How to find index of all occurrences of element in array

Question

I am trying to find the index of all the instances of an element  say   Nano   in a JavaScript array   var Cars     Nano    Volvo    BMW    Nano    VW    Nano      I tried jQuery inArray  or similarly   indexOf    but it only gave the index of the last instance of the element  i e  5 in this case   How do I get it for all instances

User · Answer

You can use Polyfill  if   Array prototype filterIndex    Array prototype filterIndex   function  func  thisArg          use strict       if     typeof func      Function     typeof func      function    amp  amp  this           throw new TypeError         let len   this length  gt  gt  gt  0          res   new Array len      preallocate array         t   this  c   0  i   -1       let kValue      if  thisArg     undefined            while    i     len                   checks to see if the key was set             if  i in this                    kValue   t i      in case t is changed in callback                 if  func t i   i  t                         res c      i                                                      else           while    i     len                   checks to see if the key was set             if  i in this                    kValue   t i                   if  func call thisArg  t i   i  t                         res c      i                                                       res length   c     shrink down array to proper size     return res          Use it like this    2 23 1 2 3 4 52 2  filterIndex element   gt  element     2   result   0  3  7

User · Answer

This worked for me   let array1    5  12  8  130  44  12  45  12  56   let numToFind   12 let indexesOf12         the number whose occurrence in the array we want to find  array1 forEach function elem  index  array        if  elem     numToFind   indexesOf12 push index       return indexesOf12     console log indexesOf12     outputs  1  5  7

User · Answer

Just to share another method  you can use Function Generators to achieve the result as well    x000D   x000D  function findAllIndexOf target  needle    x000D    return    concat     function     x000D      for  var i   0  i  lt  target length  i    if  target i      needle  yield  i   x000D           x000D    x000D   x000D  var target    hellooooo   x000D  var target2     w   o  1 3  l   o    x000D   x000D  console log findAllIndexOf target   o     x000D  console log findAllIndexOf target2   o     x000D   x000D   x000D

User · Answer

a    b    a    b       map  val  index    gt     val  index         filter   val  index     gt  val      a       map   val  index     gt  index     gt   0  2

User · Answer

Another alternative solution is to use Array prototype reduce       Nano   Volvo   BMW   Nano   VW   Nano   reduce function a  e  i        if  e      Nano           a push i       return a                0  3  5    N B   Check the browser compatibility for reduce method and use polyfill if required

User · Answer

You can write a simple readable solution to this by using both map and filter   const nanoIndexes   Cars    map  car  i    gt  car      Nano    i   -1     filter index   gt  index     -1       EDIT  If you don t need to support IE Edge  or are transpiling your code   ES2019 gave us flatMap  which lets you do this in a simple one-liner   const nanoIndexes   Cars flatMap  car  i    gt  car      Nano    i

User · Answer

findIndex retrieves only the first index which matches callback output  You can implement your own findIndexes by extending Array   then casting your arrays to the new structure     x000D   x000D  class EnhancedArray extends Array   x000D    findIndexes where    x000D      return this reduce  a  e  i    gt   where e  i    a concat i    a        x000D      x000D    x000D       ----Working with simple data structure  array of numbers  ---   x000D   x000D    existing array x000D  let myArray    1  3  5  5  4  5   x000D   x000D    cast it   x000D  myArray   new EnhancedArray    myArray   x000D   x000D    run x000D  console log  x000D     myArray findIndexes  e    gt  e   5  x000D    x000D    ----Working with Array of complex items structure-   x000D   x000D  let arr     name   Ahmed     name   Rami     name   Abdennour     x000D   x000D  arr  new EnhancedArray    arr   x000D   x000D   x000D  console log  x000D    arr findIndexes  o    gt  o name startsWith  A    x000D    x000D   x000D   x000D

User · Answer

More simple way with es6 style   const indexOfAll    arr  val    gt  arr reduce  acc  el  i    gt   el     val       acc  i    acc            Examples  var cars     Nano    Volvo    BMW    Nano    VW    Nano    indexOfAll cars   Nano       0  3  5  indexOfAll  1  2  3  1  2  3   1       0 3  indexOfAll  1  2  3   4

User · Answer

We can use Stack and push  i  into the stack every time we encounter the condition  arr i   value   Check this   static void getindex int arr    int value        Stack lt Integer gt st  new Stack lt Integer gt         int n  arr length      for int i n-1  i gt  0  i--                if arr i   value                        st push i                          while  st isEmpty                  System out println st peek                 st pop

User · Answer

The  indexOf   method has an optional second parameter that specifies the index to start searching from  so you can call it in a loop to find all instances of a particular value  function getAllIndexes arr  val        var indexes       i   -1      while   i   arr indexOf val  i 1      -1           indexes push i             return indexes     var indexes   getAllIndexes Cars   quot Nano quot     You don t really make it clear how you want to use the indexes  so my function returns them as an array  or returns an empty array if the value isn t found   but you could do something else with the individual index values inside the loop  UPDATE  As per VisioN s comment  a simple for loop would get the same job done more efficiently  and it is easier to understand and therefore easier to maintain  function getAllIndexes arr  val        var indexes       i      for i   0  i  lt  arr length  i            if  arr i      val              indexes push i       return indexes

User · Answer

Another approach using Array prototype map   and Array prototype filter     var indices   array map  e  i    gt  e     value   i       filter String

User · Answer

const indexes   cars      map  car  i    gt  car      Nano    i   null       filter i   gt  i     null

User · Answer

I just want to update with another easy method   You can also use forEach method   var Cars     Nano    Volvo    BMW    Nano    VW    Nano     var result        Cars forEach  car  index    gt  car      Nano    result push index    null

User · Answer

Note  MDN gives a method using a while loop   var indices       var array     a    b    a    c    a    d    var element    a   var idx   array indexOf element   while  idx    -1      indices push idx     idx   array indexOf element  idx   1       I wouldn t say it s any better than other answers  Just interesting

[javascript] How to find index of all occurrences of element in array?

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