for…in iterates over property names, not values (and did so in an unspecified order up until ES2020*). You shouldn’t use it to iterate over arrays. For them, there’s ES6’s Array.prototype.entries, which now has support across current browser versions:

const myArray = [123, 15, 187, 32];

for (const [i, value] of myArray.entries()) {
  console.log(`{value}`);
}

// 0: 123
// 1: 15
// 2: 187
// 3: 32

.as-console-wrapper { max-height: 100% !important; top: 0; border-top: 0 !important; }

Or, for extended compatibility with older browsers, there’s ES5’s forEach method that passes both the value and the index to the function you give it:

myArray.forEach(function (value, i) {
  console.log('%d: %s', i, value);
});

For iterables in general (where you would use a for…of loop rather than a for…in), iterator helpers are now in the language. You can use Iterator.prototype.forEach to iterate over an entire iterable with an index:

function* fibonacci() {
  let a = 0;
  let b = 1;

  for (;;) {
    yield a;
    [a, b] = [b, a + b];
  }
}

fibonacci().take(10).forEach((x, i) => {
  console.log(`F_{x}`);
});

.as-console-wrapper { max-height: 100% !important; top: 0; border-top: 0 !important; }

More generally, Iterator#map can associate the values yielded by an iterator with their indexes:

fibonacci().map((x, i) => [i, x])

Not every iterable (or iterator!) is an Iterator, but you can convert every iterable to an Iterator with Iterator.from.

Without support for iterator helpers, you can use a generator function instead:

function* enumerate(iterable) {
  let i = 0;

  for (const x of iterable) {
    yield [i, x];
    i++;
  }
}

for (const [i, obj] of enumerate(myArray)) {
  console.log(i, obj);
}

If you actually did mean for…in – enumerating properties – you would need an additional counter. Object.keys(obj).forEach could work, but it only includes own properties; for…in includes enumerable properties anywhere on the prototype chain.

* The order is still unspecified under certain circumstances, including for typed arrays, proxies, and other exotic objects, as well as when properties are added or removed during iteration.

Use Array.prototype.keys:

for (const index of ["a", "b", "c", "d", "e"].keys()) {
  console.log(index);
}

If you want to access both the key and the value, you can use Array.prototype.entries() with destructuring:

for (const [index, value] of ["a", "b", "c", "d", "e"].entries()) {
  console.log(index, value);
}

This is just meant to be an addition to jonas w's solutions.

If you need the key of the current value:

Copyconst object = {a:2, b:4, c:6, d:8};

for (const [index, [key, value]] of Object.entries(Object.entries(object))) {
  console.log(`${index}: ${key} = ${value}`);
}

Object.entries(object).forEach(([key, value], index) => {
  console.log(`${index}: ${key} = ${value}`);
});

Of course, you can leave out the key at any time:

Copyconst object = {a:2, b:4, c:6, d:8};

for (const [index, [, value]] of Object.entries(Object.entries(object))) {
  console.log(`${index}: ${value}`);
}

Object.entries(object).forEach(([, value], index) => {
  console.log(`${index}: ${value}`);
});