Generated a Custom Token from the Cloud Functions for Firebase

index.ts

import * as functions from 'firebase-functions';
import * as admin from "firebase-admin";

// Start writing Firebase Functions
// https://firebase.google.com/docs/functions/typescript

export const getCustomToken = functions.https.onRequest((request, response) => {
  if (admin.apps.length < 1) {   //Checks if app already initialized
    admin.initializeApp();
  }
  const uid = "USER_UUID"; //e.g. GVvCdXAC1FeC4WYTefcHD8So3q41

  admin.auth().createCustomToken(uid)
    .then(function(customToken) {
      console.log(customToken.toString);
      response.send(customToken);
    })
    .catch(function(error) {
      console.log("Error creating custom token:", error);
    });
});