Support If-Modified-Since header for feed caching

core/feed.ts

@@ -28,6 +28,7 @@ export type FeedValue = {
   lastPublishedAt?: number
   loader: LoaderName
   reading: 'fast' | 'slow'
+  refreshedAt?: number
   slowReader?: UsefulReaderName
   title: string
   url: string
@@ -99,7 +100,12 @@ export function getFeedLatestPosts(
   feed: FeedValue,
   task = createDownloadTask()
 ): PostsList {
-  return loaders[feed.loader].getPosts(task, feed.url)
+  return loaders[feed.loader].getPosts(
+    task,
+    feed.url,
+    undefined,
+    feed.refreshedAt
+  )
 }
 
 let testFeedId = 0

core/lib/download.ts

@@ -222,7 +222,7 @@ export function createDownloadTask(
       if (controller.signal.aborted) {
         throw new DOMException('', 'AbortError')
       }
-      if (!response.ok) {
+      if (!response.ok && response.status !== 304) {
         throw new HTTPStatusError(
           response.status,
           url,
@@ -246,6 +246,7 @@ export function createDownloadTask(
       if (controller.signal.aborted) {
         throw new DOMException('', 'AbortError')
       }
+
       return createTextResponse(text, {
         headers: response.headers,
         redirected: response.redirected,

core/loader/atom.ts

@@ -7,6 +7,7 @@ import { type OriginPost, type PostMedia, stringifyMedia } from '../post.ts'
 import { createPostsList, type PostsListResult } from '../posts-list.ts'
 import {
   buildFullURL,
+  fetchIfModified,
   findAnchorHrefs,
   findDocumentLinks,
   findHeaderLinks,
@@ -122,9 +123,12 @@ function parseFeed(
 
 async function loadFeed(
   task: DownloadTask,
-  url: string
+  url: string,
+  refreshedAt?: number
 ): Promise<PostsListResult> {
-  return parseFeed(task, await task.text(url))
+  return fetchIfModified(task, url, refreshedAt, response =>
+    parseFeed(task, response)
+  )
 }
 
 export const atom: Loader = {
@@ -143,11 +147,11 @@ export const atom: Loader = {
     }
   },
 
-  getPosts(task, url, text) {
+  getPosts(task, url, text, refreshedAt) {
     if (text) {
       return createPostsList(() => parseFeed(task, text))
     } else {
-      return createPostsList(() => loadFeed(task, url))
+      return createPostsList(() => loadFeed(task, url, refreshedAt))
     }
   },
 

core/loader/common.ts

@@ -1,7 +1,7 @@
 import type { FeedValue } from '../feed.ts'
 import type { DownloadTask, TextResponse } from '../lib/download.ts'
 import type { PostMedia } from '../post.ts'
-import type { PostsList } from '../posts-list.ts'
+import type { PostsList, PostsListResult } from '../posts-list.ts'
 
 export type Loader = {
   /**
@@ -17,7 +17,12 @@ export type Loader = {
    * Task is a way to combine multiple HTTP requests (for instance, during
    * the feed search/preview) to cancel all of them fast.
    */
-  getPosts(task: DownloadTask, url: string, text?: TextResponse): PostsList
+  getPosts(
+    task: DownloadTask,
+    url: string,
+    text?: TextResponse,
+    refreshedAt?: number
+  ): PostsList
 
   /**
    * Get source data of post from loader's API for debug purposes.
@@ -183,3 +188,20 @@ export function findMediaInText(
     } satisfies PostMedia
   })
 }
+
+/**
+ * Prevent heavy loading/parsing by `If-Modified-Since` header.
+ */
+export async function fetchIfModified(
+  task: DownloadTask,
+  url: string,
+  refreshedAt: number | undefined,
+  parseCb: (response: TextResponse) => PostsListResult
+): Promise<PostsListResult> {
+  let headers = refreshedAt
+    ? { 'If-Modified-Since': new Date(refreshedAt * 1000).toUTCString() }
+    : undefined
+  let response = await task.text(url, { headers })
+  if (response.status === 304) return [[], undefined]
+  return parseCb(response)
+}

core/loader/json-feed.ts

@@ -3,6 +3,7 @@ import { createDownloadTask, type TextResponse } from '../lib/download.ts'
 import { type OriginPost, type PostMedia, stringifyMedia } from '../post.ts'
 import { createPostsList } from '../posts-list.ts'
 import {
+  fetchIfModified,
   findAnchorHrefs,
   findDocumentLinks,
   findHeaderLinks,
@@ -142,13 +143,16 @@ export const jsonFeed: Loader = {
     return [...linksByType, ...findAnchorHrefs(text, /feed\.json/i)]
   },
 
-  getPosts(task, url, text) {
+  getPosts(task, url, text, refreshedAt) {
     if (text) {
       return createPostsList(() => [parsePosts(text), undefined])
     } else {
-      return createPostsList(async () => {
-        return [parsePosts(await task.text(url)), undefined]
-      })
+      return createPostsList(() =>
+        fetchIfModified(task, url, refreshedAt, response => [
+          parsePosts(response),
+          undefined
+        ])
+      )
     }
   },
 

core/loader/rss.ts

@@ -3,6 +3,7 @@ import { type OriginPost, type PostMedia, stringifyMedia } from '../post.ts'
 import { createPostsList } from '../posts-list.ts'
 import { findMRSS } from './atom.ts'
 import {
+  fetchIfModified,
   findAnchorHrefs,
   findDocumentLinks,
   findHeaderLinks,
@@ -60,13 +61,16 @@ export const rss: Loader = {
     ]
   },
 
-  getPosts(task, url, text) {
+  getPosts(task, url, text, refreshedAt) {
     if (text) {
       return createPostsList(() => [parsePosts(text), undefined])
     } else {
-      return createPostsList(async () => {
-        return [parsePosts(await task.text(url)), undefined]
-      })
+      return createPostsList(() =>
+        fetchIfModified(task, url, refreshedAt, response => [
+          parsePosts(response),
+          undefined
+        ])
+      )
     }
   },
 

core/refresh.ts

@@ -115,6 +115,9 @@ async function checkForNextPage(
     if (!getFeed(feed.id).deleted) {
       await addPosts(feed, pages.get().list)
     }
+    await changeFeed(feed.id, {
+      refreshedAt: Math.round(Date.now() / 1000)
+    })
     increaseKey(refreshStatistics, 'processedFeeds')
   }
 }

core/request.ts

@@ -11,20 +11,20 @@ export function setRequestMethod(method: RequestMethod): void {
 }
 
 export interface RequestWaiter {
-  (status: number, body?: string, contentType?: string): Promise<void>
+  (status: number, body?: null | string, contentType?: string): Promise<void>
   aborted?: true
 }
 
 export interface RequestMock {
   andFail(): void
-  andRespond(status: number, body?: string, contentType?: string): void
+  andRespond(status: number, body?: null | string, contentType?: string): void
   andWait(): RequestWaiter
 }
 
 interface RequestExpect {
   contentType: string
   error: boolean
-  response: string
+  response: null | string
   status: number
   url: string
   wait: Promise<void>

core/test/loader/atom.test.ts

@@ -11,6 +11,7 @@ import {
   loaders,
   mockRequest,
   parseMedia,
+  setRequestMethod,
   setupEnvironment,
   testFeed,
   type TextResponse
@@ -680,3 +681,56 @@ test('returns post source', async () => {
     undefined
   )
 })
+
+test('handles conditional request when server returns 304', async () => {
+  let callCount = 0
+  let capturedOpts: RequestInit | undefined
+
+  // Mock initial and refreshing requests
+  setRequestMethod((url, opts) => {
+    if (++callCount === 1) {
+      return Promise.resolve(
+        new Response(
+          `<?xml version="1.0"?>
+          <feed xmlns="http://www.w3.org/2005/Atom">
+            <title>Feed</title>
+            <entry>
+              <title>Test Post</title>
+              <id>test-post</id>
+              <link rel="alternate" href="https://example.com/1" />
+            </entry>
+          </feed>`,
+          {
+            headers: {
+              'Content-Type': 'application/atom+xml',
+              'Last-Modified': 'Thu, 01 Jan 2026 00:00:00 GMT'
+            }
+          }
+        )
+      )
+    } else {
+      capturedOpts = opts
+      return Promise.resolve(new Response(null, { status: 304 }))
+    }
+  })
+
+  let task = createDownloadTask()
+
+  // Initial request
+  let page1 = loaders.atom.getPosts(task, 'https://example.com/news/')
+  await page1.loading
+  equal(page1.get().list.length, 1)
+
+  // Refreshing request
+  let page2 = loaders.atom.getPosts(
+    task,
+    'https://example.com/news/',
+    undefined,
+    1767225600 // Thu, 01 Jan 2026 00:00:00 GMT
+  )
+  await page2.loading
+  deepEqual(capturedOpts?.headers, {
+    'If-Modified-Since': 'Thu, 01 Jan 2026 00:00:00 GMT'
+  })
+  equal(page2.get().list.length, 0)
+})

core/test/loader/json-feed.test.ts

@@ -10,6 +10,7 @@ import {
   expectRequest,
   loaders,
   mockRequest,
+  setRequestMethod,
   setupEnvironment,
   testFeed,
   type TextResponse
@@ -628,3 +629,58 @@ test('returns post source', async () => {
     undefined
   )
 })
+
+test('handles conditional request when server returns 304', async () => {
+  let callCount = 0
+  let capturedOpts: RequestInit | undefined
+
+  // Mock initial and refreshing requests
+  setRequestMethod((url, opts) => {
+    if (++callCount === 1) {
+      return Promise.resolve(
+        new Response(
+          JSON.stringify({
+            items: [
+              {
+                id: 'test-post',
+                title: 'Test Post',
+                url: 'https://example.com/1'
+              }
+            ],
+            title: 'Feed',
+            version: 'https://jsonfeed.org/version/1.1'
+          }),
+          {
+            headers: {
+              'Content-Type': 'application/json',
+              'Last-Modified': 'Thu, 01 Jan 2026 00:00:00 GMT'
+            }
+          }
+        )
+      )
+    } else {
+      capturedOpts = opts
+      return Promise.resolve(new Response(null, { status: 304 }))
+    }
+  })
+
+  let task = createDownloadTask()
+
+  // Initial request
+  let page1 = loaders.jsonFeed.getPosts(task, 'https://example.com/news/')
+  await page1.loading
+  equal(page1.get().list.length, 1)
+
+  // Refreshing request
+  let page2 = loaders.jsonFeed.getPosts(
+    task,
+    'https://example.com/news/',
+    undefined,
+    1767225600 // Thu, 01 Jan 2026 00:00:00 GMT
+  )
+  await page2.loading
+  deepEqual(capturedOpts?.headers, {
+    'If-Modified-Since': 'Thu, 01 Jan 2026 00:00:00 GMT'
+  })
+  equal(page2.get().list.length, 0)
+})