1. JS Namespacing

if (typeof mynamespace != "object") { var mynamespace = {}; }
mynamespace.component = function() { 
   // component safely nestled within a namespace  
};

2. Get a Class Name from Multiples

$("#content").attr("class").split(" ")[0];

3. Argument Defaulting

drawLines : function(lineheight) {
   lineheight = lineheight || "12px"; //default
   displayline(lineheight);
}

4. Toggle Display

function toggle(obj) {
   obj.style.display = obj.style.display == "none" ? "" : "none";		
}

5. Cross Browser Event Reduction

document.onkeydown = function(e) {
   //return the right event	
   e = e || window.event;	
   //get keycode
   var keyCode = e.keyCode || e.which;
}

6. Code Convention for Module Pattern Component

// the typical anatomy of a component component.init( config ) 
// initialisation - accepts JSON for configuration options	

component.init // intialisation (optional self invoking), can be used pass through, unload, onDomReady or arbitrarily 
component.addEvents // event handling
component.display // display 
component.destroy // remove component and clean up reference onunload

7. Annonymous Immediately Invoking Function (IFF)

// avoid global scope
// create a new annonymous function, to use as a wrapper
// from Pro JavaScript Techniques By John Resig
(function() {
   // The variable that would, normally, be global
   var msg = "Thanks for visiting!";
   
   // Binding a new function to a global object
   
   window.onload = function() {
   	// Which uses the 'hidden' variable
   	alert( msg );
   };
   
// Close off the annonymous function and execute it	
})();

8. Module Pattern Extended

Reference:

   
var myScript = function(){
 var pub = {};
 var massivelyLongVariableProbablyGerman = 1;
 function thisIsReallyLong(n){};
 function doStuff(n){};
 pub.init = function(){
   doStuff(massivelyLongVariableProbablyGerman);
   thisIsReallyLong(massivelyLongVariableProbablyGerman);
   pub.otherPublic();
 };
 pub.otherPublic = function(){
 };
 return pub;
}();
myScript.init();

9. Hash Table Methods or Handy NVP Data Storage

var person = {
   drinkBeer : function() {
   	alert("drinking...");
   },
   
   burp : function() {
   	alert("burping...");
   },
   
   purge : function() {
   	alert("purging...");
   }
};

// also
person.drinkBeer();

// or

person["drinkBeer"]();

// this could be a dynamic variable altered at runtime
var methodCall = "drinkBeer";

// call the relevant method	
person[methodCall]();

10. Quick 'n' Dirty ArrayContains

alert( ['where','are','my', 'keys'].indexOf("keys") != -1 );

//or

if (['where','are','my', 'keys'].indexOf("keys").toString() != -1) {

}

// or

var obj = {};
var arr = [ 'foo', 'bar', 5, obj, 10 ];
console.log( arr.indexOf('bar') ); // prints: 1

11. Front End Performance Tip for string concatenation

var stuckTogether = ["value1", "value2", "value3"].join(""); // Deprecated: use concat instead (see #34)

12. Module Pattern (public and private members within a namespace)

jQuery.noConflict()
//Note: ; before parenthesis is deliberate
;(function ( container, document, $, undefined ) { // add in more parameters for context e.g. ( container, document, jQuery, utils ), pass in undefined so it can't be re-assigned (security)

   function createModule()  { // Revealing Module Pattern with execution context passed in arguments
   	var localVariable = "local variable",
   	myPublicProperty = "public variable",
   	init = function() {
   	
   	}(); // self invoking
   	
   	function myPrivateMethod() {
   		// not in contract list so remains private
   	} // end myPrivateMethod

   	function myPublicMethod() {
   		return "public function";
   	} // end myPublicMethod

   	var contract = {
   		myPublicProperty : myPublicProperty,
   		myPublicMethod : myPublicMethod
   	}

   	// Public interface (properties and methods)
   	return contract;

   } // end module

   // Public API (assigns to my namespace)
   container.ModuleName = createModule();

})( window.mynamespace || (window.mynamespace = {}), jQuery, document ); //end mynamespace.ModuleName (create namespace and context)

13. Multi Line Alternative to String Concatenation

// careful using this with some build tools 

var myString = 	"some really long text repeated\n\
   		 	some really long text repeated\n\
   			some really long text repeated";

14. Faster Loops

var itemsLen = items.length; // cache length - prevents evaluation on each loop

for(i=itemsLen; i--;)  { // looping backwards is faster
   //note: loop works in reverse
}

15. Closure to Encapsulate Lost Scope

var listElements = document.getElementsByTagName("li");

for(var i=0; i < listElements.length; i++) {
   (function(inc) {
     listElements[inc].onclick = function() {
   	alert(inc);
   };
 }(i));
}

16. Property Checking

if ( foo == "bar" || foo == "foobar" || foo == "foo" ) {
//...
}

// can be written as
if ( foo in { bar:1, foobar:1, foo:1 } ) {
//...
}

17. jQuery Selector Caching

var foo = $(".item").bind("click", function(){
   
   foo.not(this).addClass("bar");
   	.removeClass("foobar");
   	.fadeOut(500);

});

18. Force a Boolean

function supports_canvas() {
 return !!document.createElement('canvas').getContext;
}

/*
if getContext gives you a "falsey" value, the !! will make it return the boolean value false. Otherwise it will return true.

The "falsey" values are:

false
NaN
undefined
null
"" (empty string)
0
*/

19. Append an Array to Another Array

var a = [4,5,6];
var b = [7,8,9];
Array.prototype.push.apply(a, b);

console.log(a); // is: [4, 5, 6, 7, 8, 9]

20. Order an Array and Get Highest or Lowest

var list = [5,1,7,2,9,4,6,3,8],
   lowestNumber,
   highestNumber;

lowestNumber = list.sort(function(a,b){a = parseFloat(a);b = parseFloat(b);return a - b;})[0]; 
highestNumber = list.sort(function(a,b){a = parseFloat(a);b = parseFloat(b);return a - b;})[list.length-1]; 
console.log(highestNumber);

21. Shuffle an Array

var list = [1,2,3,4,5,6,7,8,9];
list = list.sort(function() {return Math.random() - 0.5});
console.log(list); // prints something like: 4,3,1,2,9,5,6,7,8

22. SyntaxError (JavaScript 1.5)

// raised when a syntax error occurs while parsing code in eval()
try {
   eval('1 + * 5'); // will rise a SyntaxError exception
} catch( ex ) {
   console.log( ex.constructor == SyntaxError ); // Prints true
}

// JavaScript 1.7
try {
   eval('1 + * 5');
} catch( ex if ex instanceof SyntaxError ) {
   console.log( 'SyntaxError !' ); // prints: SyntaxError !
}

23. Function Hijacking

var obj = { // static object  
   // add function, supports adding 2 values  
   add: function(x, y) {  
       return x + y;  
   }  
};  
     
// move the existing function to another variable name  
obj.__add = obj.add;  
     
// override the existing function with your own, to support adding 2 or 3 values  
obj.add = function(x, y, z) {  
   var val = obj.__add(x, y);  
   return z ? val + z : val;  
}

24. Call a function in a parent class

toto.prototype = new function() {
 this.a = function() {
   Print(456);
 }
};

function toto() {
 this.a = function(){
   Print(123);
   toto.prototype.a.call(this); // or: this.__proto__.a();
 }
}

var o = new toto;
o.a();
/*
prints:
123
456
*/

25. Memoization

const memoizeFn = fn => {
 let prevResult
 let prevArg
 return arg => {
   if (prevArg === arg) {
     return prevResult
   }
   const result = fn(arg)
   prevArg = arg
   prevResult = result
   return result
 }
}

const calculateResult = a => a * 2

const memoizedFn = memoizeFn(calculateResult)
memoizedFn(1) // calls "calculateResult"
memoizedFn(1) // returns cached arg
memoizedFn(1) // returns cached arg
memoizedFn(2) // calls "calculateResult"

26. Inheritance

Reference

// define the Person Class  
function Person() {}  

Person.prototype.walk = function(){  
 alert ('I am walking!');  
};  

Person.prototype.sayHello = function(){  
 alert ('hello');  
};  

// define the Student class  
function Student() {  
 // Call the parent constructor  
 Person.call(this);  
}  

// inherit Person  
Student.prototype = new Person();  

// correct the constructor pointer because it points to Person  
Student.prototype.constructor = Student;  

// replace the sayHello method  
Student.prototype.sayHello = function(){  
 alert('hi, I am a student');  
}  

// add sayGoodBye method  
Student.prototype.sayGoodBye = function(){  
 alert('goodBye');  
}  

var student1 = new Student();  
student1.sayHello();  
student1.walk();  
student1.sayGoodBye();  

// check inheritance  
alert(student1 instanceof Person); // true   
alert(student1 instanceof Student); // true

27. Performant Strings

Reference:

// This example requires the script engine to create 21 new string objects, once for each time the length property is accessed, and once each time the charAt method is called:
var s = '0123456789';
for( var i = 0; i < s.length; i++ ) {
 s.charAt(i);
}

// This equivalent example creates just a single object, and will perform better as a result:
var s = new String('0123456789');
for( var i = 0; i < s.length; i++ ) {
 s.charAt(i);
}

28. Padding a string (without code branching)

('00' + 3).slice(-3)
// "003"
('00' + 42).slice(-3)
// "042"
('00' + 103).slice(-3)
// "103"

29. isArray()

arr = 1 // That's a simple integer, of course.
Object.prototype.toString.call(arr) == '[object Array]';
// false
arr = '1' // Just a string.
Object.prototype.toString.call(arr) == '[object Array]';
// false
arr = '[1,2,3]' // Also a string.
Object.prototype.toString.call(arr) == '[object Array]';
// false
arr = {x:1, y:2} // That's an object, no array either!
Object.prototype.toString.call(arr) == '[object Array]';
// false
arr = [1,2,3] // An array, so it should be true.
Object.prototype.toString.call(arr) == '[object Array]';
// true

30. Range

function range(start, stop) {
 for (var r = []; start < stop; r.push(start++));
 return r;
}

31. Is Array

if(!Array.isArray) { 
 Array.isArray = function(v){ 
   return Object.prototype.toString.call(v) === '[object Array]'; 
 } 
}

32. Push State & Location Hash

// Note: don't litter browser history, limit hashbang events
if (history.pushState) {
   history.pushState(null, '', '#', data);
} else {
   location.hash = data;
}

33. Convert to an Integer (instead of Math.floor) - possibly a faster alternative

~~((image.width / image.height))

34. JavaScript Templates (also executes quickly - easy to read)

var tmpl = ''.concat(
 '<div>',
   '<span>foo<span>',
 '</div>'
);

35. Quick indexOf in an Array

Reference:

var foo = [1, 2, 3, 4, 5];

if (!!~a.indexOf(5)) { //!! is used to reverse the boolean value to a positive
   console.log("found five");
}

36. Get a property from an Object (utilities)

function splat (fn) {
 return function (list) {
   return Array.prototype.map.call(list, fn)
 }
}

function get (attr) {
 return function (object) { return object[attr]; }
}

var inventories = [
 { apples: 0, oranges: 144, eggs: 36 },
 { apples: 240, oranges: 54, eggs: 12 },
 { apples: 24, oranges: 12, eggs: 42 }
];

var inventory = {
 "apples": 0,
 "oranges": 144,
 "eggs": 36
};

console.log(get('oranges')(inventory));
console.log(splat(get('oranges'))(inventories));

37. Reverse a string

function reverse(s) {
   return s.split("").reverse().join("");
}

38. JavaScript Truncation Operator

3.1|0 == 3

3.9|0 == 3

-10.9|0 == 10

39. Quick charAt in String

"foo"[2]
// returns "o"

40. Get an object by label in JSON

var arr=[
    {"time":"2016-07-26 09:02:27","type":"aa"},
    {"time":"2016-04-21 20:35:07","type":"ae"},
    {"time":"2016-08-20 03:31:57","type":"ar"},
    {"time":"2017-01-19 22:58:06","type":"ae"},
    {"time":"2016-08-28 10:19:27","type":"ae"},
    {"time":"2016-12-06 10:36:22","type":"ar"},
    {"time":"2016-07-09 12:14:03","type":"ar"}
];

var filtered = arr.filter(item=>item.type=="ar");
console.log(filtered);

41 Get first item from an array

// ok (iterate over all and return a new array)
const user = users.filter(u => u.id === id)[0];

// better (find first one and stop)
const user = users.find(u => u.id === id);

42 Nested Destructuring

const response = {
    status: 200,
    data: {
        user: {
            name: 'rozario chivers',
            description: ''
        },
        account: {},
        tag: '#tag'
    }
};

const {data: {user}} = response;

console.log('user ', user);
// {name: "rozario chivers", description: ""}

43 ES6 Remove Duplicates in an Array

const arrayDuplicates = [1,2,2,2,3,3,5,5,1,1];
const uniqueArray = [... new Set(Array)];

// uniqueArray -> [1,2,3,5];

44 Multi-line function template string

// example 1
let myString = (() => {/*
    <div id="someId">
        some content<br />
        <a href="#someRef">someRefTxt</a>
    </div>        
*/}).toString().match(/[^]*\/\*([^]*)\*\/\}$/)[1]

// example 2
let tennysonQuote = (() => {/*
  Theirs not to make reply,
  Theirs not to reason why,
  Theirs but to do and die
*/}).toString().replace(/^[^\/]+\/\*!?/, '').replace(/\*\/[^\/]+$/, '')

45 JavaScript Base64 runtime payload using ES6 template literals

Function`$${atob`YWxlcnQoMSk=`}```