javascript - 为什么 undefined var 没有添加到 window 对象 JavaScript 中？

Question

据我所知，以下声明不会为变量aa添加任何值:

var aa = undefined;

function a () {
    var aa;
    console.log(aa);  // here aa is still undefined
    if(!aa) {
        aa = 11;  // should add to the globle scope (window Object)
        bb = 12;  // should add to the globle scope (window Object)
    }
    console.log(aa);
    console.log(aa);  // should be 11
    console.log(bb);  // should be 12
}

现在，如果我想访问变量 aa 和 bb，我只能访问 bb 而不是 aa.我的问题是为什么 aa 无法从外部访问，因为在声明中我没有为它分配任何值并且它仍然是未定义的？

谢谢。

Answer 1

看我的评论

var aa = undefined; // global scope

function a () {
    if(true) { // useless
        var aa; // declare aa in the function scope and assign undefined
        // to work on the global aa you would remove the above line
        console.log(aa);  // here aa is still undefined
        if(!aa) {
            aa = 11;  // reassign the local aa to 11
            bb = 12;  // assign 12 to the global var bb
        }
        console.log(aa); // aa is 11
    }
    console.log(aa);  // still in the function scope so it output 11
    console.log(bb);  // should be 12
}
console.log(aa) // undefined nothing has change for the global aa

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