> var foo;
> foo
undefined

> function id(x) { return x }
> id()
undefined

a = 1;
a !== undefined // true

a = undefined
var b
a === b //true

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

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

//也会把 false, -0, +0, NaN 与 '' 当成“空值”
if (v) {
  // v 有值
} else {
  // v 没有值
}

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

"" == "0" // false
0 == "" // true
0 == "0" // true
false == "false" // false
false == "0" // true
false == undefined // false
false == null // false
null == 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 string

var 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是在内部作用域内有效
};
// foo在外部用于是不可见的
typeof foo; // "undefined"
f(); // "function"

from

var name = 'Chris';
var age = '34';
var status = 'single';
function createMember(){
// [...]
}
function getMemberDetails(){
// [...]
}
to

var 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 = {}; //空对象

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 !== ''

// Numbers
typeof 37 === 'number';
typeof 3.14 === 'number';
typeof Infinity === 'number';
typeof NaN === 'number'; // 尽管NaN是"Not-A-Number"的缩写,意思是"不是一个数字"

// Strings
typeof "" === 'string';
typeof "bla" === 'string';
typeof (typeof 1) === 'string'; // typeof返回的肯定是一个字符串

// Booleans
typeof true === 'boolean';
typeof false === 'boolean';

// Undefined
typeof undefined === 'undefined';
typeof blabla === 'undefined'; // 一个未定义的变量,或者一个定义了却未赋初值的变量

// Objects
typeof {a:1} === 'object';
typeof [1, 2, 4] === 'object'; // 使用Array.isArray或者Object.prototype.toString.call方法可以分辨出一个数组和真实的对象
typeof new Date() === 'object';

// Functions
typeof function(){} === 'function';
typeof Math.sin === 'function';

typeof null === 'object'; // 从JavaScript诞生以来,一直是这样的.

expression ? xxx : yyy
bad

var direction;
if(x < 200){
 direction = 1;
} else {
 direction = -1;
}
good

var direction = x < 200 ? 1 : -1;

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

//默认值
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);
}

// 循环数组，输出每项名字（译者注：这样的注释似乎有点多余吧）.
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);  // 输出 "true"

// 判断 foo 是否是 Foo 类的实例
function Foo(){}
var foo = new Foo();
console.log(foo instanceof Foo)//true

// 判断 foo 是否是 Foo 类的实例 , 并且是否是其父类型的实例
function Aoo(){}
function Foo(){}
Foo.prototype = new Aoo();//JavaScript 原型继承

var foo = new Foo();
console.log(foo instanceof Foo)//true
console.log(foo instanceof Aoo)//true

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

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