1)Boolean(value)

(Invoked as a function, not as a constructor)

2)value ? true : false

3)!!value
https://stackoverflow.com/questions/10467475/double-negation-in-javascript-what-is-the-purpose
var x = "somevalue"
var isNotEmpty = !!x.length;
Let's break it to pieces:

x.length   // 9
!x.length  // false
!!x.length // true
So it's used convert a "truethy" \"falsy" value to a boolean.

var a = "0";
var b = [];
var c = {};

var d = "";
var e = 0;
var f = null;
var g;

!!a;    // true
!!b;    // true
!!c;    // true

!!d;    // false
!!e;    // false
!!f;    // false
!!g;    // false

 alert( parseInt('100px') ); // 100
alert( parseFloat('12.5em') ); // 12.5

alert( Number("   123   ") ); // 123
alert( Number("123z") );      // NaN (error reading a number at "z")
alert( Number(true) );        // 1
alert( Number(false) );       // 0

> NaN === NaN
false

> isNaN(NaN)
true
> isNaN(33)
false

> isNaN('xyz')
true

function myIsNaN(value) {
    return typeof value === 'number' && isNaN(value);
}

function myIsNaN(value) {
    return value !== value;
}

Number('')    // 0
Number('   ') // 0
Number(true)  // 1
Number(false) // 0
Number([0])   // 0

var a = null;

(!a && typeof a === "object"); // true

When you’re trying to identify null, use triple equals so that you can correctly identify the type.
ex : console.log(undefined == null); // true
console.log(undefined === null); // false

function C() { }
C.prototype = null;
But instantiating this constructor still results in instances of Object:
var o = new C();
Object.getPrototypeOf(o) === null; // false
Object.getPrototypeOf(o) === Object.prototype; // true

> NaN && 'abc'
NaN
> 123 && 'abc'
'abc'
Or (||)
If the first operand is truthy, return it. Otherwise, return the second operand:

> 'abc' || 123
'abc'
> '' || 123
123

var a = 42;
var b = "abc";
var c = null;

a || b;     // 42
a && b;     // "abc"

c || b;     // "abc"
c && b;     // null

a || b;
// roughly equivalent to:
a ? a : b;

a && b;
// roughly equivalent to:
a ? b : a;

Number.isInteger( 42 );     // true
Number.isInteger( 42.000 ); // true
Number.isInteger( 42.3 );   // false

this behaviour can be use in :  
To polyfill Number.isInteger(..) for pre-ES6:

if (!Number.isInteger) {
    Number.isInteger = function(num) {
        return typeof num == "number" && num % 1 == 0;
    };
}

if (x == 123) ...

if (Number(x) === 123)

> 'abc' == new String('abc')
true

> new String('abc') == new String('abc')
false
Second, you should avoid wrappers anyway. 
Third, if you do use them, it is better to be explicit:

if (wrapped.valueOf() === 'abc') 

var a = "42";

// bad (will fail!):
if (a == true) {
    // ..
}

// also bad (will fail!):
if (a === true) {
    // ..
}

// good enough (works implicitly):
if (a) {
    // ..
}

// better (works explicitly):
if (!!a) {
    // ..
}

// also great (works explicitly):
if (Boolean( a )) {
    // ..
}

x < y

> typeof true
'boolean'
> typeof 'abc'
'string'
> typeof {} // empty object literal
'object'
> typeof [] // empty array literal
'object'

typeof undefined
'undefined'

typeof null
'object'

var a = null;

(!a && typeof a === "object"); // true
null is the only primitive value that is "falsy" 
but that also returns "object" from the typeof check.

function isArrayLike( obj ) {

    // Support: real iOS 8.2 only (not reproducible in simulator)
    // `in` check used to prevent JIT error (gh-2145)
    // hasOwn isn't used here due to false negatives
    // regarding Nodelist length in IE
    var length = !!obj && "length" in obj && obj.length,
        type = toType( obj );

    if ( isFunction( obj ) || isWindow( obj ) ) {
        return false;
    }

    return type === "array" || length === 0 ||
        typeof length === "number" && length > 0 && ( length - 1 ) in obj;
}

> var b = new Bar();  // object created by constructor Bar
> b instanceof Bar
true

> {} instanceof Object
true
> [] instanceof Array
true
> [] instanceof Object  // Array is a subconstructor of Object
true

> undefined instanceof Object
false
> null instanceof Object
false

function Rabbit() {}
let rabbit = new Rabbit();

// changed the prototype
Rabbit.prototype = {};
rabbit instanceof Rabbit //false

var iframe = document.createElement('iframe');
document.body.appendChild(iframe);
xArray = window.frames[window.frames.length-1].Array;
var arr = new xArray(1,2,3); // [1,2,3]

// Boom!
arr instanceof Array; // false

// Boom!
arr.constructor === Array; // false

let objectToString = Object.prototype.toString;

// what type is this?
let arr = [];

alert( objectToString.call(arr) ); // [object Array]

Object.prototype.toString.call( [1,2,3] );
Object.prototype.toString.call( null );         // "[object Null]"
Object.prototype.toString.call( undefined );    // "[object Undefined]"

function f() {
    if (condition) {
        var tmp = ...;
        ...
    }
    // tmp still exists here
    // => not what we want
}

function f() {
    if (condition) {
        (function () {  // open block
            var tmp = ...;
            ...
        }());  // close block
    }
}

var x = 23;
(function (twice) {
    console.log(twice);
}(x * 2));
This is similar to:

var x = 23;
(function () {
    var twice = x * 2;
    console.log(twice);
}());

var result = [];
for (var i=0; i < 5; i++) {
    result.push(function () { return i });  // (1)
}
console.log(result[1]()); // 5 (not 1)
console.log(result[3]()); // 5 (not 3)
The value returned in line (1) is always the current value of i, not the value it had when the function was created. After the loop is finished, i has the value 5, which is why all functions in the array return that value. If you want the function in line (1) to receive a snapshot of the current value of i, you can use an IIFE:

for (var i=0; i < 5; i++) {
    (function () {
        var i2 = i; // copy current i
        result.push(function () { return i2 });
    }());
}

var myAlerts = [];

for (var i = 0; i < 5; i++) {
    myAlerts.push(
        function inner() {
            alert(i);
        }
    );
}

myAlerts[0](); // 5
myAlerts[1](); // 5
myAlerts[2](); // 5
myAlerts[3](); // 5
myAlerts[4](); // 5


Answer :

var myAlerts = [];

for (var i = 0; i < 5; i++) {
    myAlerts.push(
       (function iife(){
         var j = i ;
         return function inner() {
            console.log(j);
          }
       })()

    );
}
myAlerts[1]()

function f(x, y) {
    console.log(x, y);
    return toArray(arguments);
}
Additional parameters will be ignored (except by arguments):

> f('a', 'b', 'c')
a b
[ 'a', 'b', 'c' ]

function pair(x, y) {
    x = x || 0;  // (1)
    y = y || 0;
    return [ x, y ];
}
In line (1), the || operator returns x if it is truthy (not null, undefined, etc.). Otherwise, it returns the second operand:

> pair()
[ 0, 0 ]
> pair(3)
[ 3, 0 ]
> pair(3, 5)
[ 3, 5 ]

function pair(x, y) {
    if (arguments.length !== 2) {
        throw new Error('Need exactly 2 arguments');
    }
    ...
}

var jane = {
    name: 'Jane',
    friends: [ 'Tarzan', 'Cheeta' ],
    logHiToFriends: function () {
        'use strict';
        this.friends.forEach(function (friend) {
            // `this` is undefined here
            console.log(this.name+' says hi to '+friend);
        });
    }
}
Calling logHiToFriends produces an error: why ?

> jane.logHiToFriends()
TypeError: Cannot read property 'name' of undefined

Ans :
 jane.logHiToFriends() runs like 

 var callback = function (friend) {
            // `this` is undefined here
            console.log(this.name+' says hi to '+friend);
        }

 var jane = {
    name: 'Jane',
    friends: [ 'Tarzan', 'Cheeta' ],
    logHiToFriends: function () {
        'use strict';
        this.friends.forEach(callback);
    }
}

as callback funs like callback() i.e without any context , this points to global object i.e window

logHiToFriends: function () {
    'use strict';
    var that = this;
    this.friends.forEach(function (friend) {
        console.log(that.name+' says hi to '+friend);
    });
}
Or, forEach has a second parameter that allows you to provide a value for this:

logHiToFriends: function () {
    'use strict';
    this.friends.forEach(function (friend) {
        console.log(this.name+' says hi to '+friend);
    }, this);
}

scoreCalculator.getScore.call(player1)

inArray: function( elem, arr, i ) {
        return arr == null ? -1 : indexOf.call( arr, elem, i );
    }

var elems = {
   length: 0,
   add: function(elem){
       Array.prototype.push.call(this, elem);
   },
   gather: function(id){
       this.add(document.getElementById(id));
   }
 };

 elems.gather("first");

 console.log(elems.length == 1 && elems[0].nodeType,
 "Verify that we have an element in our stash");

 elems.gather("second");

 console.log(elems.length == 2 && elems[1].nodeType,
 "Verify the other insertion"); 

 function smallest(array){
     return Math.min.apply(Math, array);
 }
 function largest(array){
     return Math.max.apply(Math, array);
 }

 assert(smallest([0, 1, 2, 3]) == 0,
 "Located the smallest value.");

 assert(largest([0, 1, 2, 3]) == 3,
 "Located the largest value."); 

     new (Date.bind(null, 2011, 11, 24))

     or 
     new (Date.bind.call(Date,null, 2011, 11, 24))

new (Function.prototype.bind.apply(
         Date, [null, 2011, 11, 24]))

var arr = [2011, 11, 24];
new (Function.prototype.bind.apply(
         Date, [null].concat(arr)))

'use strict';
var jane = {
    name: 'Jane',

    describe: function () {
        return 'Person named '+this.name;
    }
};

> var func = jane.describe;
> func()
TypeError: Cannot read property 'name' of undefined
The solution is to use the method bind() that all functions have. It creates a new function whose this always has the given value:

> var func2 = jane.describe.bind(jane);
> func2()
'Person named Jane'

var animals = [
  { species: 'Lion', name: 'King' },
  { species: 'Whale', name: 'Fail' }
];

for (var i = 0; i < animals.length; i++) {
  (function(i) {
    this.print = function() {
      console.log('#' + i + ' ' + this.species
                  + ': ' + this.name);
    }
    this.print();
    this.print = null ;
    delete this.print ;
  }).call(animals[i], i);
}

      function Foo() {  }

      Foo.prototype = { 
         c :2

       }; // create a new prototype object

      var a1 = new Foo();

      Foo.prototype.isPrototypeOf( a ); // true

function CSVReader(separators) {
    this.separators = separators || [","];
    this.regexp =
    new RegExp(this.separators.map(function(sep) {
      return "\\" + sep[0];
       }).join("|"));
}


CSVReader.prototype.read = function(str) {
    var lines = str.trim().split(/\n/);
    return lines.map(function(line) {
        console.log(this)
       return line.split(this.regexp); // wrong this!
     });
};
var reader = new CSVReader();
reader.read("a,b,c\nd,e,f\n");

function CSVReader(separators) {
    this.separators = separators || [","];
    this.regexp =
    new RegExp(this.separators.map(function(sep) {
      return "\\" + sep[0];
       }).join("|"));
}


CSVReader.prototype.read = function(str) {
    var lines = str.trim().split(/\n/);
    return lines.map(function(line) {
        console.log(this)
       return line.split(this.regexp); // wrong this!
     },this);
};
var reader = new CSVReader();
reader.read("a,b,c\nd,e,f\n");

      var StartStopCounter = {
          counter : 0,
          start : function(){
                StartStopCounter.stopCounter = setInterval(function(){
                    console.log(StartStopCounter.counter);
                    //que : why only counter won't work ?
                    StartStopCounter.counter=StartStopCounter.counter + 1 ;
                },1000);

          },
          stop : function(){
              clearInterval(StartStopCounter.stopCounter);

          }
      }

      function Ninja() {                                            //#1

        var feints = 0;                                             //#2

        this.getFeints = function(){                                //#3
          return feints;                                            //#3
        };                                                          //#3

        this.feint = function(){                                    //#4
          feints++;                                                 //#4
        };                                                          //#4
      }

      var ninja = new Ninja();                                      //#5

      ninja.feint();                                                //#6

      assert(ninja.feints === undefined,                            //#7
          "And the private data is inaccessible to us." );          //#7

      assert(ninja.getFeints() == 1,                                //#8
             "We're able to access the internal feint count." );    //#8

function animateIt(elementId) {

    var elem = document.getElementById(elementId);              //#2
    var tick = 0;                                               //#3

    var timer = setInterval(function(){                         //#4
      if (tick < 100) {
        elem.style.left = elem.style.top = tick + "px";
        tick++;
      }
      else {
        clearInterval(timer);
        assert(tick == 100,                                      //#5
               "Tick accessed via a closure.");
        assert(elem,
               "Element also accessed via a closure.");
        assert(timer,
               "Timer reference also obtained via a closure." );
      }
    }, 10);

  }

  animateIt('box')

 var ninjas = {
    values: ["Dean Edwards", "Sam Stephenson", "Alex Russell"]
 };

function addMethod(object, name, fn) {
 var old = object[name];
 object[name] = function(){
   if (fn.length == arguments.length)
      return fn.apply(this, arguments)
   else if (typeof old == 'function')
      return old.apply(this, arguments);
  };
}

 addMethod(ninjas, "find", function(){
          return this.values;
 });

 addMethod(ninjas, "find", function(name){
      var ret = [];
      for (var i = 0; i < this.values.length; i++)
      if (this.values[i].indexOf(name) == 0)
      ret.push(this.values[i]);
      return ret;
 });

 addMethod(ninjas, "find", function(first, last){
      var ret = [];
      for (var i = 0; i < this.values.length; i++)
      if (this.values[i] == (first + " " + last))
      ret.push(this.values[i]);
      return ret;
 });

 assert(ninjas.find().length == 3,
 "Found all ninjas");
 assert(ninjas.find("Sam").length == 1,
 "Found ninja by first name");
 assert(ninjas.find("Dean", "Edwards").length == 1,
 "Found ninja by first and last name");
 assert(ninjas.find("Alex", "Russell", "Jr") == 
 ,
 "Found nothing");

(function(){
 var numClicks = 0;
 document.addEventListener("click", function(){
 alert( ++numClicks );
 }, false);
})();

/*
Because the immediate function is executed immediately (hence its name), the click
handler is also bound right away. The important thing to note is that a closure is created
for the handler that includes numClicks, allowing the numClicks variable to persist
along with the handler, and be referenceable by the handler but nowhere else.
 This is one of the most common ways in which immediate functions are used: as
simple, self-contained wrappers for functionality. The variables needed for the unit of
functionality are trapped in the closure, but they aren’t visible anywhere else.
*/

// 2. copy has all of orig’s properties
copyOwnPropertiesFrom(copy, orig);

return copy;

      'use strict';

      const anotherObject = {};

      Object.defineProperty(anotherObject, 'a', {
        value: 2,
        writable: false
      });

      var myObject = Object.create( anotherObject );

      myObject.a = 3;

function SuperType(name){
 this.name = name;
 this.colors = ["red","green"];
}

SuperType.prototype.sayName = function(){
 console.log(this.name);
};

function SubType(name, age){

 //inherit properties
 SuperType.call(this, name);

 this.age = age;
}

//inherit methods
SubType.prototype = new SuperType();  // problem as this creates color property on prototype 

SubType.prototype.sayAge = function(){
 console.log(this.age);
};

var instance1 = new SubType("Nicholas", 29);


problems with this :

console.log(instance1) > color property on prototype 

SubType {name: "Nicholas", colors: Array(2), age: 29}
    age: 29
    colors: (2) ["red", "green"]
    name: "Nicholas"
    __proto__: SuperType
        colors: (2) ["red", "green"]
        name: undefined
        sayAge: ƒ ()
        __proto__: Object

var dict = {};
"alice" in dict; // false
"bob" in dict; // false
"chris" in dict; // false
"toString" in dict; // true
"valueOf" in dict; // true
Luckily, Object.prototype provides the hasOwnProperty method, which
is just the tool we need to avoid prototype pollution when testing for
dictionary entries:
dict.hasOwnProperty("alice"); // false
dict.hasOwnProperty("toString"); // false
dict.hasOwnProperty("valueOf"); // false

(12) ["constructor", "__defineGetter__", "__defineSetter__", "hasOwnProperty", "__lookupGetter__", "__lookupSetter__", "isPrototypeOf", "propertyIsEnumerable", "toString", "valueOf", "__proto__", "toLocaleString"]

function Rabbit() {}

/* default prototype
Rabbit.prototype = { constructor: Rabbit };
*/

function Rabbit() {}
// by default:
// Rabbit.prototype = { constructor: Rabbit }

alert( Rabbit.prototype.constructor == Rabbit ); // true

function Rabbit() {}
Rabbit.prototype = {
  jumps: true
};

let rabbit = new Rabbit();
alert(rabbit.constructor === Rabbit); // false

function Rabbit() {}

Rabbit.prototype.jumps = true

Rabbit.prototype = {
  jumps: true,
  constructor: Rabbit
};

    function Person(){

    }

    var friend = new Person();

    Person.prototype = {
     name : "Nicholas",
     age : 29,
     job : "Software Engineer",
     sayName : function () {
        console.log(this.name);
       }
    };


Constructor is Object Constructor
(new Person()).constructor
ƒ Object() { [native code] }

array.splice(start[, deleteCount[, item1[, item2[, ...]]]]) //deletecount is no of element to delete

1)
let arr = ["I", "study", "JavaScript"];

arr.splice(1, 1); // from index 1 remove 1 element

alert( arr ); // ["I", "JavaScript"]

2)
let arr = ["I", "study", "JavaScript", "right", "now"];

// remove 3 first elements and replace them with another
arr.splice(0, 3, "Let's", "dance");

alert( arr ) // now ["Let's", "dance", "right", "now"]

3)
let arr = ["I", "study", "JavaScript"];

// from index 2
// delete 0
// then insert "complex" and "language"
arr.splice(2, 0, "complex", "language");

alert( arr ); // "I", "study", "complex", "language", "JavaScript"

var arrayLike = { 0: "a", 1: "b", 2: "c", length: 3 };
var result = Array.prototype.map.call(arrayLike, function(s) {
return s.toUpperCase();
}); // ["A", "B", "C"]


function toArray(arrayLikeObject) {
    return Array.prototype.slice.call(arrayLikeObject);
}

var result = Array.prototype.map.call("abc", function(s) {
return s.toUpperCase();
}); // ["A", "B", "C"]

function namesColumn() {
  return ["Names"].concat(arguments);
}
namesColumn("Alice", "Bob", "Chris");
// ["Names", { 0: "Alice", 1: "Bob", 2: "Chris" }]

function namesColumn() {
   return ["Names"].concat([].slice.call(arguments));
}
namesColumn("Alice", "Bob", "Chris");
// ["Names", "Alice", "Bob", "Chris"]

    var tr = Array.from(document.querySelectorAll('tr'));
    for(var i =0 ;i<tr.length ;i++){
       for(var j =0 ;j<tr.length ;j++){
         if(i ==j){
           console.log("i ",i ,j);
           console.log((Array.from(tr[i].querySelectorAll('td'))[i]).style.background ='red');
           console.log('.......')
         }      
       }
    }

    // your code

var el = document.getElementById('tree');
console.log(el);
el.addEventListener("click", handler);

function handler(e){
  if(e.target.children.length == 1){
    e.target.children[0].hidden = !e.target.children[0].hidden;
  }
}

<!DOCTYPE html>
<html>
<body>

<h1>Show checkboxes:</h1>

<form action="/action_page.php">
  <input type="checkbox" name="vehicle1" value="Bike"> I have a bike<br>
  <input type="checkbox" name="vehicle2" value="Car"> I have a car<br>
  <input type="checkbox" name="vehicle3" value="Boat" checked> I have a boat<br><br>
  <input type="submit" value="Submit">
</form>

<script>
var fe = document.querySelector('form');

fe.addEventListener("mousedown", function(e){
  console.log(e);
  e.preventDefault()
});

fe.addEventListener("mouseup", function(e){
e.preventDefault()
  console.log(e)
});

fe.addEventListener("click", function(e){
e.preventDefault()
  console.log(e)
});


</script>
</body>
</html>