Last Updated: 3/7/2026

ETag Middleware

Using this middleware, you can add ETag headers easily.

Import

import { Hono } from 'hono'
import { etag } from 'hono/etag'

Usage

const app = new Hono()
 
app.use('/etag/*', etag())
app.get('/etag/abc', (c) => {
  return c.text('Hono is cool')
})

The retained headers

The 304 Response must include the headers that would have been sent in an equivalent 200 OK response. The default headers are Cache-Control, Content-Location, Date, ETag, Expires, and Vary.

If you want to add the header that is sent, you can use retainedHeaders option and RETAINED_304_HEADERS strings array variable that includes the default headers:

import { etag, RETAINED_304_HEADERS } from 'hono/etag'
 
// ...
 
app.use(
  '/etag/*',
  etag({
    retainedHeaders: ['x-message', ...RETAINED_304_HEADERS],
  })
)

Options

optional weak: boolean

Define using or not using a weak validation. If true is set, then w/ is added to the prefix of the value. The default is false.

optional retainedHeaders: string[]

The headers that you want to retain in the 304 Response.

optional generateDigest: (body: Uint8Array) => ArrayBuffer | Promise<ArrayBuffer>

A custom digest generation function. By default, it uses SHA-1. This function is called with the response body as a Uint8Array and should return a hash as an ArrayBuffer or a Promise of one.