gatsby-node.js

const report = require('gatsby-cli/lib/reporter');
const firebase = require('firebase-admin');
const crypto = require('crypto');

const getDigest = id =>
  crypto
    .createHash('md5')
    .update(id)
    .digest('hex');

exports.sourceNodes = async (
  { actions },
  { types, credential }
) => {

  try{
    if (firebase.apps || !firebase.apps.length) {
      firebase.initializeApp({ credential: firebase.credential.cert(credential) });
    }
  } catch (e) {
    report.warn('Could not initialize Firebase. Please check `credential` property in gatsby-config.js');
    report.warn(e);
    return;
  }

  const db = firebase.firestore();

  const { createNode } = actions;

  const promises = types.map(
    async ({ collection, type, map = node => node }) => {
      const snapshot = await db.collection(collection).get();
      for (let doc of snapshot.docs) {
        const contentDigest = getDigest(doc.id);
        createNode(
          Object.assign({}, map(doc.data()), {
            id: doc.id,
            parent: null,
            children: [],
            internal: {
              type,
              contentDigest,
            },
          })
        );

        Promise.resolve();
      }
    }
  );

  await Promise.all(promises);

  return;
};