javascript 作用域

     在javascript里面没有块级作用域，都是通过函数来限制的，变量在函数内部以var开头声明为函数的局部变量，如果没有关键字var 则变量为全局变量，全局变量作为window对象的属性存在的，在函数内部的局部变量只能在函数内部访问，函数内部可以访问其上一级的变量，如果内部和上一级的变量/函数重名，则内部变量/函数覆盖外部变量/函数

示例代码如下

var name = "外部的变量";
function showName () {
	var name = "内部的变量"; // 局部变量
	console.log (name); // Jack
}
console.log (name); //结果：外部的变量

 没有块级作用域

var name = "Richard";
if (name) {
	name = "Jack"; // this name is the global name variable and it is being changed to "Jack" here
	console.log (name); // Jack: still the global variable
}
// Here, the name variable is the same global name variable, but it was changed in the if statement
console.log (name); // Jack

 切记一定要在使用变量之前声明变量，局部变量优先于全局变量

var name = "Paul";
function users () {
	// Here, the name variable is local and it takes precedence over the same name variable in the global scope
var name = "Jack";

// The search for name starts right here inside the function before it attempts to look outside the function in the global scope
console.log (name); 
}
users (); // Jack

在setTimeout 执行的函数、单独函数，闭包、中this指向window。即使用this获取的变量为全局变量

 // The use of the "this" object inside the setTimeout function refers to the Window object, not to myObj
var highValue = 200;
var constantVal = 2;
var myObj = {
    highValue: 20,
    constantVal: 5,
    calculateIt: function () {
 setTimeout (function  () {
    console.log(this.constantVal * this.highValue);
}, 2000);
    }
}
// 这里面的this指向的全局对象，并不是我们期待的myObj
// because the reference to "this" in the setTimeout function refers to the global window object, not to the myObj object as we might expect.
myObj.calculateIt(); // 400

为了避免污染全局作用域，尽量少使用全局变量。

所有变量如果再函数内部声明会被提升的函数的顶部，如果再函数外部声明会被提升到当前上下文的顶部，只要在变量声明的时候才会提升，而不是在初始化的时候。

如果变量声明与函数名字相同（不分声明先后），函数声明优先于变量声明（不是变量赋值）。如果变量声明的时候并且进行了初始化，则函数被覆盖

function showName () {
console.log ("First Name: " + name);
var name = "Ford";
console.log ("Last Name: " + name);
}
showName (); 
// First Name: undefined
// Last Name: Ford
// The reason undefined prints first is because the local variable name was hoisted to the top of the function
// Which means it is this local variable that get calls the first time.
// This is how the code is actually processed by the JavaScript engine:
function showName () {
	var name; // name is hoisted (note that is undefined at this point, since the assignment happens below)
console.log ("First Name: " + name); // First Name: undefined
name = "Ford"; // name is assigned a value
// now name is Ford
console.log ("Last Name: " + name); // Last Name: Ford
}

 // 只声明外部变量为myName，同时定义名字同样为myName的函数
var myName;

function myName () {
console.log ("Rich");
}
// 函数声明覆盖变量声明
console.log(typeof myName); //function

 // 变量的声明初始化会覆盖后面的函数 这个时候函数定义无效
var myName = "Richard"; // This is the variable assignment (initialization) that overrides the function declaration.
function myName () {
console.log ("Rich");
}
console.log(typeof myName); // string 

请注意如下方式，如果函数为表达式方式，则函数会覆盖变量

var myName = "Richard"; 

var myName = function () {
console.log ("Rich");
} 
console.log(typeof myName);//function