类中嵌套函数之间的对象上下文（“this”）

希望你们都做得很好。

我有一个关于对象的上下文如何在 Node.JS（或一般的 JS）函数上工作的问题。我理解“当您在 Javascript 中调用顶级函数时，函数内的 this 关键字指的是默认对象”。例如，我有以下功能：

function nestedFunctions() {
    var a = "I am a variable"
    console.log("Initial context:")
    console.log(this)
 
    function one() {
        console.log("First nested function, this is my this context:")
        console.log(this)
        console.log("First nested function, this is my a value:")
        console.log(a)
 
        function two() {
            console.log("Second nested function, this is my this context:")
            console.log(this)
            console.log("Second nested function, this is my a value:")
            console.log(a)
        }
        two()
    }
    one()
}
nestedFunctions()

给出以下输出：

Initial context:
Object Global
First nested function, this is my this context:
Object Global
First nested function, this is my a value:
I am a variable
Second nested function, this is my this context:
Object Global
Second nested function, this is my a value:
I am a variable

到目前为止，一切正常，函数获得了全局上下文。当我尝试蒙山时：

var rex = {
    nestedFunctions() {
        var a = "I am a variable"
        console.log("Initial context:")
        console.log(this)
 
 
        function one() {
            console.log("First nested function, this is my this context:")
            console.log(this)
            console.log("First nested function, this is my a value:")
            console.log(a)
 
            function two() {
                console.log("Second nested function, this is my this context:")
                console.log(this)
                console.log("Second nested function, this is my a value:")
                console.log(a)
            }
            two()
        }
        one()
    }
}
 
rex.nestedFunctions()

输出是：

Initial context:
{ nestedFunctions: [Function: nestedFunctions] }
First nested function, this is my this context:
Object Global
First nested function, this is my a value:
I am a variable
Second nested function, this is my this context:
Object Global
Second nested function, this is my a value:
I am a variable

到目前为止一切都很好。父函数得到“this” = {nestedFunctions: [Function:nestedFunctions]} 因为是对象上下文，嵌套函数得到全局上下文因为它们没有绑定到父上下文。

但我的疑问是在下一个案例中：

class Dog {
    constructor(name) {
        this.name = name
    }
 
    bark() {
        console.log(`Woof, my name is ${this.name}`);
    }
    nestedFunctions() {
        var a = "I am a variable"
        console.log("Initial context:")
        console.log(this)
 
 
        function one() {
            console.log("First nested function, this is my this context:")
            console.log(this)
            console.log("First nested function, this is my a value:")
            console.log(a)
 
            function two() {
                console.log("Second nested function, this is my this context:")
                console.log(this)
                console.log("Second nested function, this is my a value:")
                console.log(a)
            }
            two()
        }
        one()
    }
}
 
let rex = new Dog('Rex')
rex.nestedFunctions()

输出在哪里：

Initial context:
Dog { name: 'Rex' }
First nested function, this is my this context:
undefined
First nested function, this is my a value:
I am a variable
Second nested function, this is my this context:
undefined
Second nested function, this is my a value:
I am a variable

为什么嵌套函数上的“this”上下文在这种情况下是“undefined”而不是全局上下文，就像前面的例子一样？为什么在处理类时默认上下文是未定义的（而不是“全局”）？