => {
     expect(error.request).to.equal(getURL("404"));
   });
 
-  it("retry with delay", async () => {
+  it("retry with number delay", async () => {
     const slow = $fetch<string>(getURL("408"), {
       retry: 2,
       retryDelay: 100,
@@ -287,6 +287,20 @@ describe("ofetch", () => {
     expect(race).to.equal("fast");
   });
 
+  it("retry with callback delay", async () => {
+    const slow = $fetch<string>(getURL("408"), {
+      retry: 2,
+      retryDelay: () => 100,
+    }).catch(() => "slow");
+    const fast = $fetch<string>(getURL("408"), {
+      retry: 2,
+      retryDelay: () => 1,
+    }).catch(() => "fast");
+
+    const race = await Promise.race([slow, fast]);
+    expect(race).to.equal("fast");
+  });
+
   it("abort with retry", () => {
     const controller = new AbortController();
     async function abortHandle() {