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Lecture 8

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Final Project - Milestone 1 due 12/1

No classes until 12/1! Good luck on semi-finals and happy Thanksgiving!!

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 πŸ€‘"

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:

// This corresponds to the type of data you get back in your response
type DataType = readonly string[];

const App = () => {
const [data, setData] = useState<DataType>([]);
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.
  • We declare a type definition for the useState call so that TS knows the type of data we're working with. (It can't infer type from an empty array!)
  • 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 πŸ‘§"

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

Let's talk about types! p in this case might be fetch(), which returns the type Promise<Response>. If so, then value would have the type Response. Then, if the fulfillment function (which takes in value) returns type string, then the entire expression would be type Promise<String>.

If you've taken CS 3110 or done some functional programming outside of this class/category theory, this might make you think of Monads/Applicatives/Functors. The specified behavior for Promises in JS/TS don't exactly follow the laws of what was just listed, but for learning purposes it may be helpful to roughly compare the then function to fmap or bind.

If you're interested, take a look at this snippet and notice how the types behave:

// promise: Promise<Response>
const promise = fetch('something');

// kindaFunctor: (res: Response) => number
const kindaFunctor = (res: Response) => res.status;

// kindaMonad: (res: Response) => Promise<string>
const kindaMonad = (res: Response) => res.text();

// newPromise1: Promise<number>
const newPromise1 = promise.then(kindaFunctor);

// newPromise2: Promise<string>
const newPromise2 = promise.then(kindaMonad);


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


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

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

Note that onRejected takes a parameter of type any, since we don't know the type of the error we will get.

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 many .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 β€œawaited” 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.

What if I don't like fetch? 😩

Try axios instead!

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

It has methods 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('');

In Typescript you can provide type definitions for the response you'll get from a request, similar to what we do in useState<type>. If you don't specify, you'll get the any type.

// anyResponse: AxiosResponse<any>
const anyResponse = await axios.get('url');
// anyData: any
const anyData =;

type DataType = ...;
// typedResponse: AxiosResponse<DataType>
const typedResponse = await axios.get<DataType>('url');
// typedData: DataType
const typedData =;

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.ts (backend)
import admin from 'firebase-admin';
import express from 'express';

const serviceAccount = require('./service-account.json');

credential: admin.credential.cert(serviceAccount),
databaseURL: '[DATABASE_URL]',

const db = admin.firestore();

const app = express();
const port = 8080;

type Song = {
name: string;
artist: string;
rating: number;

type SongWithID = Song & {
id: string;

const songsCollection = db.collection('songs');

app.get('/getSongs', async (req, res) => {
const songs = await songsCollection.get();
res.json( SongWithID => {
const song = as Song;
return {, id: };
});'/createSong', async (req, res) => {
const newSong: Song = req.body;
const addedSong = await songsCollection.add(newSong);
});'/updateRating', async (req, res) => {
const { id, rating } = req.query;
await songsCollection.doc(id as string).update({ rating });
res.send('Song rating updated!');

app.listen(port, () => console.log(`Example app listening on port ${port}!`));
SongList.tsx (frontend)
import React, { useState, useEffect } from 'react';
import { Song } from './Song';
import { SongAdder } from './SongAdder';
import axios from 'axios';

type Song = {
readonly name: string;
readonly artist: string;
readonly rating: number;

type SongWithID = Song & {
readonly id: string;

export const SongList = () => {
const [songs, setSongs] = useState<readonly SongWithID[]>([]);

// GET request using fetch
const fetchSongs = () => {
.then((res) => res.json())
.then((json) => setSongs(json));

// GET request using axios and async/await
// const fetchSongs = async () => {
// const res = await axios.get<readonly SongWithID[]>('/getSongs');
// setSongs(
// }

useEffect(() => fetchSongs(), []);

// POST request using fetch
const addSong = (name: string, artist: string, rating: number) => {
const body: Song = { name, artist, rating };
fetch('/createSong', {
method: 'POST',
headers: {
'Content-Type': 'application/json',
body: JSON.stringify(body),
.then((res) => res.text())
.then((id) => setSongs([...songs, { ...body, id }]));

// POST request using axios and async/await
// const addSong = async (name: string, artist: string, rating: number) => {
// const body: Song = { name, artist, rating };
// const { data: id } = await<string>('/createSong', body);
// setSongs([...songs, { name, artist, rating, id }])
// }

// POST request (update) using fetch
const updateRating = (id: string, rating: number) => {
fetch(`/updateRating?id=${id}&rating=${rating}`, {
method: 'POST',
}).then((res) =>
setSongs( => ( === id ? {, rating } : song)),

// POST request (update) using axios and async/await
// const updateRating = async (id: string, rating: number) => {
// await`/updateRating?id=${id}&rating=${rating}`);
// setSongs(
// => ( === id ? {, rating } : song))
// );
// };

return (
{ => (
{' '}
<Song key={} {} updateRating={updateRating} /> <br />{' '}
<SongAdder callback={addSong} />
SongAdder.tsx (frontend)
import React, { useState } from 'react';

type Props = {
readonly callback: (name: string, artist: string, rating: number) => void;

export const SongAdder = ({ callback }: Props) => {
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(parseInt(}
/> <br />
<button onClick={(e) => callback(name, artist, rating)}> Add song</button>
Song.tsx (frontend)
import React, { useState } from 'react';

type Props = {
readonly id: string;
readonly name: string;
readonly artist: string;
readonly rating: number;
readonly updateRating: (newId: string, newRating: number) => void;

export const Song = ({ id, name, artist, rating, updateRating }: Props) => {
const [newRating, setNewRating] = useState(rating);

return (
{' '}
The song {name} by {artist} currently has a rating of {rating}/5{' '}
placeholder="New rating"
onChange={(e) => setNewRating(parseInt(}
<button onClick={(e) => updateRating(id, newRating)}>
{' '}
Update Rating{' '}