这个JavaScript代码片段令人困惑 [英] Puzzled by this JavaScript code snippet

问题描述

对于这个片段，我并不感到惊讶全局变量'a'的评估结果是5。

For this snippet, I'm not surprised global variable 'a' evaluates to be 5.

http://jsfiddle.net/MeiJsVa23/gZSxY/ ：

var a = 10;

function func(){
  a = 5;
}

func();   // expect global variable 'a' to be modified to 5;

alert(a); // and this prints out 5 as expected. No surprise here.
​

但是为什么这个代码片段，全局变量'a'评估为10而不是5？好像 a = 5 从未发生过。

But how come for this code snippet, global variable 'a' evaluates to be 10 and not 5? It's as if the a = 5 never happened.

http://jsfiddle.net/MeiJsVa23/2WZ7w/ ：

var a = 10;

function func(){
  a = 5;
  var a = 23;
}

func();   // expect global variable 'a' to be modified to 5;

alert(a); // but this prints out 10!! why?

推荐答案

这是由于变量提升：用 var 关键字定义的任何内容都被提升到当前范围的顶部，创建一个局部变量 A 。请参阅： http://www.adequatelygood.com/2010/2/JavaScript -Scoping-and-Hanging

This is due to variable hoisting: anything defined with the var keyword gets 'hoisted' to the top of the current scope, creating a local variable a. See: http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting

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