Version: 2021sp

Lecture 1

Lecture Video

Lecture Slides

No homework this week!


What is JavaScript#

  • JavaScript is the de facto language of the web
  • Commonly used in conjunction with HTML/CSS
  • Became really popular for powering client-side logic through AJAX
    • Previously, languages like PHP had to communicate with the server before coming back with a response
  • These days, JavaScript is everywhere!

Java is to JavaScript as car is to carpet. They are very different languages!

Basic JavaScript Syntax#


There are three ways to create variables in JS:

  1. var x = 5
  2. let x = 5
  3. const x = 5

We prefer using const for immutability although let is also accepted. Never use var. We'll talk more about why in Lecture 4, where we cover best-practices in modern JavaScript (ES6).

if statements#

Nothing surprising here.

if (condition) {
// executes if condition is true
} else if (condition2) {
// executes if condition is false but condition2 is true
} else {
// executes if condition is false

for loops#

regular counter for loop#
for (let i = 0; i < 5; i++) {
for of loop#

We can use for..of loops to loop through elements of an array.

const arr = [10, 20, 30, 40];
for (const val of arr) {
console.log(val); // prints values: 10, 20, 30, 40

for in loop#

We can use loops to loop through keys of an object. A good way to remember when to use of or in is that of is for iterables, while in is for objects.

const object = { a: 1, b: 2, c: 3 };
for (const property in object) {
console.log(`${property}: ${object[property]}`);
// expected output:
// "a: 1"
// "b: 2"
// "c: 3"

while loops#

let n = 0;
while (n < 3) {
// expected output:
// "0"
// "1"
// "2"

function declaration#

We can use the function key word to define a function!

function calcRectArea(width, height) {
return width * height;
console.log(calcRectArea(5, 6)); // 30

or we can use arrow functions:

const calcRectArea = (width, height) => {
return width * height;

More details on arrow functions in a few weeks when we talk about ES6 (Lecture 4)!


JavaScript is a super powerful language and this was just a small sample of its language features. Check out Mozilla Developer Network (MDN) for the best JavaScript documentation:


TypeScript is a typed superset of JavaScript that compiles to plain JavaScript. Superset means TypeScript has everything in JavaScript and more. (Built by Microsoft!)

JavaScript Types#

JavaScript has 6 primitive types:

  • Boolean
  • String
  • Number
  • Symbol
  • undefined
  • BigInt

All JavaScript values are those 6 primitive types, or an:

  • object
  • function (JavaScript is functional!)
  • null

How are types used?#

In JavaScript we had:

let str = 'Hello, trends';
let num = 42;
let truth = true;
const someFunc = (x, s, b) => {
// do some operations...
return x;

Notice we don't have any types here! JavaScript is weakly typed, meaning that it is lenient with declaring what types variables are before you run a program with them, similarly to Python.

let str: string = 'Hello, trends';
let num: number = 42;
let truth: boolean = false;
const someFunc = (x: number, s: string, b: boolean): number => {
// do some operations...
return x;

TypeScript allows us to add type information!

Why TypeScript?#

JavaScript code can be ambiguous. We had the function:

const someFunc = (x, s, b) => {
// do some operations...
return x;

What are x, s, b? What should I pass in for those? What should I expect returned?

Adding the TypeScript types makes this code self-documenting:

const someFunc = (x: number, s: string, b: boolean): number => {
// do some operations...
return x;

JavaScript variables can also change type which can be undesirable, unexpected, and error-prone.

let str = 'Hello, trends';
let num = 42;
let truth = true;
str = 13;

None of these variables have to be any specific type! I can have str be a string and then a number.

In the end, we want to use TypeScript because it is:

  • Easier to read
  • Easier and faster to implement
  • Easier to refactor
  • Less buggy

TypeScript Types#

Basic Syntax:

let <var_name>: <type> = <something>;

We can also use const but again no var.

Basic Types#

// Boolean
let isDone: boolean = false;
// Number can be decimal, or in any base!
let decimal: number = 4.2;
let binary: number = 0b1010;
let hex: number = 0xf00d;
// String
let lang: string = 'typescript';
let templateStr: string = `We love ${lang}`;
// Boolean
let isDone: boolean = false;
// Number can be decimal, or in any base!
let decimal: number = 4.2;
let binary: number = 0b1010;
let hex: number = 0xf00d;
// String
let lang: string = 'typescript';
let templateStr: string = `We love ${lang}`;


any is a wildcard and it can be anything. any places no restrictions on type.

// Any: can be anything!
let notSure: any = 4;
notSure = 'maybe a string instead';
notSure = false; // okay, definitely a boolean

If you were to use any everywhere though you might as well just use JavaScript.

let anyList: any[] = [4, 'le string', false];

But it can be useful in specifying collections of items of different types, when you don't know the constituent types. If you did know that they could either be numbers, strings, or booleans as the above code snippet, you could have written:

let hodgePodgeList: (number | string | boolean)[] = [4, 'le string', false];


Functions can have types too!

// un-typed
const myFunc = (x, y) => x + y;
// typed
const myFunc = (x: number, y: number): number => x + y;

myFunc has type (x: number, y: number): number.

TypeScript can do some limited type inference so if you leave out the return type number, TypeScript can infer it since we are just adding two numbers which can only produce a number. If TypeScript can't infer the type, it defaults as any.

We can also have optional parameters:

const introduce = (name: string, github?: string): string => {
return github
? `Hi, I'm ${name}. Checkout my GitHub @${github}`
: `Hi, I'm ${name}. I don't have a GitHub.`;

github? designates github as an optional parameter that defaults to undefined.

Literal Types#

Literal Types are types that can be a literal set of possibilities that you specify. TypeScript allows number and string literal types:

String Literal Types#
// String literal type
type TrafficLightColors = 'red' | 'green' | 'yellow';

Any variable with TrafficLightColors type can only take on values "red", "green", "yellow".

let light1: TrafficLightColors = 'red';
light1 = 'blue'; // TypeError
Numeric Literal Types#
// Numeric literal type
type DiceRoll = 1 | 2 | 3 | 4 | 5 | 6;
const rollDice = (): DiceRoll => {
// ...

Union Types#

With union types, a variable can be of one type or another type.

const union: number | string = 5; // number
const union2: number | string = 'hello'; // string
type TrafficLightColors = 'red' | 'green' | 'yellow';
type PrimaryColors = 'red' | 'green' | 'blue';
// "red" | "green" | "yellow" | "blue"
type union = PrimaryColors | TrafficLightColors;

Intersection Types#

With union types, a variable must be of one type and another type.

// Intersection Type
type TrafficLightColors = 'red' | 'green' | 'yellow';
type PrimaryColors = 'red' | 'green' | 'blue';
type intersect = PrimaryColors & TrafficLightColors; // "red" | "green"

There's also so much more to TypeScript. Checkout TypeScript docs to learn more!


We went through a demo of writing and running code in TypeScript using the preassessment as an example. Run the following commands to first create a Node project and then install typescript as well as ts-node, a package that runs TypeScript files through the terminal. Don't worry about the files that appear when you run these commands for now; we'll explain what they mean next week.

yarn init # answer the questions as well
yarn add typescript
yarn add ts-node

We used the following example code. (note that TypeScript files have a .ts extension, as opposed to JavaScript's .js. This will allow VS Code to recognize that you are coding in TS)

const mySum = (inputArray: number[]): number => {
let sum: number = 0;
for (const num of inputArray) {
sum += num;
return sum;
console.log(mySum([1, 2, 3])); // expected 6
const isLeapYear = (year: number): boolean => {
return (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
console.log(isLeapYear(2000)); // is a leap year
console.log(isLeapYear(2100)); // is NOT a leap year;
const perfectSquares = (arr: number[]): number[] => {
const ans: number[] = [];
for (const num of arr) {
if (Math.sqrt(num) % 1 === 0) {
return ans;
console.log(perfectSquares([1, 4, 9])); // expected same as input
console.log(perfectSquares([1, 5, 9])); // expected [1, 9]

Run it with ts-node script.ts. Voilà! That's a basic introduction to TypeScript. For more language quirks and useful syntax, visit the TypeScript website and pick the tutorial that best fits you.