为什么即使稍后在代码中定义了局部变量，全局变量也无法访问 [英] why is global variable not accessible even if local variable is defined later in code

问题描述

为什么以下代码段会生成以下输出?

why does the following code segment generate the following output?

代码段:

var a = 10;
function(){
    console.log(a);
    var a = 5;
}

输出:

undefined

推荐答案

由于变量位于顶部，因此在函数中已声明变量var a = 5，该变量与以下变量相同:

Because variable is hoisted at top and in your function you have declared the variable var a = 5 which is same as following:

var a = 10;
function(){
    var a; // a = undefined
    console.log(a);//a is not defined so outputs undefined
    a = 5;
    console.log(a);//a is now 5 so outputs 5
}

并且在您的函数范围中声明了var，它看不到全局变量，而是局部变量，即var a且未定义.

And in your function scope var is being declared it doesn't see global variable but local variable i.e. var a and which is undefined.

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