Array.prototype.keys returns an iterator for all keys in the array
keys() returns an iterator
const arr = ['a', 'b'];
const iterator = arr.keys();
assert.deepEqual(iterator.next(), {value: 0, done: false});
assert.deepEqual(iterator.next(), {value: void 0, done: true});
gets all keys
const arr = ['a', 'b'];
const keys = Array.from(arr.keys());
assert.deepEqual(keys, [0, 1, 2]);
empty array contains no keys
const arr = ['empty me'];
const keys = [...arr.keys()];
assert.equal(keys.length, 0);
a sparse array without real values has keys though
const arr = [,,];
const keys = [...arr.___()];
assert.deepEqual(keys, [0, 1]);
also includes holes in sparse arrays
const arr = ['a', , 'c'];
const keys = arr.keys;
assert.deepEqual(keys, [0, 1, 2]);