js scope and advance statement

Global scope

1. Created when the page is opened, destroyed when closed.
2. In the global scope has a global object window, it can be used directly. Created by the browser, you can use the page.
3. Global variables created in the role, will generate as a property of the window object. var a = 10, available window.a call. Create a function is generated as a method of window.

var a = 10;
console.log(window.a) //等于console.log(a)
function fun() {}

Variable declaration in advance

1. Variables declared using the var keyword, it will be declared before any code is executed (but not assigned).
2. If you do not use the var keyword when declaring a variable, the variable will not be declared in advance.
   Var keyword:

console.log(a)   // undefined
var a = 123;

Equivalent to:

var a   // 声明
console.log(a)   // undefined
a = 123; //赋值

Do not use the var keyword to declare variables:

console.log(a)  // Uncaught ReferenceError: a is not defined
a = 123  

Function declaration in advance

1. Created with the function declarations in the form of a function (fun1) it will then be created before any code is executed, so you can call before the function declaration.
2. Use create function expression of the function is not declared in advance, it can not be declared before the call.

fun1()   //  控制台打印 "我是fun1"
console.log(fun2) // undefined
fun2()   // Uncaught TypeError: undefined is not a function(fun2不是函数)
function fun1() {     // 函数声明，会被提前创建
  console.log('我是fun1')
}
var fun2 = function() {  // 函数表达式，不会提前创建
   console.log('我是fun2')
}

Function scope

1. Create a function scope when the function is called function is completed destroyed.
2. Each call to the function will create a new function scope, independent of each other.
3. In the scope function can access variables global scope.
4. Global scope can not access to the function scope variables.

var a = 10
function fun() {
  var b = 10
  console.log(a)
}
fun() // 10
console.log(b) // Uncaught ReferenceError: b is not defined

5. When the manipulated variable scope function, its role is now looking domain, there are used directly, without the up to find a scope.

var a = 10
function fun() {
   var a = '函数中a'
   console.log(a) // 函数中a
}
fun() // 10

6. In the scope function declaration also advance features variable declared using the var keyword will be (will function declaration) declaration before any code is executed in the function.

var a = 10
function fun3() {
	console.log(a)    // undefined （var关键字变量声明提前）
	var a = 20
}

7. Variables do not use var declared in a function, will become global variables.

var c = 10
function fun5() {
	console.log(a)    // undefined （var关键字变量声明提前）
	c = 20  // 这里改变了全局的c
	d = 100  // d 没有使用var关键字则会设置为全局变量
}
console.log(c) // 20
console.log(d) // 100

8. Parameter corresponds to the function defined scope declared variables.

var e = 23
function fun6(e) {
	console.log(e) // undefined
}
fun6()