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 结果为 [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 结果为 [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(); // is the same thing as if (foo == 10) doSomething(); 
foo == 5 || doSomething(); // is the same thing as if (foo != 5) doSomething();

function doSomething(arg1){ 
 arg1 = arg1 || 10; // arg1 will have 10 as a default value if it's not already set
}

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

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(undefined); // false
isFinite(); // false
isFinite(null); // true，这点当特别注意

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 结果为 "{"name":"Saad","age":26,"department":{"ID":15,"name":"R&D"}}" */
var personFromString = JSON.parse(stringFromPerson);
/* personFromString 的值与 person 对象相同 */

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]; 
}

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); // 将返回 "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); 
 } 
}