Using Firebase Cloud Function calling another Firebase Cloud Function

I am using Firebase Function to call another Firebase Function on the free spark tier.

Is there a special way to call another Firebase Function? or do you just use a standard http request?

I have tried calling the other function directly like so:

exports.purchaseTicket = functions.https.onRequest((req, res) => {    
  fetch('https://us-central1-functions-****.cloudfunctions.net/validate')
    .then(response => response.json())
    .then(json => res.status(201).json(json))
})

But I get the error

FetchError: request to https://us-central1-functions-****.cloudfunctions.net/validate failed, reason: getaddrinfo ENOTFOUND us-central1-functions-*****.cloudfunctions.net us-central1-functions-*****.cloudfunctions.net:443

Which sounds like firebase is blocking the connection, despite it being a google owned, and therefore it shouldn't be locked

the Spark plan only allows outbound network requests to Google owned services.

How can I make use a Firebase Cloud Function to call another Firebase Cloud Function?

via user2427278