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 (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";
} 

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

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

setTimeout(function () {

}, 1000);

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

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

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

/* Multi line
comment */
// One line

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,
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

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
html = "";
for (var i of custOrder) {
html += "<li>" + i + "</li>";
}

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
}

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

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

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

var abc = "1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRST";
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)

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

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

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)

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

"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

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

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

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]);
}

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

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
}