> var foo;> fooundefined

> function id(x) { return x }> id()undefineda = 1;a !== undefined // truea = undefinedvar ba === b //true

(1 + null) # 1 (1 * null) # 0 (1 * null) # Infinity

(1 + undefined) # NaN (1 * undefined) # NaN (1 / undefined) # NaN

//也會(huì)把 false, -0, +0, NaN 與 '' 當(dāng)成“空值”if (v) {  // v 有值} else {  // v 沒(méi)有值}

var foo;console.log(foo == null); // trueconsole.log(foo == undefined); // trueconsole.log(foo === null); // falseconsole.log(foo === undefined); // trueconsole.log(null == undefined); // true

"" == "0" // false0 == "" // true0 == "0" // truefalse == "false" // falsefalse == "0" // truefalse == undefined // falsefalse == null // falsenull == undefined // true" /t/r/n" == 0 // true

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 */to stringvar obj = {  name: 'myObj'};JSON.stringify(obj);

var slice_func = [].slice//slice_func()var a = function() {};// a()var a = {  fun : function() {  };}// a.fun()someElement.addEventListener("click", function(e) {  // I'm anonymous!});

var f = function foo(){  return typeof foo; // foo是在內(nèi)部作用域內(nèi)有效};// foo在外部用于是不可見(jiàn)的typeof foo; // "undefined"f(); // "function"

fromvar name = 'Chris';var age = '34';var status = 'single';function createMember(){// [...]}function getMemberDetails(){// [...]}tovar myApplication = function(){var name = 'Chris';var age = '34';var status = 'single';return{createMember:function(){// [...]},getMemberDetails:function(){// [...]}}}();// myApplication.createMember() and// myApplication.getMemberDetails() now works.

var someItem = 'some string';var anotherItem = 'another string';var oneMoreItem = 'one more string';

var someItem = 'some string',  anotherItem = 'another string',  oneMoreItem = 'one more string';

var someItem = 'some string'function doSomething() {return 'something'}

var someItem = 'some string';function doSomething() {return 'something';}

var o = new Object();o.name = 'Jeffrey';o.lastName = 'Way';o.someFunction = function() {console.log(this.name);}

var o = {}; //空對(duì)象

var o = {name: 'Jeffrey',lastName = 'Way',someFunction : function() {  console.log(this.name);}};

var a = new Array();a[0] = "Joe";a[1] = 'Plumber';

var a = ['Joe','Plumber'];

typeof xx === ''typeof xx !== ''

// Numberstypeof 37 === 'number';typeof 3.14 === 'number';typeof Infinity === 'number';typeof NaN === 'number'; // 盡管NaN是"Not-A-Number"的縮寫(xiě),意思是"不是一個(gè)數(shù)字"http:// Stringstypeof "" === 'string';typeof "bla" === 'string';typeof (typeof 1) === 'string'; // typeof返回的肯定是一個(gè)字符串// Booleanstypeof true === 'boolean';typeof false === 'boolean';// Undefinedtypeof undefined === 'undefined';typeof blabla === 'undefined'; // 一個(gè)未定義的變量,或者一個(gè)定義了卻未賦初值的變量// Objectstypeof {a:1} === 'object';typeof [1, 2, 4] === 'object'; // 使用Array.isArray或者Object.prototype.toString.call方法可以分辨出一個(gè)數(shù)組和真實(shí)的對(duì)象typeof new Date() === 'object';// Functionstypeof function(){} === 'function';typeof Math.sin === 'function';typeof null === 'object'; // 從JavaScript誕生以來(lái),一直是這樣的.

expression ? xxx : yyybadvar direction;if(x < 200){ direction = 1;} else { direction = -1;}goodvar direction = x < 200 ? 1 : -1;

var foo = 10;foo == 10 && doSomething(); // 等價(jià)于 if (foo == 10) doSomething();foo == 5 || doSomething(); // 等價(jià)于 if (foo != 5) doSomething();//默認(rèn)值a = b || 'default'return b || c || d > 1 ? 0 : 2

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

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

for(var i = 0; i < someArray.length; i++) {var container = document.getElementById('container');container.innerHtml += 'my number: ' + i;console.log(i);}

var container = document.getElementById('container');for(var i = 0, len = someArray.length; i < len; i++) {container.innerHtml += 'my number: ' + i;console.log(i);}

// 循環(huán)數(shù)組，輸出每項(xiàng)名字（譯者注：這樣的注釋似乎有點(diǎn)多余吧）.for(var i = 0, len = array.length; i < len; i++) {console.log(array[i]);}

var oStringObject = new String("hello world");console.log(oStringObject instanceof String);  // 輸出 "true"http:// 判斷 foo 是否是 Foo 類(lèi)的實(shí)例function Foo(){}var foo = new Foo();console.log(foo instanceof Foo)//true// 判斷 foo 是否是 Foo 類(lèi)的實(shí)例 , 并且是否是其父類(lèi)型的實(shí)例function Aoo(){}function Foo(){}Foo.prototype = new Aoo();//JavaScript 原型繼承var foo = new Foo();console.log(foo instanceof Foo)//trueconsole.log(foo instanceof Aoo)//true

someFn.call(this, arg1, arg2, arg3);someFn.apply(this, [arg1, arg2, arg3]);

Function.call(obj,[param1[,param2[,…[,paramN]]]])