var admin = require("firebase-admin");
...
admin.auth().createCustomToken(uid);

var firebase = require("firebase");
var admin = require("firebase-admin");
...
admin.auth().createCustomToken(uid)
.then(function (customToken) {
   firebase.auth().signInWithCustomToken(customToken).then(function (usuario) {
  firebase.auth().currentUser.getIdToken().then(function (IdToken) {
    console.log(IdToken);
    console.log(firebase.auth().currentUser.refreshToken);
    ...
  });
  }
);