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JS Cheat Sheet JS CheatSheet

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Basics

Basic syntax, introduction.

On page script

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

Include external JS file

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

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

Loops

Cycle through a block of code a number of times.

For Loop

for (var i = 0; i < 10; i++) {
    document.write(i + ": " + i*3 + "<br />");
}
var sum = 0;
for (var i = 0; i < a.length; i++) {
    sum + = a[i];
}               // parsing an array

While Loop

var i = 1;                      // initialize
while (i < 100) {               // enters the cycle if statement is true
    i *= 2;                     // increment to avoid infinite loop
    document.write(i + ", ");   // output
}

Do While Loop

var i = 1;                      // initialize
do {                            // enters cycle at least once
    i *= 2;                     // increment to avoid infinite loop
    document.write(i + ", ");   // output
} while (i < 100)               // repeats cycle if statement is true at the end

Break

for (var i = 0; i < 10; i++) {
    if (i == 5) { break; }          // stops and exits the cycle
    document.write(i + ", ");       // last output number is 4
}

Continue

for (var i = 0; i < 10; i++) {
    if (i == 5) { continue; }       // skips the rest of the cycle
    document.write(i + ", ");       // skips 5
}

If - Else

If-Else and Switch statements.
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";
} 

Variables

JavaScript variables and operators
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

Bitwise operators

& AND   5 & 1 (0101 & 0001)1 (1)
| OR   5 | 1 (0101 | 0001)5 (101)
~ NOT   ~ 5 (~0101)10 (1010)
^ XOR   5 ^ 1 (0101 ^ 0001)4 (100)
<< left shift   5 << 1 (0101 << 1)10 (1010)
>> right shift   5 >> 1 (0101 >> 1)2 (10)
>>> zero fill right shift   5 >>> 1 (0101 >>> 1)2 (10)

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

JavaScript variables and objects
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

Selectors specify which elements are targeted with a style
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)

Ads

Otherwise we're starving.

Events

Event handlers in JavaScript.
<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

Numbers and Math

Working with numbers in JavaScript.
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

Working with dates.
var d = new Date();
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 zone

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)

Arrays

JavaScript arrays: multiple values in a single variable
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

concat, copyWithin, every, fill, filter, find, findIndex, forEach, indexOf, isArray, join, lastIndexOf, map, pop, push, reduce, reduceRight, reverse, shift, slice, some, sort, splice, toString, unshift, valueOf

Global Functions

Common JS 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

Regular Expressions

JavaScript regular expression cheetsheet.
var a = str.search(/CheatSheet/i);

Modifiers

i perform case-insensitive matching
g perform a global match
m perform multiline matching

Patterns

\ Escape character
\d find a digit
\s find a whitespace character
\b find match at beginning or end of a word
n+ contains at least one n
n* contains zero or more occurrences of n
n? contains zero or one occurrences of n
^ Start of string
$ End of string
\uxxxx find the Unicode character
. Any single character
(a|b) a or b
(...) Group section
[abc] In range (a, b or c)
[0-9] any of the digits between the brackets
[^abc] Not in range
\s White space
a? Zero or one of a
a* Zero or more of a
a*? Zero or more, ungreedy
a+ One or more of a
a+? One or more, ungreedy
a{2} Exactly 2 of a
a{2,} 2 or more of a
a{,5} Up to 5 of a
a{2,5} 2 to 5 of a
a{2,5}? 2 to 5 of a, ungreedy
[:punct:] Any punctu­ation symbol
[:space:] Any space character
[:blank:] Space or tab

Errors

Handling JS errors: try, catch, throw, finally
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

JavaScript Object Notation
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

An object, used for asynchronous computations. A Promise represents a value which may be available now, or in the future, or never.
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)

Useful Links

A list of free useful resources.
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Online Interactive JavaScript (JS) Cheat Sheet

JavaScript Cheat Seet contains useful code examples on a single page. Find code for JS loops, variables, objects, data types, strings, events and many other categories. Popy-paste the code you need or just quickly check the JS syntax for your projects.

Choose to display or hide the comments, clicking the command in the top right corner.

  • Basics – Introduction to JavaScript syntax. Learn how to include the scripts on a HTML page, how to declare a function, target a DOM element by it ID, how to output the data and how to write comments.
  • Loops – Most programming languages allow to work with loops, which help in executing one or more statements up to a desired number of times. Find the "for" and "while" loop syntax in this section.
  • If - Else statements – Conditional statements are used to perform different actions based on different conditions.
  • Variables – Use variables (numbers, strings, arrays etc.) and learn the operators.
  • Data types – You can declare many types of variables and declare your own objects in JavaScript.
  • Strings – Learn how to work with JS strings and find the most common functions to work with this data type.
  • Events – Use JavaScript event listeners to trigger functions.
  • Numbers and math – Work with JS numbers, predefined constants and perform math functions.
  • Dates – Get or modify current time and date.
  • Arrays – Learn how to organize your vairables in vectors and how to use them.
  • Global functions – Predefined functions that are built in every browser that supports JS.
  • Regular expressions – Use RegEx to define a search pattern.
  • Errors – JS error handling.
  • JSON – JavaScript Object Notation is syntax used for storing and exchanging data.
  • Promises – The Promise object is used for asynchronous computation. See our example on how to declare one.

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