为什么将外部函数中定义的对象重新分配给外部声明的相同变量名称会给出错误？

Why typeof of b is undefined in the below code?

为什么在下面的代码中未定义b的typeof?

var b = function() {}

var a = function() {
var b = b
console.log('typeof function_b:', typeof b) 
}
a()

2 个解决方案

#1

Because you're initialising a new variable inside the a function scope with the declaration var b.

因为您使用声明var b在函数作用域内初始化一个新变量。

var b gets initialised and ran before the value gets assigned (b = b), so it is assigning the just-initialised empty value to itself.

var b在赋值(b = b)之前初始化并运行,因此它将刚刚初始化的空值赋给自身。

To alter the output, you can just skip the var declaration and typeof b outputs "function":

要改变输出,您可以跳过var声明和typeof b输出“function”:

var b = function() {}

var a = function() {
    b = b;
    console.log('typeof function_b:', typeof b); // Outputs "function"
}
a();

#2

That's because the variable first get declared (var b), and then assigned b = b. After declaring it, it is undefined in that scope, so you're assigning undefined to it.

那是因为变量首先被声明(var b),然后被赋值b = b。在声明它之后,它在该范围内是未定义的,因此您要为其分配undefined。

#1