JavaScript 101

Quick references for fundamental things in JavaScript.

Miscellaneous #

• Using // for 1 line comment, /* */ for multi-line comment.
• End of each command by ;.
• Variables and function name are case sensitive.
• Variables / functions should be named in the form of nameOfVariable.
  • Naming a constant: const FAV_PET = 'Cat';.
  • UpperCamelCase should be used by convention for ES6 class names.
• Using \ for special characters, for example, \" for a " inside a "".
• '' can be used inside a "" and vice versa.

Practice directly on the browser #

Open the browser (I use Chrome), press F12 to open the Inspect window, then choose tab Console. Now, you can practice on this console window, for example, try with 1+1 and press Enter.

ES6 #

• "ES" = "ECMAScript" ~ "Javascript".
• Most of browsers use ES6 but not all.
• ES6 = ES2015
• JS timeline.
• New features: Arrow functions, Classes, Modules, Promises, Generators, let and const.
• Read more about ES6 on w3schools.

Concise things #

// Concise Object Literal Declarations
const getMousePosition = (x, y) => ({ x, y });

// Concise Declarative Functions
const person = {
  name: "Taylor",
  sayHello() {
    return `Hello! My name is ${this.name}.`;
  }
};

Getters and setters #

class Book {
  constructor(author) {
    this._author = author;
  }
  // getter
  get writer() {
    return this._author;
  }
  // setter
  set writer(updatedAuthor) {
    this._author = updatedAuthor;
  }
}
const lol = new Book('anonymous');
console.log(lol.writer);  // anonymous
lol.writer = 'wut';
console.log(lol.writer);  // wut

Export/Import to share code blocks #

// file ./math_funcs.js
const add = (x, y) => {return x + y};
const subtract = (x, y) => {return x - y};
export { add, subtract }; // can be imported in other scripts

// file ./math_default.js only has 1 export
export default function(x, y) { // without name
  return x + y;
}

// main.js
import {add, subtract } from './math_funcs.js';
import * as myMathModule from "./math_functions.js"; // everything

import anything from './math_default.js'; // from export default

Declare variables & Scopes #

var varName; // global scope
let varName; // ES6, block scope (inside {} or function,...)
const varName; // ES6, can't be changed

function funcName(){
  oopsGlobal = 5;  // without `var`, it's a global variable
}
oppsGlobal; // returns 5

Difference between var, let and const,

var a = 1;
var a = 2; // ok, a=2 now
a = 5; // a=5 now

let c = 1;
let c = 2; // error
c = 3; // c=3 now

const b = 1;
const b = 2; // error
b = 2 // error

const s = [1, 2, 3];
s = [1, 2, 4]; // error
s[2] = 4; // OK

Output #

myName = "Thi"
console.log("I'm " + myName + ".");
console.log(`I'm ${myName}.`); // ES6

Basic operators #

Check more operators & more operators with Math.

i = i + 1; // i++; // i += 1;
i = i - 1; // i--; // i -= 1;
i = i * 3; // i *= 3;
i = i / 2; // i /= 2;

11 % 3 // = 2, remainder

Math.random(); // give a random number between 0 and 1
Math.floor(); // round to its nearest whole number

parseInt("007a"); // give number 7
parseInt("11", 2); // give number 3, "2" is radix

Conditional statements if / switch #

• Comparison Operators: <, ==, === (strict equality), >, >=, <=, !=, !== (strict inequality).
  • Difference between == and ===: 3=='3' (true), 3==='3' (false).
  • Difference between != and !==: 3!='3' (false), 3!=='3' (true).
• Logical operators: &&, ||, !.
• Short-circuit evaluation: let a = b || 'Thi'; (if b isn't defined yet, a takes value 'Thi')
• Ternary Operator: isNightTime ? console.log('Yes!') : console.log('No!');. We can use multiple nested ternary.

if (true){
  // commands
}

if (true){
  // commands
} else if (true){
  // commands
} else{
  // commands
}

let var = 'papaya';
switch (var) {
  case 'val1':
    // commands
    break;
  case 'val2':
    // commands
    break;
  default:
    // commands
    break;
}

You can remove break; to apply the same result for multiple cases.

Functions #

// ES6 way
const rectangleArea = (width=10, height=5) => {
  let area = width * height;
  return area;
};

// if there is no parameter
const <func> = () => {};

// if there is only one parameter
const <func> = <para> => {};

// single line: no need "{}"
const sumNumbers = number => number + number;

Older ways (ES5),

function <funcName>(<para>){
  // commands
}

const last_element = (...args) => {
  return args[-1];
}
last_element(1,2,3); // 3

const arr = [6, 89, 3, 45];
const maximus = Math.max(...arr); // 89

Strings #

Check more methods.

var a = 'Anh-Thi' + 'Dinh'; // plus the strings
a.length; // length of the string
a[0]; // first letter of the string
a[3]; // 4th letter
a[a.length - 1]; // last letter

a[0] = 'D'; // !!! CAN'T change an individual word

Special characters: \' (single quote), \" (double quote), \\(backslash), \n (newline), \r (carriage return), \t (tab), \b (word boundary), \f (form feed).

Arrays #

Check more methods.

var arrName = ['a', 1, 'c'];
var nestedArr = [[1, 2], [3, 4]];

arrName[0] = 2; // 1st element is changed (different from string!)
nestedArr[1]; // gives [3, 4]

arrName.push(5); // add 5 at the END of an array
arrName.unshift(6); // add 6 at the BEGINNING of an array

popedElement = arrName.pop() // remove the LAST element of an array
shiftedElement = arrName.shift() // remove the FIRST element of an array

Objects #

var myObj = {
  top: "hat",
  "bottom": "pants"
};

// CHECK PROPERTIES
myObj.hasOwnProperty("top");    // true
myObj.hasOwnProperty("middle"); // false

// Accessing Object Properties
myObj.top; // dot
myObj["bottom"]; // bracket

var pos = "top";
myObj[pos]; // via a variable

myObj.top = "T-shirt"; // Update object properties
myObj.middle = "New shoe"; // Add new properties
delete myObj.middle; // Delete a property

We can use object for lookups instead of using if..else or switch,

var alpha = {1:"A", 2:"B", 3:"C"};
value = 2;
alpha[value]; // instead of using if value==2 or switch value...

We can create a nested object inside an object.

Prevent object mutation,

let obj = {
  name:"FreeCodeCamp",
  review:"Awesome"
};
Object.freeze(obj); // obj can't be changed

Destructuring Assignment (ES6) #

Extract values from object,

const user = {name: "Thi", age: 30};
const {name, age} = user; // name="Thi", age=30

Assign variable from object,

const {name: uName, age: uAge} = user; // uName="Thi", uAge=30

Assign Variables from Nested Objects,

const user = {
  anhThi: {
    age: 30,
    email: '[email protected]'
  }
};
const {anhThi: {age, email}} = user;
const {anhThi: {age: userAge, email: userEmail}} = user;

Assign Variables from Arrays,

const [a, b] = [1, 2, 3, 4, 5, 6];
console.log(a, b); // 1, 2

const [a, b,,, c] = [1, 2, 3, 4, 5, 6];
console.log(a, b, c); // 1, 2, 5

Assignment with the Rest Parameter to Reassign Array Elements,

const [a, b, ...arr] = [1, 2, 3, 4, 5, 7];
console.log(a, b); // 1, 2
console.log(arr); // [3, 4, 5, 7]

Pass an Object as a Function's Parameters,

const profileUpdate = ({ name, age, nationality, location }) => {
  // do something with these fields
}

JSON #

JavaScript Object Notation or JSON is a related data interchange format used to store data,

var ourMusic = [
  {
    "artist": "Daft Punk",
    "title": "Homework",
    "release_year": 1997,
    "formats": [
      "CD",
      "Cassette",
      "LP"
    ],
    "gold": true
  }, // first artist
  {
  ...
  } // second artist
];

To access "CD": ourMusic[0].formats[0].

Iterate with while / for / do..while loops #

Check more statements.

while (<conditions>){
  // commands
}

do{
  // commands
} while (<conditions>);

for (var i=0; i<5; i++){
  // commands
}

// Iterate odd numbers
for (var i=0; i<5; i+=2){
  // commands
}

// Count backwards
for (var i=10; i?5; i-=2){
  // commands
}

References #

• freeCodeCamp -- JavaScript Algorithms and Data Structures Certification (300 hours).
• w3schools -- JavaScript Tutorial.
• Event bubbling and capture