of in

const o = { foo: 1, bar: 2 }
const a = [1, 2, 3]

// iterate over keys in object
for (prop of Object.keys(o)) {
  console.log(prop)
}

// iterate over values in object
for (value of Object.values(o)) {
  console.log(value)
}

// iterate over values in array
for (value of a) {
  console.log(a)
}

// check if key is in object
console.log('foo' in o)