[javascript] Get loop counter/index using for…of syntax in JavaScript

Caution:

question still applies to for…of loops.> Don't use for…in to iterate over an Array, use it to iterate over the properties of an object. That said, this

I understand that the basic for…in syntax in JavaScript looks like this:

for (var obj in myArray) {
    // ...
}

But how do I get the loop counter/index?

I know I could probably do something like:

var i = 0;
for (var obj in myArray) {
    alert(i)
    i++
}

Or even the good old:

for (var i = 0; i < myArray.length; i++) {
    var obj = myArray[i]
    alert(i)
}

But I would rather use the simpler for-in loop. I think they look better and make more sense.

Is there a simpler or more elegant way?

In Python it's easy:

for i, obj in enumerate(myArray):
    print i

Here's a function eachWithIndex that works with anything iterable.

You could also write a similar function eachWithKey that works with objets using for...in.

// example generator (returns an iterator that can only be iterated once)
function* eachFromTo(start, end) { for (let i = start; i <= end; i++) yield i }

// convers an iterable to an array (potential infinite loop)
function eachToArray(iterable) {
    const result = []
    for (const val of iterable) result.push(val)
    return result
}

// yields every value and index of an iterable (array, generator, ...)
function* eachWithIndex(iterable) {
    const shared = new Array(2)
    shared[1] = 0
    for (shared[0] of iterable) {
        yield shared
        shared[1]++
    }
}

console.log('iterate values and indexes from a generator')
for (const [val, i] of eachWithIndex(eachFromTo(10, 13))) console.log(val, i)

console.log('create an array')
const anArray = eachToArray(eachFromTo(10, 13))
console.log(anArray)

console.log('iterate values and indexes from an array')
for (const [val, i] of eachWithIndex(anArray)) console.log(val, i)

The good thing with generators is that they are lazy and can take another generator's result as an argument.

How about this

let numbers = [1,2,3,4,5]
numbers.forEach((number, index) => console.log(`${index}:${number}`))

Where array.forEach this method has an index parameter which is the index of the current element being processed in the array.

That's my version of a composite iterator that yields an index and any passed generator function's value with an example of (slow) prime search:

const eachWithIndex = (iterable) => {_x000D_
  return {_x000D_
    *[Symbol.iterator]() {_x000D_
      let i = 0_x000D_
      for(let val of iteratable) {_x000D_
        i++_x000D_
          yield [i, val]_x000D_
      }_x000D_
    }_x000D_
  }_x000D_
_x000D_
}_x000D_
_x000D_
const isPrime = (n) => {_x000D_
  for (i = 2; i < Math.floor(Math.sqrt(n) + 1); i++) {_x000D_
    if (n % i == 0) {_x000D_
      return false_x000D_
    }_x000D_
  }_x000D_
  return true_x000D_
}_x000D_
_x000D_
let primes = {_x000D_
  *[Symbol.iterator]() {_x000D_
    let candidate = 2_x000D_
    while (true) {_x000D_
      if (isPrime(candidate)) yield candidate_x000D_
        candidate++_x000D_
    }_x000D_
  }_x000D_
}_x000D_
_x000D_
for (const [i, prime] of eachWithIndex(primes)) {_x000D_
  console.log(i, prime)_x000D_
  if (i === 100) break_x000D_
}

In ES6, it is good to use for - of loop. You can get index in for of like this

for (let [index, val] of array.entries()) {
        // your code goes here    
}

Note that Array.entries() returns an iterator, which is what allows it to work in the for-of loop; don't confuse this with Object.entries(), which returns an array of key-value pairs.

For-in-loops iterate over properties of an Object. Don't use them for Arrays, even if they sometimes work.

Object properties then have no index, they are all equal and not required to be run through in a determined order. If you want to count properties, you will have to set up the extra counter (as you did in your first example).

loop over an Array:

var a = [];
for (var i=0; i<a.length; i++) {
    i // is the index
    a[i] // is the item
}

loop over an Object:

var o = {};
for (var prop in o) {
    prop // is the property name
    o[prop] // is the property value - the item
}

Solution for small array collections:

for (var obj in arr) {
    var i = Object.keys(arr).indexOf(obj);
}

arr - ARRAY, obj - KEY of current element, i - COUNTER/INDEX

Notice: Method keys() is not available for IE version <9, you should use Polyfill code. https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/keys

To use for..of loop on array and retrieve index you can you use array1.indexOf(element) which will return the index value of an element in the loop. You can return both the index and the value using this method.

array1 = ['a', 'b', 'c']
for (element of array1) {
    console.log(array1.indexOf(element), element) // 0 a 1 b 2 c
}

As others have said, you shouldn't be using for..in to iterate over an array.

for ( var i = 0, len = myArray.length; i < len; i++ ) { ... }

If you want cleaner syntax, you could use forEach:

myArray.forEach( function ( val, i ) { ... } );

If you want to use this method, make sure that you include the ES5 shim to add support for older browsers.

Answer Given by rushUp Is correct but this will be more convenient

for (let [index, val] of array.entries() || []) {
   // your code goes here    
}

On top of the very good answers everyone posted I want to add that the most performant solution is the ES6 entries. It seems contraintuitive for many devs here, so I created this perf benchamrk.

It's ~6 times faster. Mainly because doesn't need to: a) access the array more than once and, b) cast the index.