Repository: fastify/fastify
File: lib/server.js (JavaScript)

Code before:
      if (typeof listenOptions.signal.on !== 'function' && typeof listenOptions.signal.addEventListener !== 'function') {
        throw new FST_ERR_LISTEN_OPTIONS_INVALID('Invalid options.signal')
      }

      if (listenOptions.signal.aborted) {
        this.close()
      } else {
        const onAborted = () => {
          this.close()
        }
        listenOptions.signal.addEventListener('abort', onAborted, { once: true })
      }
    }

    // If we have a path specified, don't default host to 'localhost' so we don't end up listening
    // on both path and host
    // See https://github.com/fastify/fastify/issues/4007
    let host
    if (listenOptions.path == null) {
      host = listenOptions.host ?? 'localhost'
    } else {
      host = listenOptions.host
    }
    if (Object.prototype.hasOwnProperty.call(listenOptions, 'host') === false ||
      listenOptions.host === undefined ||
      listenOptions.host === null) {
      listenOptions.host = host
    }
    if (host === 'localhost') {
      listenOptions.cb = (err, address) => {
        if (err) {
          // the server did not start
          cb(err, address)
          return
        }

        multipleBindings.call(this, server, httpHandler, options, listenOptions, () => {
          this[kState].listening = true
          cb(null, address)
          onListenHookRunner(this)
        })
      }
    } else {
      listenOptions.cb = (err, address) => {
        // the server did not start
        if (err) {
          cb(err, address)
          return
        }
        this[kState].listening = true
        cb(null, address)

Review comment (suggestion):
This will have the same behavior

```suggestion
    if (listenOptions.host == null) {
```

Code after:
      if (typeof listenOptions.signal.on !== 'function' && typeof listenOptions.signal.addEventListener !== 'function') {
        throw new FST_ERR_LISTEN_OPTIONS_INVALID('Invalid options.signal')
      }

      if (listenOptions.signal.aborted) {
        this.close()
      } else {
        const onAborted = () => {
          this.close()
        }
        listenOptions.signal.addEventListener('abort', onAborted, { once: true })
      }
    }

    // If we have a path specified, don't default host to 'localhost' so we don't end up listening
    // on both path and host
    // See https://github.com/fastify/fastify/issues/4007
    let host
    if (listenOptions.path == null) {
      host = listenOptions.host ?? 'localhost'
    } else {
      host = listenOptions.host
    }
    if (Object.prototype.hasOwnProperty.call(listenOptions, 'host') === false ||
      listenOptions.host == null) {
      listenOptions.host = host
    }
    if (host === 'localhost') {
      listenOptions.cb = (err, address) => {
        if (err) {
          // the server did not start
          cb(err, address)
          return
        }

        multipleBindings.call(this, server, httpHandler, options, listenOptions, () => {
          this[kState].listening = true
          cb(null, address)
          onListenHookRunner(this)
        })
      }
    } else {
      listenOptions.cb = (err, address) => {
        // the server did not start
        if (err) {
          cb(err, address)
          return
        }
        this[kState].listening = true
        cb(null, address)
        onListenHookRunner(this)