--- title: "ETag and Cache" description: "Hono provides robust ETag and Cache middleware features that optimize HTTP request-response lifecycles through automated cryptographic hashing, conditional validation, and Web Standard Cache API in..." last_updated: "2026-10-12T08:14:12.769531+00:00" canonical_url: "https://www.doc0.dev/docs/552ca36e-f67e-41c3-a07a-def9bd9551b0/technical/traffic-middleware/etag-and-cache" ---
Relevant source files The following files were used as context for generating this wiki page: - [src/middleware/etag/index.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts) - [src/middleware/secure-headers/secure-headers.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/secure-headers/secure-headers.ts) - [src/middleware/cache/index.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts) - [src/middleware/etag/digest.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/digest.ts) - [src/context.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/context.ts) - [src/helper/streaming/sse.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/helper/streaming/sse.ts) - [src/adapter/aws-lambda/handler.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/adapter/aws-lambda/handler.ts) - [src/middleware/serve-static/index.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/serve-static/index.ts) - [src/utils/crypto.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/utils/crypto.ts) - [src/middleware/bearer-auth/index.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/bearer-auth/index.ts) - [src/middleware/compress/index.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/compress/index.ts) - [src/request.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/request.ts) - [src/middleware/basic-auth/index.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/basic-auth/index.ts) - [src/middleware/timing/timing.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/timing/timing.ts) - [src/jsx/dom/intrinsic-element/components.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/jsx/dom/intrinsic-element/components.ts) - [src/middleware/method-override/index.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/method-override/index.ts) - [src/helper/ssg/ssg.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/helper/ssg/ssg.ts) - [src/helper/proxy/index.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/helper/proxy/index.ts) - [src/middleware/jwk/keys.test.json](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/jwk/keys.test.json) - [src/adapter/lambda-edge/handler.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/adapter/lambda-edge/handler.ts) - [src/middleware/language/language.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/language/language.ts) - [src/helper/ssg/middleware.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/helper/ssg/middleware.ts) - [src/middleware/cors/index.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cors/index.ts) - [src/adapter/cloudflare-workers/serve-static.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/adapter/cloudflare-workers/serve-static.ts) - [src/utils/compress.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/utils/compress.ts) - [src/middleware/request-id/request-id.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/request-id/request-id.ts) - [src/utils/headers.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/utils/headers.ts) - [src/utils/buffer.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/utils/buffer.ts) - [src/middleware/powered-by/index.ts](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/powered-by/index.ts)
## Overview Hono provides robust ETag and Cache middleware features that optimize HTTP request-response lifecycles through automated cryptographic hashing, conditional validation, and Web Standard Cache API integration. By computing SHA-1 or custom digests from response streams, the ETag middleware evaluates `If-None-Match` headers against strong or weak tags to short-circuit redundant transmissions with `304 Not Modified` responses while retaining specified headers. Concurrently, the cache middleware interfaces with runtime storage via `caches.open` and `cache.match`, enforcing status code constraints, vary directive checks, and `Cache-Control` directives to manage cache population and conditional execution. Sources: [src/middleware/etag/index.ts:21-36](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L21-L36), [src/middleware/etag/index.ts:79-123](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L79-L123), [src/middleware/cache/index.ts:13-35](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L13-L35), [src/middleware/cache/index.ts:64-180](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L64-L180) ## ETag generation and hashing algorithms ### Public API and Generator Initialization The ETag middleware accepts an `ETagOptions` configuration object defining parameters that control tag formatting, retained headers on validation bypass, and custom hashing logic. When no custom generator is provided via `options.generateDigest`, the initializer falls back to the native `crypto.subtle` API using the `SHA-1` algorithm. | Option | Type | Default | Purpose | | :--- | :--- | :--- | :--- | | `retainedHeaders` | `string[]` | `RETAINED_304_HEADERS` | Headers preserved on `304 Not Modified` responses. | | `weak` | `boolean` | `false` | Prepends `W/` to generated ETags for weak validation. | | `generateDigest` | `(body: Uint8Array) => ArrayBuffer \| Promise` | `crypto.subtle` (SHA-1) | Custom digest generation function for response bodies. | Sources: [src/middleware/etag/index.ts:9-13](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L9-L13), [src/middleware/etag/index.ts:38-54](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L38-L54), [src/middleware/etag/index.ts:79-82](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L79-L82) ### Digest Calculation and Hex Encoding The `generateDigest` function reads response bodies iteratively via a `ReadableStream` reader, merging incoming chunks and invoking the digest generator. Once the stream is fully consumed, the resulting `ArrayBuffer` digest is converted into a lowercase hexadecimal string by mapping each byte to a zero-padded two-character hex value. ```typescript export const generateDigest = async ( stream: ReadableStream> | null, generator: (body: Uint8Array) => ArrayBuffer | Promise ): Promise => { if (!stream) { return null } let result: ArrayBuffer | undefined = undefined const reader = stream.getReader() for (;;) { const { value, done } = await reader.read() if (done) { break } result = await generator(mergeBuffers(result, value)) } if (!result) { return null } return Array.prototype.map .call(new Uint8Array(result), (x) => x.toString(16).padStart(2, '0')) .join('') } ``` Sources: [src/middleware/etag/digest.ts:16-43](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/digest.ts#L16-L43) > [!NOTE] > If a response carries an existing `ETag` header from upstream handlers, the ETag middleware bypasses automatic digest calculation entirely and uses the pre-existing tag value. Sources: [src/middleware/etag/index.ts:90-95](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L90-L95) ## Conditional validation and weak tags ### Execution Walkthrough and Sequence Analysis Conditional validation in the ETag middleware evaluates incoming `If-None-Match` request headers against computed or pre-existing entity tags. When a request includes an `If-None-Match` header, the middleware inspects the value after downstream handlers execute and the response ETag is resolved. The validation check executes through a strict function call chain: `etag` handler invokes `etagMatches`, which in turn applies `stripWeak` to parse and normalize tags. ```mermaid sequenceDiagram participant Middleware as etag (index.ts) participant Matcher as etagMatches (index.ts) participant Stripper as stripWeak (index.ts) Middleware->>Matcher: etagMatches(etag, ifNoneMatch) Matcher->>Stripper: stripWeak(t) for each comma-split token Stripper-->>Matcher: normalized token string Matcher->>Stripper: stripWeak(etag) Stripper-->>Matcher: normalized etag string Matcher-->>Middleware: boolean match result ``` The execution sequence follows this precise path: 1. `etag` — The main middleware function extracts the `If-None-Match` request header and awaits downstream execution before reading the `ETag` response header or generating a fresh digest. Sources: [src/middleware/etag/index.ts:84-105](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L84-L105) 2. `etagMatches` — Evaluates whether `ifNoneMatch` is non-null, splits the header value by commas and optional whitespace (`/,\s*/`), and checks if any token matches the target ETag. Sources: [src/middleware/etag/index.ts:32-36](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L32-L36) 3. `stripWeak` — Strips the weak validation prefix `W/` from the beginning of a tag string using a regular expression replacement (`tag.replace(/^W\//, '')`), allowing weak and strong tags to be compared equivalently during validation. Sources: [src/middleware/etag/index.ts:30-30](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L30-L30) ### 304 Response Generation and Retained Headers When `etagMatches` returns true, the middleware short-circuits the response by constructing a new `304 Not Modified` response instance. This response discards the response body while preserving specific metadata headers defined by `retainedHeaders`. By default, the middleware references `RETAINED_304_HEADERS` to determine which response headers survive the validation bypass. | Header Constant | Value / Identifier | Purpose | | :--- | :--- | :--- | | `RETAINED_304_HEADERS` | `'cache-control'` | Retains caching directives on 304 responses. | | `RETAINED_304_HEADERS` | `'content-location'` | Retains resource location metadata. | | `RETAINED_304_HEADERS` | `'date'` | Retains server response timestamp. | | `RETAINED_304_HEADERS` | `'etag'` | Retains the entity tag for validation tracking. | | `RETAINED_304_HEADERS` | `'expires'` | Retains expiration time limits. | | `RETAINED_304_HEADERS` | `'vary'` | Retains cache negotiation criteria. | Sources: [src/middleware/etag/index.ts:21-28](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L21-L28), [src/middleware/etag/index.ts:107-119](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L107-L119) > [!NOTE] > When iterating over response headers during a 304 short-circuit, header keys are converted to lowercase via `key.toLowerCase()` before checking against the `retainedHeaders` index array, ensuring case-insensitive header preservation. Sources: [src/middleware/etag/index.ts:115-119](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/index.ts#L115-L119) ## Cache middleware and storage adapter ### Cache Storage Initialization and Availability Check The cache middleware integrates with the Web Standard `Cache API` via `globalThis.caches`. Upon initialization, the `cache` function verifies if `globalThis.caches` is defined in the current runtime. If the storage adapter is unavailable, the middleware checks the `onCacheNotAvailable` option: when set to `false`, it silently suppresses logging; when provided as a custom function, it invokes that handler; otherwise, it logs a default message to the console and returns a pass-through middleware handler that immediately invokes `next()`. ```typescript if (!globalThis.caches) { if (options.onCacheNotAvailable === false) { // suppress log } else if (options.onCacheNotAvailable) { options.onCacheNotAvailable() } else { console.log('Cache Middleware is not enabled because caches is not defined.') } return async (_c, next) => await next() } ``` Sources: [src/middleware/cache/index.ts:73-82](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L73-L82) When `globalThis.caches` is present, the middleware resolves the cache store name. The `cacheName` option accepts either a static string or a dynamic function `(c: Context) => Promise | string`. The resolved store name is passed to `caches.open(cacheName)` to obtain the cache instance. ```typescript const cacheName = typeof options.cacheName === 'function' ? await options.cacheName(c) : options.cacheName const cache = await caches.open(cacheName) ``` Sources: [src/middleware/cache/index.ts:155-157](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L155-L157) ### Request Matching and Key Generation Lifecycle Before executing downstream handlers, the middleware intercepts incoming requests. Requests are filtered by method and authorization headers: non-`GET` requests or requests containing an `Authorization` header bypass the cache middleware entirely and proceed directly to `next()`. ```typescript if (c.req.method !== 'GET' || c.req.raw.headers.has('Authorization')) { await next() return } ``` Sources: [src/middleware/cache/index.ts:139-142](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L139-L142) For valid `GET` requests, the cache key is constructed starting from the request URL (`c.req.url`). If a custom `keyGenerator` option is supplied, it executes to replace the base key string. When `vary` directives are configured, the middleware appends each specified header name and its URI-encoded request header value to the cache key using a double-colon delimiter format: `::directive=encodedValue`. ```typescript let key = c.req.url if (options.keyGenerator) { key = await options.keyGenerator(c) } if (varyDirectives) { for (const directive of varyDirectives) { const value = c.req.raw.headers.get(directive) ?? '' key += `::${directive}=${encodeURIComponent(value)}` } } ``` Sources: [src/middleware/cache/index.ts:144-153](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L144-L153) Once the cache key is assembled, the middleware queries the storage adapter via `cache.match(key)`. If a matching response is located, a new `Response` is instantiated from the cached response body and headers, short-circuiting further request execution and returning immediately. ```typescript const response = await cache.match(key) if (response) { return new Response(response.body, response) } ``` Sources: [src/middleware/cache/index.ts:158-161](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L158-L161) > [!WARNING] > Requests bearing an `Authorization` header automatically bypass the cache lookup and storage lifecycle, preventing authenticated user data from being improperly stored in shared cache tiers. Sources: [src/middleware/cache/index.ts:139-142](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L139-L142) ### Writing to the Cache Store When a cache miss occurs, the middleware awaits downstream execution (`await next()`). It then verifies whether the resulting response status code is present in the allowed `cacheableStatusCodes` set (which defaults to `[200]`). If the status code is ineligible, the middleware returns without caching. ```typescript await next() if (!cacheableStatusCodes.has(c.res.status)) { return } ``` Sources: [src/middleware/cache/index.ts:163-166](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L163-L166) Eligible responses undergo header augmentation and skip-cache evaluations. If checks pass, the response is cloned via `c.res.clone()`. The write operation to the cache store is governed by the `wait` boolean option: - When `wait` is set to `true`, Hono awaits the resolution of `cache.put(key, res)` before finishing the middleware handler execution (a requirement for runtimes like Deno). - When `wait` is `false` (the default), the promise returned by `cache.put(key, res)` is passed to `c.executionCtx.waitUntil()` to allow background storage without blocking the response delivery. ```typescript const res = c.res.clone() if (options.wait) { await cache.put(key, res) } else { c.executionCtx.waitUntil(cache.put(key, res)) } ``` Sources: [src/middleware/cache/index.ts:174-179](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L174-L179) > [!NOTE] > The `wait` option defaults to `false`. Explicitly setting `wait: true` is mandatory in Deno and other runtime environments that require pending promises to resolve within the request scope before termination. Sources: [src/middleware/cache/index.ts:44-44](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L44-L44), [src/middleware/cache/index.ts:84-86](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L84-L86) ## Cache control and skipping heuristics ### Heuristics and Skip Inspection The cache middleware evaluates several inspection rules before deciding whether an eligible response should be stored in the cache. These heuristics examine the `Cache-Control` header, `Set-Cookie` headers, and `Vary` directives to prevent writing private or sensitive data. ```typescript const shouldSkipCacheControl = (cacheControl: string | null): boolean => !!cacheControl && /(?:^|,\s*)(?:private|no-(?:store|cache))(?:\s*(?:=|,|$))/i.test(cacheControl) ``` Sources: [src/middleware/cache/index.ts:15-16](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L15-L16) The `shouldSkipCache` function consolidates these checks, returning `true` if any skipping criterion is satisfied: ```typescript const shouldSkipCache = ( res: Response, optionsVaryDirectives: Set | undefined, responseVary: string[] ): boolean => (responseVary.length && (!optionsVaryDirectives || responseVary.some((name) => !optionsVaryDirectives.has(name)))) || shouldSkipCacheControl(res.headers.get('Cache-Control')) || res.headers.has('Set-Cookie') ``` Sources: [src/middleware/cache/index.ts:27-35](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L27-L35) > [!WARNING] > Responses containing a `Set-Cookie` header or a `Cache-Control` directive matching `private`, `no-store`, or `no-cache` are automatically excluded from cache writes to prevent leaking user sessions or private records. > Sources: [src/middleware/cache/index.ts:32-35](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L32-L35) ### Skip Inspection Call Chain When determining whether to skip caching a response after downstream execution, Hono invokes a dedicated evaluation sequence. ```mermaid sequenceDiagram participant Index as src/middleware/cache/index.ts Index->>Index: cache(c, next) Index->>Index: shouldSkipCache(res, optionsVaryDirectives, responseVary) Index->>Index: shouldSkipCacheControl(cacheControl) ``` Sources: [src/middleware/cache/index.ts:15-17](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L15-L17), [src/middleware/cache/index.ts:27-35](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L27-L35), [src/middleware/cache/index.ts:170-172](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L170-L172) 1. `cache` invokes `shouldSkipCache` passing the response and vary directives. 2. `shouldSkipCache` checks vary conditions and calls `shouldSkipCacheControl`. 3. `shouldSkipCacheControl` tests the `Cache-Control` header string against the regex pattern for `private`, `no-store`, or `no-cache`. Sources: [src/middleware/cache/index.ts:15-17](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L15-L17), [src/middleware/cache/index.ts:27-35](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L27-L35), [src/middleware/cache/index.ts:170-172](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L170-L172) ### Header Augmentation Options The middleware can inject configured `Cache-Control` and `Vary` headers into outgoing responses prior to storage evaluation via the internal `addHeader` function. ```typescript const addHeader = (c: Context, responseVary: string[]) => { if (cacheControlDirectives) { const existingDirectives = c.res.headers .get('Cache-Control') ?.split(',') .map((d) => d.trim().split('=', 1)[0]) ?? [] for (const directive of cacheControlDirectives) { let [name, value] = directive.trim().split('=', 2) name = name.toLowerCase() if (!existingDirectives.includes(name)) { c.header('Cache-Control', `${name}${value ? `=${value}` : ''}`, { append: true }) } } } if (varyDirectives) { if (responseVary.length === 0) { c.header('Vary', Array.from(varyDirectives).join(', ')) } else { const merged = new Set(varyDirectives) for (const directive of responseVary) { merged.add(directive) } if (merged.has('*')) { c.header('Vary', '*') } else { c.header('Vary', Array.from(merged).join(', ')) } } } } ``` Sources: [src/middleware/cache/index.ts:105-136](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L105-L136) > [!NOTE] > If `varyDirectives` includes the wildcard token `*`, the middleware throws an error upon initialization because RFC 7231 Section 7.1.4 disallows effective caching when `*` is specified in the `Vary` header. > Sources: [src/middleware/cache/index.ts:93-99](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/cache/index.ts#L93-L99) ## Buffer management for digest hashing ### Chunk Concatenation and Memory Merging The `generateDigest` utility in `src/middleware/etag/digest.ts` processes streamed response payloads by reading chunks iteratively from a `ReadableStream>`. When building the complete hash input across multiple chunks, the internal `mergeBuffers` function combines an existing `ArrayBuffer` with an incoming `Uint8Array` chunk. If `buffer1` is undefined, it returns `buffer2` directly; otherwise, it allocates a new `ArrayBuffer` matching the combined byte lengths, writes the initial buffer at offset `0`, and copies the incoming chunk at offset `buffer1.byteLength`. ```typescript const mergeBuffers = ( buffer1: ArrayBuffer | undefined, buffer2: Uint8Array ): Uint8Array => { if (!buffer1) { return buffer2 } const merged = new Uint8Array( new ArrayBuffer(buffer1.byteLength + buffer2.byteLength) ) merged.set(new Uint8Array(buffer1), 0) merged.set(buffer2, buffer1.byteLength) return merged } ``` Sources: [src/middleware/etag/digest.ts:1-14](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/digest.ts#L1-L14) > [!NOTE] > Successive chunks from the stream reader are accumulated iteratively via `mergeBuffers`, feeding the result into the custom or default `generator` function on every read cycle until the stream signals completion. > Sources: [src/middleware/etag/digest.ts:26-34](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/digest.ts#L26-L34) ### Stream Reader Loop and Hex Encoding Once all stream chunks are processed and hashed, the resulting `ArrayBuffer` is converted into a lowercase hexadecimal string by mapping over its bytes with `toString(16).padStart(2, '0')` and joining them. If the input stream is null or yields no result, the generator returns `null`. ```typescript export const generateDigest = async ( stream: ReadableStream> | null, generator: (body: Uint8Array) => ArrayBuffer | Promise ): Promise => { if (!stream) { return null } let result: ArrayBuffer | undefined = undefined const reader = stream.getReader() for (;;) { const { value, done } = await reader.read() if (done) { break } result = await generator(mergeBuffers(result, value)) } if (!result) { return null } return Array.prototype.map .call(new Uint8Array(result), (x) => x.toString(16).padStart(2, '0')) .join('') } ``` Sources: [src/middleware/etag/digest.ts:16-43](https://github.com/blade47/hono/blob/e50df01453e71b071c3e6136b161b160b9fdf916/src/middleware/etag/digest.ts#L16-L43) ## Related - [Response Compression](https://www.doc0.dev/docs/552ca36e-f67e-41c3-a07a-def9bd9551b0/technical/traffic-middleware/response-compression) - [Request Lifecycle](https://www.doc0.dev/docs/552ca36e-f67e-41c3-a07a-def9bd9551b0/technical/core-engine/request-lifecycle) ## Sitemap See the full [sitemap](https://www.doc0.dev/docs/552ca36e-f67e-41c3-a07a-def9bd9551b0/llms.txt) for all pages in this wiki.