var foo = (12.4 / 4.13) | 0;//结果为3
var bar = ~~(12.4 / 4.13);//结果为3

var eee="eee";
alert(!!eee)

(function() {
 var oldAlert = window.alert,
 count = 0;
 window.alert = function(a) {
 count++;
 oldAlert(a + "\n You've called alert " + count + " times now. Stop, it's evil!");
 };
})();
alert("Hello Haorooms");

var a=1,b=2;a=[b,b=a][0];
console.log('a:'+a+',b:'+b);

 //输出a:2,b:1

var day=(new Date).getDay()===0;
//传统if语句
if (day) {
 alert('Today is Sunday!');
};
//运用逻辑与代替if
day&&alert('Today is Sunday!');

if(conditoin) alert(1),alert(2),console.log(3);

[10] === 10 // is false
[10] == 10 // is true
'10' == 10 // is true
'10' === 10 // is false
 [] == 0 // is true
 [] === 0 // is false
 '' == false // is true but true == "a" is false
 '' === false // is false

function Person(name, age) {
 this.getName = function() { return name; };
 this.setName = function(newName) { name = newName; };
 this.getAge = function() { return age; };
 this.setAge = function(newAge) { age = newAge; };

 //未在构造函数中初始化的属性
 var occupation;
 this.getOccupation = function() { return occupation; };
 this.setOccupation = function(newOcc) { occupation = 
  newOcc; };
}

function Person(firstName, lastName){
 this.firstName = firstName;
 this.lastName = lastName;
}

var Saad = new Person("Saad", "Mousliki");

var arr = ["a", "b", "c"];
typeof arr; // return "object"
arr instanceof Array // true
arr.constructor(); //[]

(function(){
 // some private code that will be executed automatically
})();
(function(a,b){
 var result = a+b;
 return result;
})(10,20)

var items = [12, 548 , 'a' , 2 , 5478 , 'foo' , 8852, , 'Doe' , 2145 , 119];

var randomItem = items[Math.floor(Math.random() * items.length)];

var x = Math.floor(Math.random() * (max - min + 1)) + min;

var numbersArray = [] , max = 100;

for( var i=1; numbersArray.push(i++) < max;); // numbers = [0,1,2,3 ... 100]

function generateRandomAlphaNum(len) {
 var rdmstring = "";
 for( ; rdmString.length < len; rdmString += Math.random().toString(36).substr(2));
 return rdmString.substr(0, len);
}

//调用方法generateRandomAlphaNum(15);

var numbers = [5, 458 , 120 , -215 , 228 , 400 , 122205, -85411];
numbers = numbers.sort(function(){ return Math.random() - 0.5});
/* the array numbers will be equal for example to [120, 5, 228, -215, 400, 458, -85411, 122205] */

String.prototype.trim = function(){return this.replace(/^\s+|\s+$/g, "");};

var array1 = [12 , "foo" , {name: "Joe"} , -2458];

var array2 = ["Doe" , 555 , 100];
Array.prototype.push.apply(array1, array2);
/* array1 will be equal to [12 , "foo" , {name "Joe"} , -2458 , "Doe" , 555 , 100] */

var argArray = Array.prototype.slice.call(arguments);
【译者注：arguments对象是一个类数组对象，但不是一个真正的数组】

function isNumber(n){
 return !isNaN(parseFloat(n)) && isFinite(n);
}

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

Array.isArray(obj); // 这是一个新的array的方法

var myFrame = document.createElement('iframe');
document.body.appendChild(myFrame);

var myArray = window.frames[window.frames.length-1].Array;
var arr = new myArray(a,b,10); // [a,b,10]

// instanceof will not work correctly, myArray loses his constructor
// constructor is not shared between frames
arr instanceof Array; // false

var myArray = [12 , 222 , 1000 ];
myArray.length = 0; // myArray will be equal to [].

var items = [12, 548 ,'a' , 2 , 5478 , 'foo' , 8852, , 'Doe' ,2154 , 119 ];
items.length; // return 11
delete items[3]; // return true
items.length; // return 11
/* items will be equal to [12, 548, "a", undefined × 1, 5478, "foo", 8852, undefined × 1, "Doe", 2154, 119] */

var items = [12, 548 ,'a' , 2 , 5478 , 'foo' , 8852, , 'Doe' ,2154 , 119 ];
items.length; // return 11
items.splice(3,1) ;
items.length; // return 10
/* items will be equal to [12, 548, "a", 5478, "foo", 8852, undefined × 1, "Doe", 2154, 119] */

var myArray = [12 , 222 , 1000 , 124 , 98 , 10 ];
myArray.length = 4; // myArray will be equal to [12 , 222 , 1000 , 124].

myArray.length = 10; // the new array length is 10
myArray[myArray.length - 1] ; // undefined

var foo = 10;
foo == 10 && doSomething(); // 等价于 if (foo == 10) doSomething();
foo == 5 || doSomething(); // 等价于 if (foo != 5) doSomething();

function doSomething(arg1){
 Arg1 = arg1 || 10; // 如果arg1没有被设置的话，Arg1将被默认设成10
}

var squares = [1,2,3,4].map(function (val) {
 return val * val;
});
// squares will be equal to [1, 4, 9, 16]

var num =2.443242342;
num = num.toFixed(4); // num will be equal to 2.4432

0.1 + 0.2 === 0.3 // is false
9007199254740992 + 1 // is equal to 9007199254740992
9007199254740992 + 2 // is equal to 9007199254740994

for (var name in object) {
 if (object.hasOwnProperty(name)) {
 // do something with name
 }
}

var a = 0;
var b = ( a++, 99 );
console.log(a); // a will be equal to 1
console.log(b); // b is equal to 99

var navright = document.querySelector('#right');
var navleft = document.querySelector('#left');
var navup = document.querySelector('#up');
var navdown = document.querySelector('#down');

isFinite(0/0) ; // false
isFinite("foo"); // false
isFinite("10"); // true
isFinite(10); // true
isFinite(undifined); // false
isFinite(); // false
isFinite(null); // true !!!

var numbersArray = [1,2,3,4,5];
var from = numbersArray.indexOf("foo") ; // from is equal to -1
numbersArray.splice(from,2); // will return [5]

var person = {name :'Saad', age : 26, department : {ID : 15, name : "R&D"} };
var stringFromPerson = JSON.stringify(person);
/* stringFromPerson is equal to "{"name":"Saad","age":26,"department":{"ID":15,"name":"R&D"}}" */
var personFromString = JSON.parse(stringFromPerson);
/* personFromString is equal to person object */

var func1 = new Function(functionCode);
var func2 = eval(functionCode);

var sum = 0;
for (var i in arrayNumbers) {
 sum += arrayNumbers[i];
}

var sum = 0;
for (var i = 0, len = arrayNumbers.length; i < len; i++) {
 sum += arrayNumbers[i];
}

for (var i = 0; i < arrayNumbers.length; i++)

setInterval('doSomethingPeriodically()', 1000);
setTimeOut('doSomethingAfterFiveSeconds()', 5000)

setInterval(doSomethingPeriodically, 1000);
setTimeOut(doSomethingAfterFiveSeconds, 5000);

function getCategory(age) {
 var category = "";
 switch (true) {
 case isNaN(age):
 category = "not an age";
 break;
 case (age >= 50):
 category = "Old";
 break;
 case (age <= 20):
 category = "Baby";
 break;
 default:
 category = "Young";
 break;
 };
 return category;
}
getCategory(5); // will return "Baby"

function clone(object) {
 function OneShotConstructor(){};
 OneShotConstructor.prototype= object;
 return new OneShotConstructor();
}
clone(Array).prototype ; // []

function escapeHTML(text) {
 var replacements= {"<": "<", ">": ">","&": "&", "\"": """};
 return text.replace(/[<>&"]/g, function(character) {
 return replacements[character];
 });
}

var object = ['foo', 'bar'], i;
for (i = 0, len = object.length; i <len; i++) {
 try {
 // do something that throws an exception
 }
 catch (e) {
 // handle exception
 }
}

var object = ['foo', 'bar'], i;
try {
 for (i = 0, len = object.length; i <len; i++) {
 // do something that throws an exception
 }
}
catch (e) {
 // handle exception
}

var xhr = new XMLHttpRequest ();
xhr.onreadystatechange = function () {
 if (this.readyState == 4) {
 clearTimeout(timeout);
 // do something with response data
 }
}
var timeout = setTimeout( function () {
 xhr.abort(); // call error callback
}, 60*1000 /* timeout after a minute */ );
xhr.open('GET', url, true); 

xhr.send();

var timerID = 0;
function keepAlive() {
 var timeout = 15000;
 if (webSocket.readyState == webSocket.OPEN) {
 webSocket.send('');
 }
 timerId = setTimeout(keepAlive, timeout);
}
function cancelKeepAlive() {
 if (timerId) {
 cancelTimeout(timerId);
 }
}

var min = Math.min(a,b);
A.push(v);

var min = a < b ? a b;
A[A.length] = v;

var num=Math.floor(Math.random()*9+2)

1. Math.ceil()用作向上取整。
2. Math.floor()用作向下取整。
3. Math.round() 我们数学中常用到的四舍五入取整。

console.log(0.6|0)//0
console.log(1.1|0)//1
console.log(3.65555|0)//3
console.log(5.99999|0)//5
console.log(-7.777|0)//-7

100.456001.toFixed(2); //100.47

100.456001.toFixed(3); //100.456

Number.prototype.toFixed.call(100.456001,2); //100.47

99.456001.toPrecision(5); //99.456

100.456001.toPrecision(5); //100.46

Number.prototype.toPrecision.call(10.456001,5); //10.456

console.log(3|4); //7
console.log(4|4);//4
console.log(8|3);//11
console.log(5.3|4.1);//5
console.log(9|3455);//3455

3|4
转换为二进制之后011|100 相加得到111=7

4|4
转换为二进制之后100 |100 相加得到100=4

8|3
转换为二进制之后1000 |011 相加得到1011=11