why these 'this' keywords are referring to different objects

Question

const person = {
    name: "aman",
    age: 19,
    hello: [2,123,13,1,23],
    showDetails(){
        this.hello.forEach(function(items){
            console.log(this)   // referring to window object
        })
    }
}
person.showDetails()

and other is

const person = {
    name: "aman",
    age: 19,
    hello: [2,123,13,1,23],
    showDetails(){
        this.hello.forEach((items)=>{
            console.log(this)  // referring to person object
        })
    }
}
person.showDetails()

here in both cases the program is logging different output.