JS cheat sheet

Basics

On page script

<script type="text/javascript">  ...
</script>

Include external JS file

<script src="filename.js"></script>

Delay - 1 second timeout

setTimeout(function () {

}, 1000);

Functions

function addNumbers(a, b) {
return a + b; ;
}
x = addNumbers(1, 2);

Edit DOM element

document.getElementById("elementID").innerHTML = "Hello World!";

Output

console.log(a);             // write to the browser console
document.write(a);          // write to the HTML
alert(a);                   // output in an alert box
confirm("Really?");         // yes/no dialog, returns true/false depending on user click
prompt("Your age?","0");    // input dialog. Second argument is the initial value

Comments

/* Multi line
comment */
// One line

If - Else

if ((age >= 14) && (age < 19)) {        // logical condition
status = "Eligible.";               // executed if condition is true
} else {                                // else block is optional
status = "Not eligible.";           // executed if condition is false
}

Switch Statement

switch (new Date().getDay()) {      // input is current day
case 6:                         // if (day == 6)
	text = "Saturday";          
	break;
case 0:                         // if (day == 0)
	text = "Sunday";
	break;
default:                        // else...
	text = "Whatever";
}

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...of loop

const fruits = ["apple", "orange", "banana"];

for (let fruit of fruits) {
  console.log(fruit);
}
// => apple
// => orange
// => banana

for...in loop

const fruits = ["apple", "orange", "banana"];

for (let index in fruits) {
  console.log(index);
}
// => 0
// => 1
// => 2

for...in loop (Object)

let user = {
  firstName: 'Lady',
  lastName: 'Gaga',
  gender: 'female'
}


for (let index in user) {
  alert(index); // firstName, lastName, gender
  alert(user[index]); // 'Lady', 'Gaga', 'female'
}

Variables x

var a;                          // variable
var b = "init";                 // string
var c = "Hi" + " " + "Joe";     // = "Hi Joe"
var d = 1 + 2 + "3";            // = "33"
var e = [2,3,5,8];              // array
var f = false;                  // boolean
var g = /()/;                   // RegEx
var h = function(){};           // function object
const PI = 3.14;                // constant
var a = 1, b = 2, c = a + b;    // one line
let z = 'zzz';                  // block scope local variable

Strict mode

"use strict";   // Use strict mode to write secure code
x = 1;          // Throws an error because variable is not declared

Values

false, true                     // boolean
18, 3.14, 0b10011, 0xF6, NaN    // number
"flower", 'John'                // string
undefined, null , Infinity      // special

Operators

a = b + c - d;      // addition, substraction
a = b * (c / d);    // multiplication, division
x = 100 % 48;       // modulo. 100 / 48 remainder = 4
a++; b--;           // postfix increment and decrement

Arithmetic

a * (b + c)         // grouping
person.age          // member
person[age]         // member
!(a == b)           // logical not
a != b              // not equal
typeof a            // type (number, object, function...)
x << 2  x >> 3      // minary shifting
a = b               // assignment
a == b              // equals
a != b              // unequal
a === b             // strict equal
a !== b             // strict unequal
a < b   a > b       // less and greater than
a <= b  a >= b      // less or equal, greater or eq
a += b              // a = a + b (works with - * %...)
a && b              // logical and
a || b              // logical or

Data types

var age = 18;                           // number 
var name = "Jane";                      // string
var name = {first:"Jane", last:"Doe"};  // object
var truth = false;                      // boolean
var sheets = ["HTML","CSS","JS"];       // array
var a; typeof a;                        // undefined
var a = null;                           // value null

Objects

var student = {                 // object name
firstName:"Jane",           // list of properties and values
lastName:"Doe",
age:18,
height:170,
fullName : function() {     // object function
   return this.firstName + " " + this.lastName;
}
}; 
student.age = 19;           // setting value
student[age]++;             // incrementing
name = student.fullName();  // call object function

Strings

var abc = "abcdefghijklmnopqrstuvwxyz";
var esc = 'I don\'t \n know';   // \n new line
var len = abc.length;           // string length
abc.indexOf("lmno");            // find substring, -1 if doesn't contain 
abc.lastIndexOf("lmno");        // last occurance
abc.slice(3, 6);                // cuts out "def", negative values count from behind
abc.replace("abc","123");       // find and replace, takes regular expressions
abc.toUpperCase();              // convert to upper case
abc.toLowerCase();              // convert to lower case
abc.concat(" ", str2);          // abc + " " + str2
abc.charAt(2);                  // character at index: "c"
abc[2];                         // unsafe, abc[2] = "C" doesn't work
abc.charCodeAt(2);              // character code at index: "c" -> 99
abc.split(",");                 // splitting a string on commas gives an array
abc.split("");                  // splitting on characters
128.toString(16);               // number to hex(16), octal (8) or binary (2)

Numbers & Math

var pi = 3.141;
pi.toFixed(0);          // returns 3
pi.toFixed(2);          // returns 3.14 - for working with money
pi.toPrecision(2)       // returns 3.1
pi.valueOf();           // returns number
Number(true);           // converts to number
Number(new Date())      // number of milliseconds since 1970
parseInt("3 months");   // returns the first number: 3
parseFloat("3.5 days"); // returns 3.5
Number.MAX_VALUE        // largest possible JS number
Number.MIN_VALUE        // smallest possible JS number
Number.NEGATIVE_INFINITY// -Infinity
Number.POSITIVE_INFINITY// Infinity

Math.

var pi = Math.PI;       // 3.141592653589793
Math.round(4.4);        // = 4 - rounded
Math.round(4.5);        // = 5
Math.pow(2,8);          // = 256 - 2 to the power of 8
Math.sqrt(49);          // = 7 - square root 
Math.abs(-3.14);        // = 3.14 - absolute, positive value
Math.ceil(3.14);        // = 4 - rounded up
Math.floor(3.99);       // = 3 - rounded down
Math.sin(0);            // = 0 - sine
Math.cos(Math.PI);      // OTHERS: tan,atan,asin,acos,
Math.min(0, 3, -2, 2);  // = -2 - the lowest value
Math.max(0, 3, -2, 2);  // = 3 - the highest value
Math.log(1);            // = 0 natural logarithm 
Math.exp(1);            // = 2.7182pow(E,x)
Math.random();          // random number between 0 and 1
Math.floor(Math.random() * 5) + 1;  // random integer, from 1 to 5

Constants like Math.PI:

E, PI, SQRT2, SQRT1_2, LN2, LN10, LOG2E, Log10E

Dates

//Fri Mar 24 2023 13:37:29 GMT+0200 (EET)
var d = new Date();
//1679657849400 miliseconds passed since 1970
Number(d) 
Date("2017-06-23");                 // date declaration
Date("2017");                       // is set to Jan 01
Date("2017-06-23T12:00:00-09:45");  // date - time YYYY-MM-DDTHH:MM:SSZ
Date("June 23 2017");               // long date format
Date("Jun 23 2017 07:45:00 GMT+0100 (Tokyo Time)"); // time

Get Times

var d = new Date();
a = d.getDay();     // getting the weekday

getDate();          // day as a number (1-31)
getDay();           // weekday as a number (0-6)
getFullYear();      // four digit year (yyyy)
getHours();         // hour (0-23)
getMilliseconds();  // milliseconds (0-999)
getMinutes();       // minutes (0-59)
getMonth();         // month (0-11)
getSeconds();       // seconds (0-59)
getTime();          // milliseconds since 1970

Setting part of a date

var d = new Date();
d.setDate(d.getDate() + 7); // adds a week to a date

setDate();          // day as a number (1-31)
setFullYear();      // year (optionally month and day)
setHours();         // hour (0-23)
setMilliseconds();  // milliseconds (0-999)
setMinutes();       // minutes (0-59)
setMonth();         // month (0-11)
setSeconds();       // seconds (0-59)
setTime();          // milliseconds since 1970)

Events

<button onclick="myFunction();">
Click here
</button>

Mouse
onclick, oncontextmenu, ondblclick, onmousedown, onmouseenter, onmouseleave, onmousemove, onmouseover, onmouseout, onmouseup

Keyboard
onkeydown, onkeypress, onkeyup

Frame
onabort, onbeforeunload, onerror, onhashchange, onload, onpageshow, onpagehide, onresize, onscroll, onunload

Form
onblur, onchange, onfocus, onfocusin, onfocusout, oninput, oninvalid, onreset, onsearch, onselect, onsubmit

Drag
ondrag, ondragend, ondragenter, ondragleave, ondragover, ondragstart, ondrop

Clipboard
oncopy, oncut, onpaste

Media
onabort, oncanplay, oncanplaythrough, ondurationchange, onended, onerror, onloadeddata, onloadedmetadata, onloadstart, onpause, onplay, onplaying, onprogress, onratechange, onseeked, onseeking, onstalled, onsuspend, ontimeupdate, onvolumechange, onwaiting

Animation
animationend, animationiteration, animationstart

Miscellaneous
transitionend, onmessage, onmousewheel, ononline, onoffline, onpopstate, onshow, onstorage, ontoggle, onwheel, ontouchcancel, ontouchend, ontouchmove, ontouchstart

Arrays

var dogs = ["Bulldog", "Beagle", "Labrador"]; 
var dogs = new Array("Bulldog", "Beagle", "Labrador");  // declaration

alert(dogs[1]);             // access value at index, first item being [0]
dogs[0] = "Bull Terier";    // change the first item

for (var i = 0; i < dogs.length; i++) {     // parsing with array.length
console.log(dogs[i]);
}

Methods

dogs.toString();                        // convert to string: results "Bulldog,Beagle,Labrador"
dogs.join(" * ");                       // join: "Bulldog * Beagle * Labrador"
dogs.pop();                             // remove last element
dogs.push("Chihuahua");                 // add new element to the end
dogs[dogs.length] = "Chihuahua";        // the same as push
dogs.shift();                           // remove first element
dogs.unshift("Chihuahua");              // add new element to the beginning
delete dogs[0];                         // change element to undefined (not recommended)
dogs.splice(2, 0, "Pug", "Boxer");      // add elements (where, how many to remove, element list)
var animals = dogs.concat(cats,birds);  // join two arrays (dogs followed by cats and birds)
dogs.slice(1,4);                        // elements from [1] to [4-1]
dogs.sort();                            // sort string alphabetically
dogs.reverse();                         // sort string in descending order
x.sort(function(a, b){return a - b});   // numeric sort
x.sort(function(a, b){return b - a});   // numeric descending sort
highest = x[0];                         // first item in sorted array is the lowest (or highest) value
x.sort(function(a, b){return 0.5 - Math.random()});     // random order sort
dogs.forEach(element => {               // Перебор элементов массива через foreach
  console.log(element);
});

Global functions

eval();                     // executes a string as if it was script code
String(23);                 // return string from number
(23).toString();            // return string from number
Number("23");               // return number from string
decodeURI(enc);             // decode URI. Result: "my page.asp"
encodeURI(uri);             // encode URI. Result: "my%page.asp"
decodeURIComponent(enc);    // decode a URI component
encodeURIComponent(uri);    // encode a URI component
isFinite();                 // is variable a finite, legal number
isNaN();                    // is variable an illegal number
parseFloat();               // returns floating point number of string
parseInt();                 // parses a string and returns an integer

Errors

try {                           // block of code to try
undefinedFunction();
}
catch(err) {                    // block to handle errors
console.log(err.message);
}

Throw error

throw "My error message";    // throw a text

Input validation

var x = document.getElementById("mynum").value; // get input value
try { 
if(x == "")  throw "empty";                 // error cases
if(isNaN(x)) throw "not a number";
x = Number(x);
if(x > 10)   throw "too high";
}
catch(err) {                                    // if there's an error
document.write("Input is " + err);          // output error
console.error(err);                         // write the error in console
}
finally {
document.write("</br />Done");              // executed regardless of the try / catch result
}

Error name values

RangeError 
A number is "out of range"
ReferenceError 
An illegal reference has occurred
SyntaxError 
A syntax error has occurred
TypeError 
A type error has occurred
URIError 
An encodeURI() error has occurred

JSON j

var str = '{"names":[' +                    // crate JSON object
'{"first":"Hakuna","lastN":"Matata" },' +
'{"first":"Jane","lastN":"Doe" },' +
'{"first":"Air","last":"Jordan" }]}';
obj = JSON.parse(str);                      // parse
document.write(obj.names[1].first);         // access

Send

var myObj = { "name":"Jane", "age":18, "city":"Chicago" };  // create object
var myJSON = JSON.stringify(myObj);                         // stringify
window.location = "demo.php?x=" + myJSON;                   // send to php

Storing and retrieving

myObj = { "name":"Jane", "age":18, "city":"Chicago" };
myJSON = JSON.stringify(myObj);                 // storing data
localStorage.setItem("testJSON", myJSON);   
text = localStorage.getItem("testJSON");        // retrieving data 
obj = JSON.parse(text);
document.write(obj.name);

Promises

function sum (a, b) {
return Promise(function (resolve, reject) { 
 setTimeout(function () {                                       // send the response after 1 second
   if (typeof a !== "number" || typeof b !== "number") {        // testing input types
	 return reject(new TypeError("Inputs must be numbers"));
   }
   resolve(a + b);
 }, 1000);
});
}
var myPromise = sum(10, 5);
myPromsise.then(function (result) {
document.write(" 10 + 5: ", result);
return sum(null, "foo");              // Invalid data and return another promise
}).then(function () {                   // Won't be called because of the error
}).catch(function (err) {               // The catch handler is called instead, after another second
console.error(err);                   // => Please provide two numbers to sum.
});

States
pending, fulfilled, rejected
Properties
Promise.length, Promise.prototype
Methods
Promise.all(iterable), Promise.race(iterable), Promise.reject(reason), Promise.resolve(value)

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 ❌

// 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.

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';

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
});

Modules

Export/Import

// 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 / Require Module

// Export
// 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
// 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

Requests

JSON

const jsonObj = {
  "name": "Rick",
  "id": "11A",
  "level": 4  
};

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)
  }
}

Last updated 1 year ago