Cloud Functions で HTTP POST リクエストを受けて Firestore に書き込む例

index.js

const functions = require('firebase-functions');
const admin = require('firebase-admin');

admin.initializeApp(functions.config().firebase);

var fireStore = admin.firestore();

exports.helloWorld = functions.https.onRequest((request, response) => {
    if (request.method !== 'POST') {
        response.status(405).send('method not allowed');
        return;
    }
    fireStore.collection("test").doc("docTest").set({ hoge: "fuga" });
    response.status(200).send("Hello from Firebase!");
});