关于Javascript提升的问题 [英] Questions on Javascript hoisting

问题描述

当我读到有关Javascript提升的内容时，我尝试了以下内容。我不确定为什么第一个和第二个输出不同。提前致谢。 （我甚至不确定这与吊装有关。）

While I was reading about Javascript hoisting, I tried the following. I am not sure why the first one and second one output differently. Thanks in advance. (I am not even sure that this is related to hoisting. )

var me = 1;
function findme(){
   if(me){
    console.log( me );//output 1
    }
  console.log( me ); //output 1
}
findme();

但是以下输出未定义。

However the followings output undefined.

var me = 1;
function findme(){
 if(me){
    var me = 100;
    console.log(me); 
    }
  console.log(me); 
}
findme();
// undefined

推荐答案

变量声明被提升到每个函数的顶部，但值 assignments 保持原样。所以第二个例子运行如下：

Variable declarations get hoisted to the top of every function, but the value assignments stay where they are. So the second example is run like this:

var me = 1;
function findme () {
    var me;  // (typeof me === 'undefined') evaluates to true
    if (me) { // evaluates to false, doesn't get executed
        me = 100;
        console.log(me); 
    }
    console.log(me); 
}
findme();

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