Discussion on: Code Smell 53 - Explicit Iteration

1. You want to use of with iterators, in is for string keys of objects (including inherited).
2. for in and for of syntax in js looks like for (const ident of expression) (where const could be let or var and of could be in)
3. there's a missing ) in the last statement
4. most commonly available print fn in js is console.log
5. Array method is called forEach (capitalization)

so, all in all:

const colors = ['red', 'green']
const {log} = console

for (const color of colors)
  log(color)

// closure/arrow function
colors.forEach(color => log(color))

if for some reason you have to use in to iterate over values of an object, it's typically done like this (to avoid iterating over inherited keys)

for (const key in obj)
  if (obj.hasOwnProperty(key))
    do_thing(key, dict[key])

however you should prefer the new iterators, such as Object.keys(), Object.values() and Object.entries().
As such, the above becomes:

for (const [key, val] of Object.entries(obj))
    do_thing(key, val)

Finally, if you see yourself operating on keys of an Object generically like this, you might instead be looking for Map, which naturally iterates by "entries", and supports non-string keys.

const ref = {two: 2}
const m = new Map([[1, "val1"], [ref, "val2"]])
for (const [k,v] of m)
  console.log("key", k, "value", v)

See also Set, WeakMap, WeakSet