Get loop counter index using for of syntax in JavaScript

Question

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  lt  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

User · Answer

As others have said  you shouldn t be using for  in to iterate over an array   for   var i   0  len   myArray length  i  lt  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

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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 lt 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

User · Answer

for   in iterates over property names  not values  and does so in an unspecified order  yes  even after ES6   You shouldn   t use it to iterate over arrays  For them  there   s ES5   s forEach method that passes both the value and the index to the function you give it   var myArray    123  15  187  32    myArray forEach function  value  i        console log   d   s   i  value           Outputs     0  123    1  15    2  187    3  32   Or ES6   s Array prototype entries  which now has support across current browser versions   for  const  i  value  of myArray entries          console log   d   s   i  value       For iterables in general  where you would use a for   of loop rather than a for   in   there   s nothing built-in  however   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       demo  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

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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

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Answer Given by rushUp Is correct but this will be more convenient    for  let  index  val  of array entries                  your code goes here

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How about this  let numbers    1 2 3 4 5  numbers forEach  number  index    gt  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

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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

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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  lt   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

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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    x000D   x000D  const eachWithIndex    iterable    gt    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    gt    x000D    for  i   2  i  lt  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    x000D   x000D   x000D

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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   x000D   x000D  array1     a    b    c   for  element of array1        console log array1 indexOf element   element     0 a 1 b 2 c   x000D   x000D   x000D

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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  lt 9  you should use Polyfill code  https   developer mozilla org en docs Web JavaScript Reference Global Objects Object keys

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

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