Simple static response for Azure Function LetsEncrypt Domain Verification

function.json

{
  "bindings": [
    {
      "authLevel": "Anonymous",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "route": ".well-known/acme-challenge/{hash}",
      "methods": [
        "get"
      ]
    },
    {
      "type": "http",
      "direction": "out",
      "name": "$return"
    }
  ]
}

index.js

const {join} = require('path')
const {existsSync, readFileSync} = require('fs')
module.exports = async function (context, req) {
    const {hash} = req.params

    const lookupFor = join(__dirname, '../.well-known/acme-challenge/', hash)

    if (!existsSync(lookupFor)) {
        return {
            status: 404,
            body: 'not found'
        }
    }
    const content = readFileSync(lookupFor)
    return {
        body: content
    }
}