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Version: 2022fa

Lecture 2

Lecture Slides

Assignment 1: Due on CMS by 10/13 by 11:59pm

How to build a web app

Intro to Node.js and Yarn

How websites work

Websites are accessed by HTTP requests, and these requests go to the server, where it fetches the information queried and sends the data back to the client. This cycle between sending requests back and forth between client and server then repeats!


Node.js is an open source, cross platform JavaScript V8 runtime environment using a single-threaded event loop.

Let's break it down...

Open Source

All of the code is available to you to view on Github! Anyone can contribute-- this democratizes the development process!

Cross Platform JavaScript Runtime Environment

  • Historically you were only able to run JavaScript on the browser or client
  • Node.js takes the V8 JavaScript engine powering Google Chrome outside of the browser allowing you to run Node.js anywhere
    • V8 JavaScript engine is a fast JavaScript engine created by Google. Learn more about it here!
  • Can run Node.js on your terminal as well
  • Now, we can use JavaScript as a universal language!


  • Threads are a separate line of execution and can be ran in parallel - i.e. several at the same time.
  • However, Node.js uses a single-threaded event loop
    • Run in a single process
    • Requests do not spawn new threads
  • Non-blocking

How does Node.js handle multiple requests?

  • Node.js is asynchronous
  • When a request is sent, it is dispatched to the server
  • Instead of blocking the thread and wasting CPU cycles waiting for the request to finish, Node.js continues its operations
  • Once the request is complete, a callback is triggered and information is sent back

Event Loop

  • Client can send requests into the event loop
  • We can register callbacks to server when doing things that might take time (ie. search, query, intensive computation)
  • After operation completes, callback will fire and return to requests

A callback is a function that you can pass to another function to be executed later. This is a common pattern in web development, since lots of data goes from the client to the server, and we want to implement certain behavior that fires after the data is received.

Why Node.js?

  • Unites front-end and back-end in one language/framework
    • TypeScript/JavaScript
    • Frontend and backend in the same language
  • Extremely performant
  • Asynchronous and non-blocking
  • NPM (Node Package Manager)
    • a directory of many libraries and packages
    • access to huge libraries to use in projects and build upon
    • similar to pip in Python, Gradle/Maven in Java, etc (it's okay if you've never heard of these!)

Node Package Manager (NPM)

NPM is a dependency manager, like pip for python or maven for java. Think of node packages as recipes made by other people you want to use in your site. Also part of the open sourced community!


The package.json is kind of like a directory for your Node project. It contains various metadata and information about it, as well as details on what it depends on, so others can reproduce the behavior of your project.

  • Tracks which node packages you use
  • Dependencies: packages needed at run-time
npm install --save <pkg_name>
yarn add <pkg_name>
  • devDependencies: packages used during development (before pushing to production). When a "production" or real version gets built prior to deployment, these dependencies will not be included. Only install certain tools that ease development in this manner.
npm install --save-dev <pkg_name>
yarn add --dev <pkg_name>

We will use Yarn!

  • Faster at installing dependencies in practice
  • More optimized

NPM vs Yarn commands

  • NPM
npm init
npm install <pkg_name>
npm uninstall <pkg_name>
npm update <pkg_name>
npm audit
  • Yarn
yarn init
yarn add <pkg_name>
yarn remove <pkg_name>
yarn upgrade <pkg_name>
yarn audit
  • Very similar
  • Audit: checks for vulnerabilities in dependencies

var, let, and const

Before we had ES6, there was only one way to declare variable: var. (Well, there is actually another way, but it's very bad and you should never use it.)

For example, if you want to define a variable a initialized to 21 * 2, you write

var a = 21 * 2;

However, var has a big problem: it is function scoped instead of block scoped. Therefore, you might accidentally use a local variable that should not be used anymore!

function someComplexFunction() {
var foo = 42;
if (someCondition()) {
var abc = 41 * 2;
foo += abc;
// You can still use abc here!

In ES6, we finally have let and const that are block scoped. Using block scoped variables outside of the block will be a runtime error. You should use const whenever possible since it defines an immutable variable, only use let if the variable has to be mutable. As mentioned above, you should never use var.

Here is a rewrite of the someComplexFunction above using only let and const:

function someComplexFunction() {
let foo = 42; // foo might be re-assigned. We need to use `let`.
if (someCondition()) {
const abc = 41 * 2; // abc is never mutated. We can use `const`.
foo += abc;
// You can NOT use abc here! This is nice!

Sometimes you have to use let even if the variable is only assigned one. This usually happens when then they are separately assigned inside two branches. For example,

let myMood;
if (hasPrelim()) {
myMood = 'bad';
} else {
myMood = 'good';

To prevent this, people use the ternary operator.

const myMood = hasPrelim() ? 'bad' : 'good';

Arrow Functions

Before ES6 we wrote functions as such:

function myFunction(x) {
return x + 1;


const myFunction = function (x) {
return x + 1;

Now we have this shorthand:

const myFunction = (x) => {
return x + 1;

The syntax is

const functionName = (parameter1, parameter2, parameter3) => {
// function body

For this simple function, we have even shorter shorthand:

const myFunction = (x) => x + 1;

This shorthand works if the function body is just one line that will be immediately returned. In this case, we are simply incrementing x and returning the result.

Anonymous Arrow Functions: just don't include [function name]

(parameter1, parameter2, parameter3) => {
// function body

Functional Programming

map runs function on each element of arrayΒ and returns an array containing the results.

Example: [1, 4, 9].map(x => Math.sqrt(x)) will return [1, 2, 3].


array.filter(function) runs function on each element of arrayΒ and return an array containing all elements that satisfy the function requirements.

Example: [1, 4, 9].filter(x => x > 3) will return [4, 9]


array.forEach(function) runs function on each element of array. The difference between map and forEach is that map returns a value, whereas forEach just applies the function to each element of the array.

Example: [1, 4, 9].forEach(x => console.log(x)) will print out each element to the console.


array.every(function) runs function on each element of array and returns whether every element of the array satisfies the function requirements.

Example: [1, 4, 9].every(x => x > 0) will return true. However, [1, 4, 9].every(x => x > 1) will return false.


array.some(function) runs function on each element of array and returns whether any element of the array satisfies the function requirements.

Example: [1, 4, 9].some(x => x == 1) will return true. However, [1, 4, 9].some(x => x == 2) will return false.


array.reduce(function) runs function on each element of arrayΒ and returns a single value.

Example: [1, 4, 9].reduce((sum, curr) => sum + curr) will return 14.

reduce can take an optional second parameter to change the value that the accumulator starts at.

Example: [1, 4, 9].reduce((sum, curr) => sum + curr, 500) will return 514.

Spreading and Destructuring

Say we have a function:

const add3 = (a: number, b: number, c: number) => a + b + c;

Now if we had an array:

const arr = [1, 2, 3];

We can use the spread operator ... to destructure each element of the array as one of the arguments:

add3(...arr); // same as add3(arr[0], arr[1], arr[2]) output 6

Now if we had an object:

const add3Object = {
a: 3,
b: 4,
c: 7,

add3(...add3Object) is illegal, since the order of fields in the object is not guaranteed. However, you can refactor the add3 function to be:

type ABC = { a: number; b: number; c: number };

const add3 = ({ a, b, c }: ABC) => a + b + c;

// equivalent to:
const add3Uglier = (abc: ABC) => abc.a + abc.b + abc.c;

Then we can do


The spread operator and destructuring is especially useful in destructuring assignment.

const [a, b] = [1, 2];

console.log(a); // 1
console.log(b); // 2

const arr1 = [1, 2, 3, 4, 5];
const [c, d,] = arr1;

console.log(c); // 1
console.log(d); // 2
console.log(rest); // [3, 4, 5]

Ugly Pieces of JavaScript

Truthy, falsy

JavaScript values can be classified into 'truthy' and 'falsy'. Of course, true is truthy and false is falsy. Most values are truthy, except:

  • false
  • ''
  • 0
  • {}
  • null
  • undefined
  • NaN

The if guard in JavaScript checks whether a value is truthy rather than whether the value is true. Similar mechanism applies to && and ||. Therefore, we have

  • '' && 'haha' evaluates to ''
  • 'haha' || '' evaluates to 'haha'.

Global variables

You were told because that there are only one way to define a variable before ES6: var. This is a white lie. You can actually define a variable without var, let, and const:

foo = 3;

If you do this, then you just define a global variable. It means you can use the variable foo everywhere. If you have a local variable, then you might accidentally use or override it with the wrong value.

Lessons learned: never use or define global variables.

Type coercion

Like most languages, JavaScript coerces types to better suit the operations that are being applied.

Example 1

If we execute true + false we get 1. This is because there is an addition operator, and true gets coerced to 1 while falseΒ gets coerced to 0.

Example 2

{} + [] + {} + [1] returns 0[object Object]1 because {} + [] gets evaluated to 0, {} gets evaluated to [object Object], and they both get coerced to strings. Then, adding a list to a string simply adds the contents of the list to the string, so 1 gets appended to the end.

Example 3

const zero = +[]; // + coerce [] into 0
const one = +!![]; // ! coerce [] into false, got inverted, then coerce to 1
const two = +!![] + +!![]; // 2 = 1 + 1

const fib2 = (__) =>
__ === zero || __ === one ? __ : fib2(__ - one) + fib2(__ - two);

This is the Fibonacci sequence implemented using type coercion.

Why Linters Are Necessary

You have just learned some darkest aspects of JavaScript. We must use it because it's the only language that can be understood by browser. To mitigate the problem, people wrote linters that try to automatically find these problems. They are well integrated with editors so you can directly see the warnings. If you follow the 'Setup your editor' section, you are good to go. You will see warnings when you accidentally write some wrong code.


I recommend the You Don't Know JS series by Kyle Simpson. The ebooks are available for free on GitHub. The series is comprehensive and will teach you everything you want to know.

Additionally, the MDN Web Docs are a great resource for quickly looking up the documentation for various features in Javascript, complete with examples.


We mentioned Mozilla Developer Network as a site for documentation about the JavaScript language, but it's also a great way to get familiar with the language.

Here is their JavaScript guide: JavaScript Guide - JavaScript | MDN (

It's a bit long, so I recommend skimming through the first few parts, up to and including the Objects section (Working with objects - JavaScript | MDN (

JavaScript objects will show up a good amount in this course, so make sure you understand the basics!


The official TypeScript website is a great resource to get familiar with the language. There are different guides that assume different programming backgrounds. Choose the article that best suits your background.

Take a look at the Get Started section here: TypeScript: The starting point for learning TypeScript (

If you've gone through the MDN JavaScript guide, fill in your TypeScript knowledge with this: TypeScript: Documentation - TypeScript for JavaScript Programmers (

There is also a Handbook that you can chug through if interested (not necessary at all): TypeScript: Handbook - The TypeScript Handbook (

Hope this is helpful for you all! This will be the language you'll be working with all-semester, so being comfortable with the language will pay off.

Demo Code

Need more examples? This code from last year's lecture's live coding demo rewrites some functions from the preassessment first using loops then with map, filter, and reduce.

// getSqrts: takes in an array and returns an array with all the square roots
// of those numbers
// example: [1, 4, 9] => [1, 2, 3]
const getSqrts = (arr: number[]): number[] => {
const result: number[] = [];
for (const num of arr) {
return result;
const getSqrtsMap = (arr: number[]): number[] => {

// perfectSquares: takes in an array and returns an array with only the
// elements that are perfect squares
// example: [1, 2, 3] => [1]
const perfectSquares = (arr: number[]): number[] => {
const result: number[] = [];
for (const num of arr) {
if (Math.sqrt(num) % 1 === 0) {
return result;
const isPerfectSquare = (num: number) => Math.sqrt(num) % 1 === 0;
const perfectSquaresFilter = (arr: number[]): number[] => {
return arr.filter(isPerfectSquare);

// mySum: takes in an array and returns the sum of the elements
// example: [1, 2, 3] => 6
const mySum = (arr: number[]): number => {
let sum = 0;
for (const num of arr) {
sum += num;
return sum;
const mySumReduce = (arr: number[]): number => {
return arr.reduce((acc: number, curr: number) => acc + curr);

// testing!
const input = [1, 2, 3];