为什么变量定义的全局是未定义的？ [英] Why a variable defined global is undefined?

问题描述

大家好，我有一个简单的函数和一个全局变量。

为什么 myname undefined 而不是字符串global？

  var myname =global; //全局变量
函数func（）{
 alert（myname）; //undefined
 var myname =local; 
 alert（myname）; //本地
} 
 func（）; 
  

不可能引用在该函数范围外定义的外部变量吗？并在这个全局变量...

我怎样才能解决这个问题，所以我没有得到 undefined 从一个全局变量？

解决方案

你刚刚偶然发现了一个叫做hoisting的js功能

  var myname =global; //全局变量
函数func（）{
 alert（myname）; //undefined
 var myname =local; 
 alert（myname）; //本地
} 
 func（）; 
  

在定义 func 时，编译器查看函数体。它看到您正在声明一个名为 myname 的变量。

Javascript 通过将声明移动到函数的顶部来提升 变量和函数声明。

因为吊起你的代码被重写为以下内容。

  var myname =global; //全局变量
函数func（）{
 var myname; //声明局部变量并将其赋值undefined 
 alert（myname）; //undefined
 myname =local; //将本地var myname分配给本地
 alert（myname）; //本地
} 
 func（）; 
  

这个覆盖全局变量。如果你想访问函数范围内的全局变量，可以使用 this 关键字。

  var myname =global; //全局变量
函数func（）{
 var myname =local; 
 alert（this.myname）; //全局
 alert（myname）; //本地
} 
 func（）; 
  

请注意，这只适用于调用函数而不是方法或构造函数，因为

编辑：为了完整性

如果您想在任何上下文中访问全局变量而不考虑函数类型，则声明一个通常不会涵盖的全局变量。

  var global = this; //在全球范围内。 
 var myname =global; 
 var obj = {f：function（）{
 var myname =local; 
 console.log（global.myname）; 
}}; 
 obj.f（）; //global
  

请注意，这是在方法位置， this 关键字直接指向obj，因此没有定义myname。

Hi guys here I have a simple function and a global variable.

Why is myname undefined and not the string "global"?

var myname = "global"; // global variable
function func() {
    alert(myname); // "undefined"
    var myname = "local";
    alert(myname); // "local"
}
func();

Is not possible to refer to a outer variable that is define outside the scope of that function? and in this a global variable...

And how I can fix this so I don't get a undefined from a global variable?

解决方案

You have just stumbled on a js "feature" called hoisting

var myname = "global"; // global variable
function func() {
    alert(myname); // "undefined"
    var myname = "local";
    alert(myname); // "local"
}
func();

In this code when you define func the compiler looks at the function body. It sees that you are declaring a variable called myname.

Javascript Hoists variable and function declarations, by moving the declaration to the top of the function.

Because of hoisting your code is rewritten to the following.

var myname = "global"; // global variable
function func() {
   var myname; //declare local variable and assign it undefined
   alert(myname); // "undefined"
   myname = "local"; // assign local var myname to "local"
   alert(myname); // "local"
}
func();

This "Covers" the global variable. If you want access to the global variable within the scope of a function use the this keyword.

var myname = "global"; // global variable
function func() {
    var myname = "local";
    alert(this.myname); // "global"
    alert(myname); // "local"
}
func();

Note that this only works in calling a function not a method or constructor because the this keyword changes what its bound to based on how you call a function.

EDIT: For completeness

If you want to get access to global variables in any context regardless of function type then declare a global variable that by convention you never cover.

var global = this; // in global scope.
var myname = "global";
var obj = {f: function () {
    var myname = "local";
    console.log(global.myname);
}};
obj.f(); // "global"

Note that this is in method position and the this keyword refers to obj directly and therefore doesn't have myname defined.

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