箭头函数返回未定义

Question

我想了解为什么这段代码返回一个值，而第二个代码返回undefined;

function personName(){
  console.log(this.name);
}

const obj = 
{
    name: "David",
    greetings: function() {
      return `Hello ${this.name}`
    }
}

console.log( obj.greetings() );

function personName(){
  console.log(this.name);
}

const obj = {
  name: "David",
  greetings: () => `Hello ${this.name}`
}

console.log( obj.greetings() );