How to get array keys in Javascript

Question

I have an array created with this code   var widthRange   new Array    widthRange 46      min 0   max 52    widthRange 66      min 52  max 70    widthRange 90      min 70  max 94      I want to get each of the values 46  66  90 in a loop  I tried for  var key in widthRange  but this gives me a whole bunch of extra properties  I assume they are functions on the object   I can t use a regular for loop since the values are not sequential

User · Answer

Alternatively  if you have a list of items you want to use     var range    46  66  90        widthRange          write          widthRange 46      min 0  max 52         widthRange 66      min 52  max 70         widthRange 90      min 70  max 94      for var x 0  x lt range length  x    var key  wr       key   range x        wr   widthRange key     false       if wr   false  continue        write push   key     key     min     wr min   max    wr max  join

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I wrote a function what works fine with every instance of Objects  Arrays are those    Object prototype toArray   function         if  this              return null             var c            for  var key in this                 if     this instanceof Array  amp  amp  this constructor     Array  amp  amp  key      length        this hasOwnProperty key                           continue                     c push this key               return c       Usage   var a       1  2  3    a 11      4  a  js     5   console log a toArray      var b     one  1  two  2  three  3  f  function     return 4     five  5    b 7    7   console log b toArray       Output    gt    1  2  3  4  5    gt    7  1  2  3  function      return 4     5     It may be useful for anyone

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for  var i   0  i  lt  widthRange length    i      if  widthRange i     null           do something         You can t really get just the keys you ve set because that s not how an Array works  Once you set element 46  you also have 0 through 45 set too  though they re null    You could always have two arrays   var widthRange       widths       newVal   function n      widths push n     return n     widthRange newVal 26       whatever   hello there      for  var i   0  i  lt  widths length    i      doSomething widthRange widths i         edit well it may be that I m all wet here

User · Answer

The question is pretty old  but nowadays you can use forEach  which is efficient  and will retain the keys as numbers  let keys   widthRange map  v k    gt  k  filter i  gt i   undefined    This loops through widthRange and makes a new array with the value of the keys  and then filters out all sparce slots by only taking the values that are defined   Bad idea  but for thorughness  If slot 0 was always empty  that could be shortened to filter i  gt i  or filter Boolean  And  it may be less efficient  but the numbers can be cast with let keys   Object keys array  map i  gt i 1

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Seems to work   var widthRange   new Array    widthRange 46      sel 46  min 0   max 52    widthRange 66      sel 66  min 52  max 70    widthRange 90      sel 90  min 70  max 94     for  var key in widthRange        document write widthRange key  sel     lt br   gt         document write widthRange key  min     lt br   gt         document write widthRange key  max     lt br   gt

User · Answer

If you are doing any kind of array collection manipulation or inspection I highly recommend using Underscore js  It s small  well-tested and will save you days weeks years of javascript headache  Here is its keys function   Keys  Retrieve all the names of the object s properties     keys  one   1  two   2  three   3      gt    one    two    three

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The stringified keys can be queried with Object keys array

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Say your array looked like arr       a  1  b  2  c  3      a  4  b  5  c  6      a  7  b  8  c  9      or possibly other keys  you could do  arr map  o    gt        return Object keys o     reduce  prev  curr    gt        return prev concat curr     filter  col  i  array    gt        return array indexOf col      i         a    b    c

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I think you should use an Object      and not an array      for this   A set of data is associated with each key  It screams for using an object  Do   var obj       obj 46      sel 46  min 0   max 52    obj 666      whatever true        This is what for  in is for for  var prop in obj      console log obj prop        Maybe some utility stuff like this can help   window WidthRange    function        var obj         return       getObj  function     return obj         add  function  key  data            obj key    data          return this     enabling chaining                       Usage  using chaining calls   WidthRange add 66   foo  true    add 67   bar  false    add 69   baz   maybe   bork  absolutely      var obj   WidthRange getObj    for  var prop in obj      console log obj prop

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Your original example works just fine for me    lt html gt   lt head gt   lt  head gt   lt body gt   lt script gt  var widthRange   new Array    widthRange 46      sel 46  min 0   max 52    widthRange 66      sel 66  min 52  max 70    widthRange 90      sel 90  min 70  max 94     var i   1  for  var key in widthRange        document write  Key      i           key       amp nbsp  amp nbsp  amp nbsp  min max       widthRange key  min         widthRange key  max     lt br   gt         i       lt  script gt   lt  html gt    Results in the browser  Firefox 3 6 2 on Windows XP    Key  1   46      min max   0 52 Key  2   66      min max   52 70 Key  3   90      min max   70 94

User · Answer

For your input data   let widthRange   new Array   widthRange 46      min 0   max 52   widthRange 61      min 52  max 70   widthRange 62      min 52  max 70   widthRange 63      min 52  max 70   widthRange 66      min 52  max 70   widthRange 90      min 70  max 94     Declarative approach   const relevantKeys    46 66 90  const relevantValues   Object keys widthRange       filter index   gt  relevantKeys includes parseInt index         map relevantIndex   gt  widthRange relevantIndex     Object keys to get the keys  using parseInt to cast them as numbers  filter to get only ones you want  map to build an array from the original object of just the indices you re after  since Object keys loses the object values   Debug   console log widthRange  console log relevantKeys  console log relevantValues

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You need to call the hasOwnProperty function to check whether the property is actually defined on the object itself  as opposed to its prototype   like this   for  var key in widthRange        if  key      length      widthRange hasOwnProperty key   continue      var value   widthRange key       Note that you need a separate check for length  However  you shouldn t be using an array here at all  you should use a regular object   All Javascript objects function as associative arrays     For example   var widthRange           Or new Object   widthRange 46      sel 46  min 0   max 52    widthRange 66      sel 66  min 52  max 70    widthRange 90      sel 90  min 70  max 94

User · Answer

widthRange map function    i    return i       or  widthRange map     i    gt  i

[javascript] How to get array keys in Javascript?

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