Getting Started
Introduction
JavaScript is a lightweight, interpreted programming language.
- JSON cheatsheet (cheatsheets.zip)
- Regex in JavaScript (cheatsheets.zip)
Console
// => Hello world!
console.log('Hello world!');
// => Hello CheatSheets.zip
console.warn('hello %s', 'CheatSheets.zip');
// Prints error message to stderr
console.error(new Error('Oops!'));
Numbers
let amount = 6;
let price = 4.99;
Variables
let x = null;
let name = "Tammy";
const found = false;
// => Tammy, false, null
console.log(name, found, x);
var a;
console.log(a); // => undefined
Strings
let single = 'Wheres my bandit hat?';
let double = "Wheres my bandit hat?";
// => 21
console.log(single.length);
Arithmetic Operators
5 + 5 = 10 // Addition
10 - 5 = 5 // Subtraction
5 * 10 = 50 // Multiplication
10 / 5 = 2 // Division
10 % 5 = 0 // Modulo
Comments
// This line will denote a comment
/*
The below configuration must be
changed before deployment.
*/
Assignment Operators
let number = 100;
// Both statements will add 10
number = number + 10;
number += 10;
console.log(number);
// => 120
String Interpolation
let age = 7;
// String concatenation
'Tommy is ' + age + ' years old.';
// String interpolation
`Tommy is ${age} years old.`;
let Keyword
let count;
console.log(count); // => undefined
count = 10;
console.log(count); // => 10
const Keyword
const numberOfColumns = 4;
// TypeError: Assignment to constant...
numberOfColumns = 8;
JavaScript Conditionals
if Statement
const isMailSent = true;
if (isMailSent) {
console.log('Mail sent to recipient');
}
Ternary Operator
var x=1;
// => true
result = (x == 1) ? true : false;
Operators
true || false; // true
10 > 5 || 10 > 20; // true
false || false; // false
10 > 100 || 10 > 20; // false
Logical Operator &&
true && true; // true
1 > 2 && 2 > 1; // false
true && false; // false
4 === 4 && 3 > 1; // true
Comparison Operators
1 > 3 // false
3 > 1 // true
250 >= 250 // true
1 === 1 // true
1 === 2 // false
1 === '1' // false
Logical Operator !
let lateToWork = true;
let oppositeValue = !lateToWork;
// => false
console.log(oppositeValue);
Nullish coalescing operator ??
null ?? 'I win'; // 'I win'
undefined ?? 'Me too'; // 'Me too'
false ?? 'I lose' // false
0 ?? 'I lose again' // 0
'' ?? 'Damn it' // ''
else if
const size = 10;
if (size > 100) {
console.log('Big');
} else if (size > 20) {
console.log('Medium');
} else if (size > 4) {
console.log('Small');
} else {
console.log('Tiny');
}
// Print: Small
switch Statement
const food = 'salad';
switch (food) {
case 'oyster':
console.log('The taste of the sea');
break;
case 'pizza':
console.log('A delicious pie');
break;
default:
console.log('Enjoy your meal');
}
== vs ===
0 == false // true
0 === false // false, different type
1 == "1" // true, automatic type conversion
1 === "1" // false, different type
null == undefined // true
null === undefined // false
'0' == false // true
'0' === false // false
The ==
just check the value, ===
check both the value and the type.
JavaScript Functions
Functions
// Defining the function:
function sum(num1, num2) {
return num1 + num2;
}
// Calling the function:
sum(3, 6); // 9
Anonymous Functions
// Named function
function rocketToMars() {
return 'BOOM!';
}
// Anonymous function
const rocketToMars = function() {
return 'BOOM!';
}
Arrow Functions (ES6)
With two arguments
const sum = (param1, param2) => {
return param1 + param2;
};
console.log(sum(2,5)); // => 7
With no arguments
const printHello = () => {
console.log('hello');
};
printHello(); // => hello
With a single argument
const checkWeight = weight => {
console.log(`Weight : ${weight}`);
};
checkWeight(25); // => Weight : 25
Concise arrow functions
const multiply = (a, b) => a * b;
// => 60
console.log(multiply(2, 30));
Arrow function available starting ES2015
return Keyword
// With return
function sum(num1, num2) {
return num1 + num2;
}
// The function doesn't output the sum
function sum(num1, num2) {
num1 + num2;
}
Calling Functions
// Defining the function
function sum(num1, num2) {
return num1 + num2;
}
// Calling the function
sum(2, 4); // 6
Function Expressions
const dog = function() {
return 'Woof!';
}
Function Parameters
// The parameter is name
function sayHello(name) {
return `Hello, ${name}!`;
}
Function Declaration
function add(num1, num2) {
return num1 + num2;
}
JavaScript Scope
Scope
function myFunction() {
var pizzaName = "Margarita";
// Code here can use pizzaName
}
// Code here can't use pizzaName
Block Scoped Variables
const isLoggedIn = true;
if (isLoggedIn == true) {
const statusMessage = 'Logged in.';
}
// Uncaught ReferenceError...
console.log(statusMessage);
Global Variables
// Variable declared globally
const color = 'blue';
function printColor() {
console.log(color);
}
printColor(); // => blue
let vs var
for (let i = 0; i < 3; i++) {
// This is the Max Scope for 'let'
// i accessible ✔️
}
// i not accessible ❌
for (var i = 0; i < 3; i++) {
// i accessible ✔️
}
// i accessible ✔️
var
is scoped to the nearest function block, and let
is scoped to the nearest enclosing block.
Loops with closures
// Prints 3 thrice, not what we meant.
for (var i = 0; i < 3; i++) {
setTimeout(_ => console.log(i), 10);
}
// Prints 0, 1 and 2, as expected.
for (let j = 0; j < 3; j++) {
setTimeout(_ => console.log(j), 10);
}
The variable has its own copy using let
, and the variable has shared copy using var
.
JavaScript Arrays
Arrays
const fruits = ["apple", "orange", "banana"];
// Different data types
const data = [1, 'chicken', false];
Property .length
const numbers = [1, 2, 3, 4];
numbers.length // 4
Index
// Accessing an array element
const myArray = [100, 200, 300];
console.log(myArray[0]); // 100
console.log(myArray[1]); // 200
Mutable chart
add | remove | start | end | |
---|---|---|---|---|
push |
✔ | ✔ | ||
pop |
✔ | ✔ | ||
unshift |
✔ | ✔ | ||
shift |
✔ | ✔ | ||
{.show-header} |
Method .push()
// Adding a single element:
const cart = ['apple', 'orange'];
cart.push('pear');
// Adding multiple elements:
const numbers = [1, 2];
numbers.push(3, 4, 5);
Add items to the end and returns the new array length.
Method .pop()
const fruits = ["apple", "orange", "banana"];
const fruit = fruits.pop(); // 'banana'
console.log(fruits); // ["apple", "orange"]
Remove an item from the end and returns the removed item.
Method .shift()
let cats = ['Bob', 'Willy', 'Mini'];
cats.shift(); // ['Willy', 'Mini']
Remove an item from the beginning and returns the removed item.
Method .unshift()
let cats = ['Bob'];
// => ['Willy', 'Bob']
cats.unshift('Willy');
// => ['Puff', 'George', 'Willy', 'Bob']
cats.unshift('Puff', 'George');
Add items to the beginning and returns the new array length.
Method .concat()
const numbers = [3, 2, 1]
const newFirstNumber = 4
// => [ 4, 3, 2, 1 ]
[newFirstNumber].concat(numbers)
// => [ 3, 2, 1, 4 ]
numbers.concat(newFirstNumber)
if you want to avoid mutating your original array, you can use concat.
JavaScipt Set
Create Set
// Empty Set Object
const emptySet = new Set()
// Set Object with values
const setObj = new Set([1, true, "hi"])
Add
const emptySet = new Set()
// add values
emptySet.add('a') // 'a'
emptySet.add(1) // 'a', 1
emptySet.add(true) // 'a', 1, true
emptySet.add('a') // 'a', 1, true
Delete
const emptySet = new Set([1, true, 'a'])
// delete values
emptySet.delete('a') // 1, true
emptySet.delete(true) // 1
emptySet.delete(1) //
Has
const setObj = new Set([1, true, 'a'])
// returns true or false
setObj.has('a') // true
setObj.has(1) // true
setObj.has(false) // false
Clear
const setObj = new Set([1, true, 'a'])
// clears the set
console.log(setObj) // 1, true, 'a'
setObj.clear() //
Size
const setObj = new Set([1, true, 'a'])
consoloe.log(setObj.size) // 3
ForEach
const setObj = new Set([1, true, 'a'])
setObj.forEach(function(value){
console.log(value)
})
// 1
// true
// 'a'
JavaScript Loops
While Loop
while (condition) {
// code block to be executed
}
let i = 0;
while (i < 5) {
console.log(i);
i++;
}
Reverse Loop
const fruits = ["apple", "orange", "banana"];
for (let i = fruits.length - 1; i >= 0; i--) {
console.log(`${i}. ${fruits[i]}`);
}
// => 2. banana
// => 1. orange
// => 0. apple
Do…While Statement
x = 0
i = 0
do {
x = x + i;
console.log(x)
i++;
} while (i < 5);
// => 0 1 3 6 10
For Loop
for (let i = 0; i < 4; i += 1) {
console.log(i);
};
// => 0, 1, 2, 3
Looping Through Arrays
for (let i = 0; i < array.length; i++){
console.log(array[i]);
}
// => Every item in the array
Break
for (let i = 0; i < 99; i += 1) {
if (i > 5) {
break;
}
console.log(i)
}
// => 0 1 2 3 4 5
Continue
for (i = 0; i < 10; i++) {
if (i === 3) { continue; }
text += "The number is " + i + "<br>";
}
Nested
for (let i = 0; i < 2; i += 1) {
for (let j = 0; j < 3; j += 1) {
console.log(`${i}-${j}`);
}
}
for…in loop
const fruits = ["apple", "orange", "banana"];
for (let index in fruits) {
console.log(index);
}
// => 0
// => 1
// => 2
for…of loop
const fruits = ["apple", "orange", "banana"];
for (let fruit of fruits) {
console.log(fruit);
}
// => apple
// => orange
// => banana
JavaScript Iterators
Functions Assigned to Variables
let plusFive = (number) => {
return number + 5;
};
// f is assigned the value of plusFive
let f = plusFive;
plusFive(3); // 8
// Since f has a function value, it can be invoked.
f(9); // 14
Callback Functions
const isEven = (n) => {
return n % 2 == 0;
}
let printMsg = (evenFunc, num) => {
const isNumEven = evenFunc(num);
console.log(`${num} is an even number: ${isNumEven}.`)
}
// Pass in isEven as the callback function
printMsg(isEven, 4);
// => The number 4 is an even number: True.
Array Method .reduce()
const numbers = [1, 2, 3, 4];
const sum = numbers.reduce((accumulator, curVal) => {
return accumulator + curVal;
});
console.log(sum); // 10
Array Method .map()
const members = ["Taylor", "Donald", "Don", "Natasha", "Bobby"];
const announcements = members.map((member) => {
return member + " joined the contest.";
});
console.log(announcements);
Array Method .forEach()
const numbers = [28, 77, 45, 99, 27];
numbers.forEach(number => {
console.log(number);
});
Array Method .filter()
const randomNumbers = [4, 11, 42, 14, 39];
const filteredArray = randomNumbers.filter(n => {
return n > 5;
});
JavaScript Objects
Accessing Properties
const apple = {
color: 'Green',
price: { bulk: '$3/kg', smallQty: '$4/kg' }
};
console.log(apple.color); // => Green
console.log(apple.price.bulk); // => $3/kg
Naming Properties
// Example of invalid key names
const trainSchedule = {
// Invalid because of the space between words.
platform num: 10,
// Expressions cannot be keys.
40 - 10 + 2: 30,
// A + sign is invalid unless it is enclosed in quotations.
+compartment: 'C'
}
Non-existent properties
const classElection = {
date: 'January 12'
};
console.log(classElection.place); // undefined
Mutable
const student = {
name: 'Sheldon',
score: 100,
grade: 'A',
}
console.log(student)
// { name: 'Sheldon', score: 100, grade: 'A' }
delete student.score
student.grade = 'F'
console.log(student)
// { name: 'Sheldon', grade: 'F' }
student = {}
// TypeError: Assignment to constant variable.
Assignment shorthand syntax
const person = {
name: 'Tom',
age: '22',
};
const {name, age} = person;
console.log(name); // 'Tom'
console.log(age); // '22'
Delete operator
const person = {
firstName: "Matilda",
age: 27,
hobby: "knitting",
goal: "learning JavaScript"
};
delete person.hobby; // or delete person[hobby];
console.log(person);
/*
{
firstName: "Matilda"
age: 27
goal: "learning JavaScript"
}
*/
Objects as arguments
const origNum = 8;
const origObj = {color: 'blue'};
const changeItUp = (num, obj) => {
num = 7;
obj.color = 'red';
};
changeItUp(origNum, origObj);
// Will output 8 since integers are passed by value.
console.log(origNum);
// Will output 'red' since objects are passed
// by reference and are therefore mutable.
console.log(origObj.color);
Shorthand object creation
const activity = 'Surfing';
const beach = { activity };
console.log(beach); // { activity: 'Surfing' }
this Keyword
const cat = {
name: 'Pipey',
age: 8,
whatName() {
return this.name
}
};
console.log(cat.whatName()); // => Pipey
Factory functions
// A factory function that accepts 'name',
// 'age', and 'breed' parameters to return
// a customized dog object.
const dogFactory = (name, age, breed) => {
return {
name: name,
age: age,
breed: breed,
bark() {
console.log('Woof!');
}
};
};
Methods
const engine = {
// method shorthand, with one argument
start(adverb) {
console.log(`The engine starts up ${adverb}...`);
},
// anonymous arrow function expression with no arguments
sputter: () => {
console.log('The engine sputters...');
},
};
engine.start('noisily');
engine.sputter();
Getters and setters
const myCat = {
_name: 'Dottie',
get name() {
return this._name;
},
set name(newName) {
this._name = newName;
}
};
// Reference invokes the getter
console.log(myCat.name);
// Assignment invokes the setter
myCat.name = 'Yankee';
JavaScript Classes
Static Methods
class Dog {
constructor(name) {
this._name = name;
}
introduce() {
console.log('This is ' + this._name + ' !');
}
// A static method
static bark() {
console.log('Woof!');
}
}
const myDog = new Dog('Buster');
myDog.introduce();
// Calling the static method
Dog.bark();
Class
class Song {
constructor() {
this.title;
this.author;
}
play() {
console.log('Song playing!');
}
}
const mySong = new Song();
mySong.play();
Class Constructor
class Song {
constructor(title, artist) {
this.title = title;
this.artist = artist;
}
}
const mySong = new Song('Bohemian Rhapsody', 'Queen');
console.log(mySong.title);
Class Methods
class Song {
play() {
console.log('Playing!');
}
stop() {
console.log('Stopping!');
}
}
extends
// Parent class
class Media {
constructor(info) {
this.publishDate = info.publishDate;
this.name = info.name;
}
}
// Child class
class Song extends Media {
constructor(songData) {
super(songData);
this.artist = songData.artist;
}
}
const mySong = new Song({
artist: 'Queen',
name: 'Bohemian Rhapsody',
publishDate: 1975
});
JavaScript Modules
Export
// myMath.js
// Default export
export default function add(x,y){
return x + y
}
// Normal export
export function subtract(x,y){
return x - y
}
// Multiple exports
function multiply(x,y){
return x * y
}
function duplicate(x){
return x * 2
}
export {
multiply,
duplicate
}
Import
// main.js
import add, { subtract, multiply, duplicate } from './myMath.js';
console.log(add(6, 2)); // 8
console.log(subtract(6, 2)) // 4
console.log(multiply(6, 2)); // 12
console.log(duplicate(5)) // 10
// index.html
<script type="module" src="main.js"></script>
Export Module
// myMath.js
function add(x,y){
return x + y
}
function subtract(x,y){
return x - y
}
function multiply(x,y){
return x * y
}
function duplicate(x){
return x * 2
}
// Multiple exports in node.js
module.exports = {
add,
subtract,
multiply,
duplicate
}
Require Module
// main.js
const myMath = require('./myMath.js')
console.log(myMath.add(6, 2)); // 8
console.log(myMath.subtract(6, 2)) // 4
console.log(myMath.multiply(6, 2)); // 12
console.log(myMath.duplicate(5)) // 10
JavaScript Promises
Promise states
const promise = new Promise((resolve, reject) => {
const res = true;
// An asynchronous operation.
if (res) {
resolve('Resolved!');
}
else {
reject(Error('Error'));
}
});
promise.then((res) => console.log(res), (err) => console.error(err));
Executor function
const executorFn = (resolve, reject) => {
resolve('Resolved!');
};
const promise = new Promise(executorFn);
setTimeout()
const loginAlert = () =>{
console.log('Login');
};
setTimeout(loginAlert, 6000);
.then() method
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('Result');
}, 200);
});
promise.then((res) => {
console.log(res);
}, (err) => {
console.error(err);
});
.catch() method
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
reject(Error('Promise Rejected Unconditionally.'));
}, 1000);
});
promise.then((res) => {
console.log(value);
});
promise.catch((err) => {
console.error(err);
});
Promise.all()
const promise1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve(3);
}, 300);
});
const promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve(2);
}, 200);
});
Promise.all([promise1, promise2]).then((res) => {
console.log(res[0]);
console.log(res[1]);
});
Avoiding nested Promise and .then()
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('*');
}, 1000);
});
const twoStars = (star) => {
return (star + star);
};
const oneDot = (star) => {
return (star + '.');
};
const print = (val) => {
console.log(val);
};
// Chaining them all together
promise.then(twoStars).then(oneDot).then(print);
Creating
const executorFn = (resolve, reject) => {
console.log('The executor function of the promise!');
};
const promise = new Promise(executorFn);
Chaining multiple .then()
const promise = new Promise(resolve => setTimeout(() => resolve('dAlan'), 100));
promise.then(res => {
return res === 'Alan' ? Promise.resolve('Hey Alan!') : Promise.reject('Who are you?')
}).then((res) => {
console.log(res)
}, (err) => {
console.error(err)
});
Fake http Request with Promise
const mock = (success, timeout = 1000) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
if(success) {
resolve({status: 200, data:{}});
} else {
reject({message: 'Error'});
}
}, timeout);
});
}
const someEvent = async () => {
try {
await mock(true, 1000);
} catch (e) {
console.log(e.message);
}
}
JavaScript Async-Await
Asynchronous
function helloWorld() {
return new Promise(resolve => {
setTimeout(() => {
resolve('Hello World!');
}, 2000);
});
}
const msg = async function() { //Async Function Expression
const msg = await helloWorld();
console.log('Message:', msg);
}
const msg1 = async () => { //Async Arrow Function
const msg = await helloWorld();
console.log('Message:', msg);
}
msg(); // Message: Hello World! <-- after 2 seconds
msg1(); // Message: Hello World! <-- after 2 seconds
Resolving Promises
let pro1 = Promise.resolve(5);
let pro2 = 44;
let pro3 = new Promise(function(resolve, reject) {
setTimeout(resolve, 100, 'foo');
});
Promise.all([pro1, pro2, pro3]).then(function(values) {
console.log(values);
});
// expected => Array [5, 44, "foo"]
Async Await Promises
function helloWorld() {
return new Promise(resolve => {
setTimeout(() => {
resolve('Hello World!');
}, 2000);
});
}
async function msg() {
const msg = await helloWorld();
console.log('Message:', msg);
}
msg(); // Message: Hello World! <-- after 2 seconds
Error Handling
let json = '{ "age": 30 }'; // incomplete data
try {
let user = JSON.parse(json); // <-- no errors
console.log( user.name ); // no name!
} catch (e) {
console.error( "Invalid JSON data!" );
}
Aysnc await operator
function helloWorld() {
return new Promise(resolve => {
setTimeout(() => {
resolve('Hello World!');
}, 2000);
});
}
async function msg() {
const msg = await helloWorld();
console.log('Message:', msg);
}
msg(); // Message: Hello World! <-- after 2 seconds
JavaScript Requests
JSON
const jsonObj = {
"name": "Rick",
"id": "11A",
"level": 4
};
Also see: JSON cheatsheet
XMLHttpRequest
const xhr = new XMLHttpRequest();
xhr.open('GET', 'mysite.com/getjson');
XMLHttpRequest
is a browser-level API that enables the client to script data transfers via JavaScript, NOT part of the JavaScript language.
GET
const req = new XMLHttpRequest();
req.responseType = 'json';
req.open('GET', '/getdata?id=65');
req.onload = () => {
console.log(xhr.response);
};
req.send();
POST
const data = {
fish: 'Salmon',
weight: '1.5 KG',
units: 5
};
const xhr = new XMLHttpRequest();
xhr.open('POST', '/inventory/add');
xhr.responseType = 'json';
xhr.send(JSON.stringify(data));
xhr.onload = () => {
console.log(xhr.response);
};
fetch api
fetch(url, {
method: 'POST',
headers: {
'Content-type': 'application/json',
'apikey': apiKey
},
body: data
}).then(response => {
if (response.ok) {
return response.json();
}
throw new Error('Request failed!');
}, networkError => {
console.log(networkError.message)
})
}
JSON Formatted
fetch('url-that-returns-JSON')
.then(response => response.json())
.then(jsonResponse => {
console.log(jsonResponse);
});
promise url parameter fetch api
fetch('url')
.then(
response => {
console.log(response);
},
rejection => {
console.error(rejection.message);
);
Fetch API Function
fetch('https://api-xxx.com/endpoint', {
method: 'POST',
body: JSON.stringify({id: "200"})
}).then(response => {
if(response.ok){
return response.json();
}
throw new Error('Request failed!');
}, networkError => {
console.log(networkError.message);
}).then(jsonResponse => {
console.log(jsonResponse);
})
async await syntax
const getSuggestions = async () => {
const wordQuery = inputField.value;
const endpoint = `${url}${queryParams}${wordQuery}`;
try{
const response = await fetch(endpoint, {cache: 'no-cache'});
if(response.ok){
const jsonResponse = await response.json()
}
}
catch(error){
console.log(error)
}
}