Version: 2020sp

Lecture 8

Lecture Slides

Lecture Video

Final Project

Bridging the Frontend & Backend#

We've worked with Express/Firebase on the backend and React on the frontend. So far, we've been learning about them separately. How can we bring both ends together to make one working product?

Data Fetching#

"data is the new oil 🤑"

What IS data fetching?#

Data fetching is getting information (data) from an outside source (e.g. REST API)

The frontend wants to fetch data from the backend.

Frontend tells Backend what it wants. Backend sends the appropriate data to Frontend. Frontend displays the data to the user!

In Trends, our React website will be fetching data from our Express server.

How do we fetch (in React)?#

So how do we fetch data with our React frontend?

Two important things to note:

  1. Modern browsers have a nifty little JavaScript function called fetch that you can use to call API endpoints. Libraries such as axios provide similar functionality.

  2. The useEffect React hook allows you to trigger side effects, such as fetching data!

We want to keep track of our data in our component state, and use hooks like useEffect to fetch the data and update the state accordingly!

Fetching Example#

Consider this snippet of code:

const App = () => {
const [data, setData] = useState([]);
useEffect(() => {
.then((response) => response.json())
.then((d) => setData(d));
// ... other methods, return, etc

Here are the important parts:

  • We have data in our component state. Later, we call setData on what the backend sends to us.
  • The useEffect hook is used to subscribe to new data.
  • fetch(...) is called on an API link, followed by .then(...) calls that format the response into json and then setData to the response. The .then() calls exist because fetch() returns a Promise (this is explained below)

Let's take a deeper look at fetch!


"stop trying to make fetch happen 👧"

fetch(resource, [init]) is a native browser function for making web requests.

Its params are:

  • resource: URL of the site you are fetching from
  • init: optional object containing any custom settings you want to apply to the request.
// your init object might look like this
// HTTP request method
method: 'GET', // | 'POST' | 'PUT' | 'DELETE' | etc
// Any request headers you want to add
headers: {
'content-type': 'application/json'
// Request body (remember to stringify!)
body: JSON.stringify(requestBody)
// ... other settings
  • For more on the init object, refer to this link!

IMPORTANT: fetch() returns a PROMISE!


Operations like web requests don't complete instantly! You want to do other stuff while the operation is still going on.

Promises represent the eventual completion (or failure) of an async operation.

Promises are in one of three possible states:

  • pending: initial state; neither fulfilled nor rejected
  • fulfilled: operation completed successfully
  • rejected: operation failed


.then() is a function on Promises that return a promise.

p.then(onFulfilled[, onRejected])

Let's break this down!

  • p is a Promise.
  • onFulfilled is the callback function that is run when p is fulfilled
  • onRejected (OPTIONAL) is the callback for when p is rejected
(value) => {
// fulfillment
(reason) => {
// rejection


.catch() is a function on Promises that catches a rejection.


For example, you might want to console.log errors:

.catch((err) => console.log(err))

Using .then() with fetch()#

fetch() returns a Promise that resolves to a Response object.

Consider this snippet, similar to one shown above:

.then((response) => response.json())
.then((d) => setData(d));
.catch((err) => console.log(err))

Here we are getting the response from an endpoint and then calling .json() on the response and then calling setData on the result of json().

If a promise gets rejected anywhere along this chain, we will log the error in our console.


If you have too much .then() calls within each other, you might build a PYRAMID OF DOOM ☠.

Adding the async keyword to a function designates that function as an asynchronous function

Within these async functions we can use await to designate which lines need to be “awaited” upon to resolve

// .then
const fetchData = () => {
.then((response) => response.json())
.then((d) => setData(d));
// async/await
const fetchData = async () => {
const response = await fetch('');
const posts = await response.json();

I still don't understand Promises#

Let's say you are at a store and you want to know whether the store has something in stock. So you ask an employee named Joe and he promises you that he'll be back with the results.

You are now waiting for Joe to come back. The Promise will be pending for as long as Joe is gone.

Let's say Joe comes back with the results. Now you know whether the store is in stock or not. Now the Promise is fulfilled.

Let's say Joe doesn't come back with the results. Unfortunately an anvil fell on him or something. Now you know that Joe will not come back with the result. Now the Promise is rejected.

I don't like fetch#

Try axios instead!

Axios is an npm package that can give you a better experience fetching data.

It's got stuff like:

  • axios.get(url[, config])
  •[, data[, config]])
  • axios.put(url[, data[, config]])
  • axios.delete(url[, config])

Axios also returns a promise and can be resolved with .then().

Here's an example:

// fetch()
const fetchData = async () => {
const response = await fetch('');
const posts = await response.json();
// axios
const fetchData = async () => {
const posts = await axios.get('');

CORS workaround: proxy#

When you are testing your backend and frontend together on localhost, you will come across some sort of CORS policy error when you try to call the backend from the frontend.

One way to get around this is to add this line to your frontend's package.json:

"proxy": "http://localhost:8080",

where 8080 is the the port of your backend (change it accordingly).

After adding the proxy config, remove the base URL from your requests:

// do this
const posts = await axios.get('/getAllPosts');
// instead of this
const posts = await axios.get('http://localhost:8080/getAllPosts');


You keep track of your data in component state. Your app displays whatever data you have.

You update your data by calling an endpoint within useEffect and setting your data to the response that you get back.

You can call endpoints using fetch() or axios and handle the responses asynchronously.

Demo Code#


index.js (backend)
const express = require('express');
const admin = require('firebase-admin');
const serviceAccount = require('./serviceAccount.json');
credential: admin.credential.cert(serviceAccount),
databaseURL: '',
const app = express();
const db = admin.firestore();
const songsCollection = db.collection('songs');
app.get('/getSongs', async (req, res) => {
const songs = await songsCollection.get();
res.json( => ({, id: })));
});'/createSong', async (req, res) => {
const newSong = req.body;
const addedSong = await songsCollection.add(newSong);
});'/updateRating', async (req, res) => {
const { id, rating } = req.query;
await songsCollection.doc(id).update({ rating });
res.send('Song rating updated!');
app.listen(8080, () => console.log('backend started'));
SongList.jsx (frontend)
import React, { useState, useEffect } from 'react';
import Song from './Song';
import SongAdder from './SongAdder';
import axios from 'axios';
export default () => {
const [songs, setSongs] = useState([]);
// GET request using fetch
const fetchSongs = () => {
.then(res => res.json())
.then(json => setSongs(json));
// GET request using axios
// const fetchSongs = () => {
// axios.get('/getSongs')
// .then(res => setSongs(;
// }
useEffect(() => fetchSongs(), []);
// POST requset using fetch
const addSong = (name, artist, rating) => {
fetch('/createSong', {
method: 'POST',
headers: {
'Content-Type': 'application/json'
body: JSON.stringify({ name, artist, rating })
.then(res => res.text())
.then(id => setSongs([...songs, { name, artist, rating, id }]))
// POST request using axios
// const addSong = (name, artist, rating) => {
//'/createSong', { name, artist, rating })
// .then(res => setSongs([...songs, { name, artist, rating, id: }]))
// }
// POST request (update) using fetch
const updateRating = (id, rating) => {
fetch(`/updateRating?id=${id}&rating=${rating}`, {
method: 'POST'
.then(res => setSongs( => === id ? { name:, artist: song.artist, rating, id } : song)))
// POST request (update) using axios
// const updateRating = (id, rating) => {
// .then(res => setSongs( => === id ? { name:, artist: song.artist, rating, id } : song)))
// }
return (
{ => (<div> <Song key={} {} updateRating={updateRating} /> <br /> </div>))}
<SongAdder callback={addSong} />
SongAdder.jsx (frontend)
import React, { useState } from 'react';
export default ({ callback }) => {
const [name, setName] = useState('');
const [artist, setArtist] = useState('');
const [rating, setRating] = useState(0);
return (
<h3> Add a new song! </h3>
placeholder="Song name"
onChange={(e) => setName(}
/> <br />
placeholder="Artist name"
onChange={(e) => setArtist(}
/>{' '}
<br />
onChange={(e) => setRating(}
/> <br />
<button onClick={(e) => callback(name, artist, rating)}> Add song</button>
Song.jsx (frontend)
import React, { useState } from 'react';
export default ({ id, name, artist, rating, updateRating }) => {
const [newRating, setNewRating] = useState(rating);
return (
{' '}
The song {name} by {artist} currently has a rating of {rating}/5{' '}
placeholder="New rating"
onChange={(e) => setNewRating(}
<button onClick={(e) => updateRating(id, newRating)}>
{' '}
Update Rating{' '}