How to set and read cookies using Cloudflare Workers

addEventListener("fetch", event => {
  event.respondWith(
    handleRequest(event.request).catch(
      err => new Response(err.stack, { status: 500 })
    )
  )
})

async function handleRequest(request) {
  const { pathname } = new URL(request.url)

  if (pathname.startsWith("/__example/getCookie")) {
    const { searchParams } = new URL(request.url)
    let key = searchParams.get("key")
    if (key) {
      let cookieString = request.headers.get("Cookie")
      const value = getCookie(cookieString, key)
      if (value) {
        const payload = {
          key,
          value,
          message: "Pulled your HTTP Cookie from a cloudflare worker!",
        }
        return new Response(JSON.stringify({ payload }, null, 2), {
          headers: { "Content-Type": "application/json" },
        })
      }
    }
  }

  if (pathname.startsWith("/__example/setCookie")) {
    const { searchParams } = new URL(request.url)
    let key = searchParams.get("key")
    let value = searchParams.get("value")
    if (key && value) {
      let cookieString = request.headers.get("Cookie")
      const newCookie = `${key}=${value}; path=/__example; secure; HttpOnly; SameSite=Strict`
      const payload = {
        key,
        value: getCookie(cookieString, key),
        message: `We set your HTTP Cookie using the following values '${key}/${value}.`,
        getCookieUrl: `https://talke.dev/__example/getCookie?key=${key}`,
      }
      const response = new Response(JSON.stringify({ payload }), {
        headers: { "Content-Type": "application/json" },
      })
      response.headers.set("Set-Cookie", newCookie)

      return response
    }
  }

  return new Response(
    JSON.stringify({ message: "What are you doing here?" }, null, 2),
    {
      headers: { "Content-Type": "application/json" },
    }
  )
}

/**
 * Takes a cookie string
 * @param {String} cookieString - The cookie string value: "val=key; val2=key2; val3=key3;"
 * @param {String} key - The name of the cookie we are reading from the cookie string
 * @returns {(String|null)} Returns the value of the cookie OR null if nothing was found.
 */
function getCookie(cookieString, key) {
  if (cookieString) {
    const allCookies = cookieString.split("; ")
    const targetCookie = allCookies.find(cookie => cookie.includes(key))
    if (targetCookie) {
      const [_, value] = targetCookie.split("=")
      return value
    }
  }

  return null
}