JavaScript Basics

Data types

// Numbers:
let length = 16;
let weight = 7.5;

// Strings:
let color = "Yellow";
let lastName = "Johnson";

// Booleans
let x = true;
let y = false;

// Object:
const person = {
    firstName:"John",
    lastName:"Doe"
};

// Array object:
const cars = ["Saab", "Volvo", "BMW"];

// Date object:
const date = new Date("2022-03-25");

let, const

let age = 30;
age = 31;
// Works - Data can be changed

let now, birthYear;
now = 2025;
birthYear = 2001;
// Works

const birthYear = 2005;
birthYear = 2001;
// Error - Data can't be changed

const name;
// Error - Should use

It's good practice to avoid variable mutation so you should use 'const', unless the variable is supposed to be changed later

You should always declare variables, avoid doing this:

lastName = 'Tran';
console.log(lastName)
// Still works, but should be avoided

Basic operators

Transform, combine, ... work with values

Math

const now = 2025;
const ageUser = now - 1994;
// 31

console.log(ageUser * 2, ageUser / 10);

console.log(2 ** 3);
// 2^3 = 8

+ can be used to concat variables:

let x = "Volvo" + 16 + 4;
// Volvo164
x = 16 + 4 + "Volvo";
// 20Volvo

Assignment

let x = 10 + 5;
x += 10; // x = x + 10 = 25
x *= 2; // x = x*2 = 50

x++; // x = x + 1
x--; // x = x - 1

Comparision

Produce boolean

const birthYear = 2006;
const isAdults = 2025  - birthYear >= 18 // true
console.log(2025  - birthYear > 2025  - 2020) // true
// Operators: >,<,>=,<=

Equality

Precedence

Check MDN page

Strings & Template Literals

const name = 'Loc'
const university = 'UMP'

console.log("I'm" + " " + name + ", who study at" + " " + university);
console.log(`I'm ${name}, who study at ${university}`);
// Same

console.log('String with \n\
multiple \n\
lines')
console.log(`String with
multiple
lines`)
// Sames, you should favor Template Literals

if - else Statements

let century;
const birthYear = 2001;
if (birthYear <=2000) {
    century = 20;
} else {
    century = 21;
}
console.log(century);

Type Conversion & Coercion

// type conversion - manual conversion
const inputYear = '2000';
console.log(inputYear + 18); // 200018 - Doesn't work as expected
console.log(Number(inputYear) + 18); // 2018

console.log(Number('Loc')); // NaN - not a number

console.log(typeof NaN); // still type: number

console.log(String(23))

// type coercion- automatic conversion
console.log("I'm " + 10 + "years old");
console.log("I'm " + '10' + "years old");
//  The same

console.log('23' - '10' + 5); // 18
console.log('23' + '10' + 5); // 23105

console.log('10' / '2'); // 5

let n = '1' + 1;
n = n - 1; // 10

console.log(2+3+4+'5'); // 95

Truthy & Falsy values

5 falsy values: 0, '', undefined, null, NaN -> convert to false when convert to boolean

console.log(Boolean(0), Boolean(undefined));
// false
console.log(Boolean('loc'), Boolean({}));
// true

// Prefer type coercion
const money = 0;
if (money) {
console.log('Don\'t spend it all')
} else {
    console.log('You should get a job!')
}

let height;
if (height) {
    console.log('Height is defined')
} else {
    console.log('You should define height')
}
// Going to bug when you set height = 0;

Miscs

'use strict'