Object.keys() return Object enumerable properties but Looping over Object.keys() return Object methods?

Question

Javascript Object.keys(my_obj) return the keys (ie enumerable property) of my_obj in an array.

But looping in Object.keys(my_obj) array return the methods of my_obj.

Does somebody understand this behaviour ?

Object.prototype.log_key = function() {
  keys = Object.keys(this)
  console.log('log Object.keys() array return enumerable property', keys)
  for (const k in keys) {
    console.log('loop in Object.keys() array return method', k)
  }
}

var my_obj = {
  testkey: 'testvalue'
}

my_obj.log_key();

Output:

log Object.keys() array return enumerable property [ 'testkey' ]
loop in Object.keys() array return methods 0
loop in Object.keys() array return methods log_key

Answer 1

for...in loops give you the index of the array. That's why you're getting 0 when you log k. It's the first and only item in the array.

If you want the actual item you can use a for...of instead.