Do you use Object.keys often?

Can you explain what's going on in this code what would be the key, index.

At first I was still thing they was object not arrays because that's what they looked like. I know the key was username1/2/3/ and the value is ben/kevin/deku but they don't have index and removing index from the parameters changed nothing, but arrays have index but not keys. from the cosole.log to get the key we console.log key, and to get the value its obj[key] but why?

I tried console.log(Object.keys(obj)) but that mad it more confusing because now the names(values) where gone and it was just an array of the username(keys)

let obj ={
    username1: "ben",
    username2: "kevin",
    username3: "deku"
}
Object.keys(obj).forEach((key, index) =>{
    console.log(key, obj[key]);
})
// username1 ben
// username2 kevin
// username3 deku



let obj ={
    username1: "ben",
    username2: "kevin",
    username3: "deku"
}
Object.keys(obj).forEach((key) =>{
    console.log(key, obj[key]);
})
// username1 ben
// username2 kevin
// username3 deku



let obj ={
    username1: "ben",
    username2: "kevin",
    username3: "deku"
}
Object.keys(obj).forEach((key, index) =>{
    console.log(obj[key]);
})
// ben
// kevin
// deku



let obj ={
    username1: "ben",
    username2: "kevin",
    username3: "deku"
}
Object.keys(obj).forEach((key) =>{
    console.log(key);
})
// username1
// username2
// username3