JavaScript cheatsheet

A JavaScript cheat sheet with the most important concepts, functions, methods, and more. A complete quick reference for beginners.

#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`		✔	✔

#Array.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.

#Array.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.

#Array.shift()

let cats = ["Bob", "Willy", "Mini"];

cats.shift(); // ['Willy', 'Mini']

Remove an item from the beginning and returns the removed item.

#Array.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.

#Array.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.

#JavaScript 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.reduce()

const numbers = [1, 2, 3, 4];

const sum = numbers.reduce((accumulator, curVal) => {
  return accumulator + curVal;
});

console.log(sum); // 10

#Array.map()

const members = ["Taylor", "Donald", "Don", "Natasha", "Bobby"];

const announcements = members.map((member) => {
  return member + " joined the contest.";
});

console.log(announcements);

#Array.forEach()

const numbers = [28, 77, 45, 99, 27];

numbers.forEach((number) => {
  console.log(number);
});

#Array.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!");
    },
  };
};

#Object 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);
  },
);

#Promise.catch()

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);
  }
};