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

// Include external JS file
<script src="filename.js"></script>

function add(a, b) {return a + b;}

console.log(add(1, 2)); // 3

const add = function(a, b) {return a + b;}

console.log(add(1, 2)); // 3

const add = (a, b) => {return a + b;}

console.log(add(1, 2)); // 3

// One parameter

const cube = a => {return a * a * a;}

console.log(cube(2)); // 8

// No parameters

const rnd = _ => {return Math.random();}

console.log(rnd()); // Some random number

// 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 dialogue, returns true/false depending on user click
prompt("Your age?","0"); // Input dialogue. Second argument is the initial value

// Comments
/* Multi line
   comment */

// Delay 1 second
setTimeout(function () { ...  }, 1000);

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

var student={ // Object name
  firstName: "Jane", // List of properties and values
  lastName: "Doe",
  age: 18,
  fullName: function() { // Object function
    return this.firstName+" "+this.lastName;
  }
}
student.age=19;          // Setting value
name=student.fullName(); // Call object function

var b="init";       // String
var d=1+2+"3";      // "33"
var e=[2,3,5,8];    // Array
var g=/()/;         // RegEx
var h=function(){}; // Function object
const PI=3.14;      // Constant
let z='zzz';        // Block scope local variable
let a??=10;         // Only assign is a is Nullish (null or undefined)

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

false, true                  // Boolean
18, 3.14, 0b10011, 0xF6, NaN // Number
"flower", 'John'             // String
undefined, null ,Infinity    // Special

x=100%48; // 4 Modulo 100/48
&  // AND         5 & 1  (0101 & 0001) (1)     1
|  // OR          5 | 1  (0101 | 0001) (101)   5
~  // NOT         ~ 5    (~0101)       (1010) 10
^  // XOR         5 ^ 1  (0101 ^ 0001) (100)   4
<< // left shift  5 << 1 (0101 << 1)   (1010) 10
>> // right shift 5 >> 1 (0101 >> 1)   (10)    2

person.age === person[age] // Member
!(a == b)   // Logical not
typeof a    // Type (number, object, function...)
a == b      // Equals
a != b      // Unequal
a === b     // Strict equal (datatypes must also match)
a !== b     // Strict unequal (value or datatypes don't match)
a && b
a || b

if ((age >= 14) && (age < 19)) {
  group = 1;
} else if (age < 60) {
  group = 2;
} else {
  group = 3;
}

console.log(a>b?'a biggest':'b biggest');

switch (new Date().getDay()) {
    case 6:
        text="Saturday";
        break;
    case 0:
        text="Sunday";
        break;
    default:
        text="Whatever";
}

let val;

if (val) { Will not go here }

if (!val) { Will go here }

const person_obj = 'nonsense';

console.log(person.obj.name.first); // Error!

// Check if it's defined
console.log(person_obj && person_obj.name && person_obj.name.first); // undefined.

// Cleaner option.
console.log(person_obj?.name?.first); // undefined.

var arr=[1, 2, 4]; var sum=0;
for (var i=0; i<a.length; i++) {
    sum += a[i];
}
html="";
for (var i of custOrder) { ... }

while (i<100) { ...  }
do { ... } while i<100);

for (var i=0; i<10; i++) {
  if (i == 5) { break; }
}

for (var i=0; i<10; i++) {
  if (i == 5) { continue; } // Skips the rest of the cycle
  document.write(i + ", "); // 5 not output
}

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 occurrence
"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)
"abc".padEnd(4,'-');        // Rpad
"abc".padStart(4,'-');      // Lpad

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
Number.POSITIVE_INFINITY

var pi=Math.PI;
Math.round(4.4);       // 4 - rounded
Math.pow(2,8);         // 256 - 2 to the power of 8
Math.sqrt(49);
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 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);           // 2.7182 pow(E,x)
Math.random();         // Random number between 0 and 1

Math.floor(Math.random() * 5) + 1; // Random integer, from 1 to 5

Date("2017-06-23"); // Date declaration. Also: Date("June 23 2017")
Date("2017");       // Is set to Jan 01
Date("2017-06-23T12:00:00-09:45");
Date("Jun 23 2017 07:45:00 GMT+0100 (Tokyo Time)");
Date(2012,10,23,17,34,32) // 2012-11-23T17:34:32.000Z

Date.now());         // Milliseconds passed since 1970

date = new Date();
date.toISOString().replace(/[ZT:-]/g,'').replace(/\..*/g,'');

var d=new Date();    // Thu Aug 22 2019 16:26:11 GMT+0100 (British Summer Time)
Number(d)            // Milliseconds passed since 1970
d.getDay();          // Getting the weekday
d.getDate();         // Day as a number (1-31)
d.getDay();          // Weekday as a number (0-6)
d.getFullYear();     // Four digit year (yyyy)
d.getHours();        // Hour (0-23)
d.getMilliseconds(); // Milliseconds (0-999)
d.getMinutes();      // Minutes (0-59)
d.getMonth();        // Month (0-11)
d.getSeconds();      // Seconds (0-59)
d.getTime();         // Milliseconds since 1970

d.setDate();         // Day as a number (1-31)
d.setFullYear();     // Year (optionally month and day)
d.setHours();        // Hour (0-23)
d.setMilliseconds(); // Milliseconds (0-999)
d.setMinutes();      // Minutes (0-59)
d.setMonth();        // Month (0-11)
d.setSeconds();      // Seconds (0-59)
d.setTime();         // Milliseconds since 1970)

d.setDate(d.getDate()+7); // Adds a week to a date

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 Terrier"; // Change the first item

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

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.includes('Boxer');              // Returns True / False
dogs.includes('Boxer', 3);           // Includes from position 3 onwards
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

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

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

var a=str.search(/CheatSheet/i);

i  // Perform case-insensitive matching
g  // Perform a global match
m  // Perform multiline matching

\         // 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 bar 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

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

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

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
}

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

var str='{"names":[{"first":"Hakuna","last":"Matata"},{"first":"Air","last":"Jordan"}]}';

obj = JSON.parse(str);               // Parse
document.write(obj.names[1].first);  // Access

let json = { "name": "Martin", "age": 21 };
for (let j in json) {
  console.log(j, json[j]);  // Yields name Martin etc
}

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

myJSON=JSON.stringify(myObj);            // Storing data
localStorage.setItem("testJSON", myJSON);
text=localStorage.getItem("testJSON");   // Retrieving data
obj=JSON.parse(text);
document.write(obj.name);

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.
});

pending, fulfilled, rejected

Promise.length, Promise.prototype

Promise.all(iterable), Promise.race(iterable), Promise.reject(reason), Promise.resolve(value)

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

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

onkeydown, onkeypress, onkeyup

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

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

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

oncopy, oncut, onpaste

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

animationend, animationiteration, animationstart

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

function saveText(text, fileName) {
    var a=document.createElement("a");
    document.body.appendChild(a);
    a.style="display: none";

    var json=JSON.stringify(text),
        blob=new Blob([text], {type: "text/plain;charset=utf-8"}),
        url=window.URL.createObjectURL(blob);
    a.href=url;
    a.download=fileName;
    a.click();
    window.URL.revokeObjectURL(url);
}

/**
 * Adds two numbers together.
 * @param {int} num1 The first number.
 * @param {int} num2 The second number.
 * @returns {int} The sum of the two numbers.
 */
function sum(num1, num2) {
    return num1 + num2;
}