JavaScript中的全局变量如何工作？ [英] How does global variables in JavaScript work?

问题描述

我是一个JavaScript的新手。我通常用Java编程。我很困惑这个下面的代码片段。

 < script> 
 x =foo; 
 
函数栏（p）{
 if（p）{
 document.writeln（x =+ x）; 
} else {
 var x =baz; 
} 
} 
 
 bar（baz）; 
 
< / script> 
  

当我运行上面的代码时，它会缩小它的打印范围

  x = undefined 
  

为什么打印undefined，因为x是一个全局变量，它应该打印foo的权利？任何人都可以解释吗？

解决方案

因为x是一个全局变量，它应该打印foo正确

如果它没有被 var x =baz; 在你的函数中进一步声明;由于提升，它会执行函数，就好像你写了一样

 函数栏（p）{
 var x; // = undefined 
 
 if（p）{
 document.writeln（x =+ x）; 
} else {
 x =baz; 
 
 
 
 
 $ b 
为了让代码做到你想要的，你可以简单地写 x =baz; 而不是 var x =baz; 。
 
I m a newbie to javascript. I usually program in Java. I am confused by this following code snippet.   
<script>
 x = "foo";

function bar(p){
  if (p){
    document.writeln("x = " + x);
  } else {
    var x = "baz";
  }
}

bar("baz");

</script>
When I run the above code snipped its printing
 x = undefined
Why does it print undefined, since x is a global variable it should print foo right ? Can anyone explain ?
解决方案

  
since x is a global variable it should print foo right
It would if it wasn't shadowed by the var x = "baz"; declaration further up in your function; due to hoisting it will execute the function as if you wrote
function bar(p){
  var x; // = undefined

  if (p){
    document.writeln("x = " + x);
  } else {
    x = "baz";
  }
}
To make the code do what you want, you could simply write x = "baz"; instead of var x = "baz";.

                        
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