storage-example.js

const functions = require('firebase-functions');
const gcs = require('@google-cloud/storage')();

exports.listener = functions.database.ref('/test/storage').onWrite(() => {
  const bucket = gcs.bucket(functions.config().firebase.storageBucket);
  const filePath = __dirname + '/image.png';

  return bucket.upload(filePath, { destination: 'storage-folder/sample/test-image.png' })
    .then(files => {
      const file = files[0];

      return file.getSignedUrl({ action: 'read', expires: '01-01-2020' });
    })
    .then(url => {
      console.log('Signed URL: ', url);
    })
    .catch(err => {
      console.error('Error: ', err);
    });
});

/**
Error:  { SigningError: Could not get credentials without a JSON, pem, or p12 keyfile.
    at /user_code/node_modules/@google-cloud/storage/src/file.js:1479:16
    at getAuthClient (/user_code/node_modules/@google-cloud/storage/node_modules/google-auto-auth/index.js:132:9)
    at runCallback (timers.js:672:20)
    at tryOnImmediate (timers.js:645:5)
    at processImmediate [as _immediateCallback] (timers.js:617:5)
  message: 'Could not get credentials without a JSON, pem, or p12 keyfile.' }
**/

Appears to be related to https://github.com/GoogleCloudPlatform/google-cloud-common/issues/180