let obj = {};
let arr = [];

console.log(typeof obj === 'object'); //true
console.log(typeof arr === 'object'); //true
console.log(typeof null === 'object'); //true

let obj = {};
let arr = [];

console.log(Object.prototype.toString.call(obj)); //[object Object]
console.log(Object.prototype.toString.call(arr)); //[object Array]
console.log(Object.prototype.toString.call(null)); //[object Null]

(function(){
 var a = b = 3;
})();

console.log("a defined? " + (typeof a !== 'undefined')); 
console.log("b defined? " + (typeof b !== 'undefined'));

console.log(b); //3
console,log(typeof a); //undefined

var myObject = {
 foo: "bar",
 func: function() {
 var self = this;
 console.log("outer func: this.foo = " + this.foo);
 console.log("outer func: self.foo = " + self.foo);
 (function() {
 console.log("inner func: this.foo = " + this.foo);
 console.log("inner func: self.foo = " + self.foo);
 }());
 }
};
myObject.func();

(function(test) {
 console.log("inner func: this.foo = " + test.foo); //'bar'
 console.log("inner func: self.foo = " + self.foo);
}(self));

for(var i = 0; i < 5; i++) {
 setTimeout(function() {
 console.log(i); 
 }, 1000);
}

for(var i = 0; i < 5; i++) {
 (function(i) {
 setTimeout(function() {
 console.log(i); 
 }, 1000);
 })(i)
}

(function($) { 
 //代码
 } )(jQuery);

function foo1()
{
 return {
 bar: "hello"
 };
}

function foo2()
{
 return
 {
 bar: "hello"
 };
}

var test = 1 + 
2
console.log(test); //3

function foo2()
{
 return;
 {
 bar: "hello"
 };
}

console.log(isNaN(NaN)); //true
console.log(isNaN(23)); //false
console.log(isNaN('ds')); //true
console.log(isNaN('32131sdasd')); //true
console.log(NaN === NaN); //false
console.log(NaN === undefined); //false
console.log(undefined === undefined); //false
console.log(typeof NaN); //number
console.log(Object.prototype.toString.call(NaN)); //[object Number]

console.log(0.1 + 0.2); //0.30000000000000004
console.log(0.1 + 0.2 == 0.3); //false

function add(num1, num2){
 let r1, r2, m;
 r1 = (''+num1).split('.')[1].length;
 r2 = (''+num2).split('.')[1].length;

 m = Math.pow(10,Math.max(r1,r2));
 return (num1 * m + num2 * m) / m;
}
console.log(add(0.1,0.2)); //0.3
console.log(add(0.15,0.2256)); //0.3756

function add(x, y) {
 return x.toPrecision() + y.toPrecision()
}
console.log(add(0.1,0.2)); //"0.10.2"

function isInteger(x) { 
 return parseInt(x, 10) === x; 
}

Number.isInteger(25) // true
Number.isInteger(25.0) // true
Number.isInteger(25.1) // false
Number.isInteger("15") // false
Number.isInteger(true) // false

(function() {
 console.log(1); 
 setTimeout(function(){console.log(2)}, 1000); 
 setTimeout(function(){console.log(3)}, 0); 
 console.log(4);
})();

function isPalindrome(str) {
 str = str.replace(/\W/g, '').toLowerCase();
 return (str == str.split('').reverse().join(''));
}

console.log(sum(2,3)); // Outputs 5
console.log(sum(2)(3)); // Outputs 5

function sum() {
 var fir = arguments[0];
 if(arguments.length === 2) {
 return arguments[0] + arguments[1]
 } else {
 return function(sec) {
 return fir + sec;
 }
 }
 
}

for (var i = 0; i < 5; i++) {
 var btn = document.createElement('button');
 btn.appendChild(document.createTextNode('Button ' + i));
 btn.addEventListener('click', function(){ console.log(i); });
 document.body.appendChild(btn);
}

var arr1 = "john".split(''); j o h n
var arr2 = arr1.reverse(); n h o j
var arr3 = "jones".split(''); j o n e s
arr2.push(arr3);
console.log("array 1: length=" + arr1.length + " last=" + arr1.slice(-1));
console.log("array 2: length=" + arr2.length + " last=" + arr2.slice(-1));

console.log(1 + "2" + "2");
console.log(1 + +"2" + "2");
console.log(1 + -"1" + "2");
console.log(+"1" + "1" + "2");
console.log( "A" - "B" + "2");
console.log( "A" - "B" + 2);

console.log(2 + 1 + '3'); / /‘33'
console.log('3' + 2 + 1); //'321'

console.log(typeof '3'); // string
console.log(typeof +'3'); //number

console.log(typeof 3); // number
console.log(typeof (''+3)); //string

console.log('a' * 'sd'); //NaN
console.log('A' - 'B'); // NaN

var list = readHugeList();

var nextListItem = function() {
 var item = list.pop();

 if (item) {
 // process the list item...
 nextListItem();
 }
};

var list = readHugeList();

var nextListItem = function() {
 var item = list.pop();

 if (item) {
 // process the list item...
 setTimeout( nextListItem, 0);
 }
};

for (var i = 0; i < 5; i++) {
 setTimeout(function() { console.log(i); }, i * 1000 );
}

console.log("0 || 1 = "+(0 || 1));
console.log("1 || 2 = "+(1 || 2));
console.log("0 && 1 = "+(0 && 1));
console.log("1 && 2 = "+(1 && 2));

console.log(1 && 2 && 0); //0
console.log(1 && 0 && 1); //0
console.log(1 && 2 && 3); //3

console.log(1 || 2 || 0); //1
console.log(0 || 2 || 1); //2
console.log(0 || 0 || false); //false

console.log(1 && 2 || 0); //2
console.log(0 || 2 && 1); //1
console.log(0 && 2 || 1); //1

console.log([] == false) //true
console.log({} == false) //false
console.log(Boolean([])) //true
console.log(Boolean({})) //true

console.log(false == '0')
console.log(false === '0')

var a={},
 b={key:'b'},
 c={key:'c'};

a[b]=123;
a[c]=456;

console.log(a[b]);

console.log((function f(n){return ((n > 1) ? n * f(n-1) : n)})(10));

(function(x) {
 return (function(y) {
 console.log(x);
 })(2)
})(1);

var hero = {
 _name: 'John Doe',
 getSecretIdentity: function (){
 return this._name;
 }
};

var stoleSecretIdentity = hero.getSecretIdentity;

console.log(stoleSecretIdentity());
console.log(hero.getSecretIdentity());

var stoleSecretIdentity = function (){
 return this._name;
}
stoleSecretIdentity 

function Traverse(p_element,p_callback) {
 p_callback(p_element);
 var list = p_element.children;
 for (var i = 0; i < list.length; i++) {
 Traverse(list[i],p_callback); // recursive call
 }
}