hono/deno_dist/middleware/cors/index.ts

import type { Context } from '../../context.ts'
import type { Next } from '../../hono.ts'

type CORSOptions = {
  origin: string
  allowMethods?: string[]
  allowHeaders?: string[]
  maxAge?: number
  credentials?: boolean
  exposeHeaders?: string[]
}

export const cors = (options?: CORSOptions) => {
  const defaults: CORSOptions = {
    origin: '*',
    allowMethods: ['GET', 'HEAD', 'PUT', 'POST', 'DELETE', 'PATCH'],
    allowHeaders: [],
    exposeHeaders: [],
  }
  const opts = {
    ...defaults,
    ...options,
  }

  return async (c: Context, next: Next) => {
    await next()

    function set(key: string, value: string) {
      c.res.headers.append(key, value)
    }

    set('Access-Control-Allow-Origin', opts.origin)

    // Suppose the server sends a response with an Access-Control-Allow-Origin value with an explicit origin (rather than the "*" wildcard).
    // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin
    if (opts.origin !== '*') {
      set('Vary', 'Origin')
    }

    if (opts.credentials) {
      set('Access-Control-Allow-Credentials', 'true')
    }

    if (opts.exposeHeaders?.length) {
      set('Access-Control-Expose-Headers', opts.exposeHeaders.join(','))
    }

    if (c.req.method === 'OPTIONS') {
      // Preflight

      if (opts.maxAge != null) {
        set('Access-Control-Max-Age', opts.maxAge.toString())
      }

      if (opts.allowMethods?.length) {
        set('Access-Control-Allow-Methods', opts.allowMethods.join(','))
      }

      let headers = opts.allowHeaders
      if (!headers?.length) {
        const requestHeaders = c.req.headers.get('Access-Control-Request-Headers')
        if (requestHeaders) {
          headers = requestHeaders.split(/\s*,\s*/)
        }
      }
      if (headers?.length) {
        set('Access-Control-Allow-Headers', headers.join(','))
        set('Vary', 'Access-Control-Request-Headers')
      }

      c.res.headers.delete('Content-Length')
      c.res.headers.delete('Content-Type')

      c.res = new Response(null, {
        headers: c.res.headers,
        status: 204,
        statusText: c.res.statusText,
      })
    }
  }
}