How can I pull the just the keys out of object.values();?

I’ve been stuck on a teaching exercise for the better part of 2 days, and I decided I needed some help. I have to write a function called keys that returns an array of all the keys in the object without using object.keys();. I was able to turn everything into a string simply enough, and print everything using object.values(); but now I want to take what I obtained using object.values, and return just the key(As right now it returns key and value I.E – A:1, B:2, C:3 I want just ABC).

So far, this is a code I was trying. Please be kind, I am doing my best!

function keys(json) {
    var obj = JSON.parse(json);

*//My code starts here*
    const keys = Object.values(obj);
    keys.toString();
    for (const [key] of Object.Entries(obj)) {
        return(key);
    }
*//My code ends here*

}

There is “inputs” that test my code, they are below and represented in obj

first input:

{"a":"1","b":"2","c":"3"}

second input:

{"first":"Matt","last":"Lane"}

third input:

{}