How to get a key in a JavaScript object by its value

Question

I have a quite simple JavaScript object  which I use as an associative array  Is there a simple function allowing me to get the key for a value  or do I have to iterate the object and find it out manually

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Really straightforward   const CryptoEnum   Object freeze                        Bitcoin   0   Ethereum   1                        Filecoin   2   Monero   3                        EOS   4   Cardano   5                        NEO   6   Dash   7                        Zcash   8   Decred   9                         Object entries CryptoEnum  0  0     output   gt   Bitcoin

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Or  easier yet - create a new object with the keys and values in the order you want then do look up against that object  We have had conflicts using the prototype codes above  You don t have to use the String function around the key  that is optional    newLookUpObj          each oldLookUpObj function key value           newLookUpObj value    String key

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I use this function   Object prototype getKey   function value     for var key in this       if this key     value         return key              return null       Usage      ISO 639  2-letter codes var languageCodes       DA   Danish     DE   German     DZ   Bhutani     EL   Greek     EN   English     EO   Esperanto     ES   Spanish      var key   languageCodes getKey  Greek    console log key      EL

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this worked for me to get key value of object    x000D   x000D  let obj     x000D           key1    value1   x000D           key2    value2   x000D           key3    value3   x000D           key4    value4  x000D        x000D      Object keys obj  map function k    x000D      console log  key with value    k        obj k       x000D       x000D         x000D       x000D   x000D   x000D

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ES6 methods   Object fromEntries Object entries a  map b   gt  b reverse      value you look for

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Non-iteratable solution   Main function   var keyByValue   function value         var kArray   Object keys greetings             Creating array of keys     var vArray   Object values greetings           Creating array of values     var vIndex   vArray indexOf value              Finding value index       return kArray vIndex                           Returning key by value index     Object with keys and values   var greetings         english      hello       ukranian                  Test   keyByValue                 gt   ukranian

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http   jsfiddle net rTazZ 2     var a   new Array         a push   1    apple    2    banana          a push   3    coconut    4    mango          GetIndexByValue a   coconut         function GetIndexByValue arrayName  value          var keyName           var index   -1      for  var i   0  i  lt  arrayName length  i              var obj   arrayName i                for  var key in obj                              if  obj key     value                         keyName   key                       index   i                                                        console log index            return index

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As if this question hasn t been beaten to a pulp     Here s one just for whatever curiosity it brings you     If you re sure that your object will have only string values  you could really exhaust yourself to conjure up this implementation   var o     a    A   b    B   c    C        json   JSON stringify o      split   json split         nosj   split reverse       o2   nosj join       var reversed   o2 replace        g  function   1    return                         1          object   JSON parse reversed      value     B      eulav   value split     reverse   join       console log   gt  gt    object eulav      Maybe there s something useful to build off of here     Hope this amuses you

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Given input   a   x    b   y    c   x         To use the first value  e g  output   x   a   y   b        x000D   x000D  input     x000D     a    x   x000D     b    y   x000D     c    x  x000D    x000D  output   Object keys input  reduceRight function accum  key  i    x000D    accum input key     key  x000D    return accum  x000D         x000D  console log output  x000D   x000D   x000D     To use the last value  e g  output   x   c   y   b        x000D   x000D  input     x000D     a    x   x000D     b    y   x000D     c    x  x000D    x000D  output   Object keys input  reduce function accum  key  i    x000D    accum input key     key  x000D    return accum  x000D         x000D  console log output  x000D   x000D   x000D     To get an array of keys for each value  e g  output   x    c   a    y    b         x000D   x000D  input     x000D     a    x   x000D     b    y   x000D     c    x  x000D    x000D  output   Object keys input  reduceRight function accum  key  i    x000D    accum input key      accum input key          concat key   x000D    return accum  x000D         x000D  console log output  x000D   x000D   x000D

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I typically recommend lodash rather than underscore   If you have it  use it   If you don t  then you should consider using the lodash invert npm package  which is pretty tiny   Here s how you can test it using gulp   1  Create a file called gulpfile js with the following contents      Filename  gulpfile js var gulp   require  gulp    var invert   require  lodash invert       gulp task  test-invert   function        var hash         foo  1      bar  2        var val   1    var key    invert hash   val        lt  lt  Here s where we call invert    console log  key for val     val         key         2  Install the lodash invert package and gulp    npm i --save lodash invert  amp  amp  npm install gulp   3  Test that it works     gulp test-invert  17 17 23  Using gulpfile   dev npm lodash-invert gulpfile js  17 17 23  Starting  test-invert     key for val 1   foo  17 17 23  Finished  test-invert  after 511   s   References  https   www npmjs com package lodash invert  https   lodash com   Differences between lodash and underscore  https   github com gulpjs gulp

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Since the values are unique  it should be possible to add the values as an additional set of keys  This could be done with the following shortcut   var foo       foo foo apple    an apple      apple   foo foo pear    a pear      pear     This would permit retrieval either via the key or the value   var key    apple   var value    an apple    console log foo value        apple  console log foo key        an apple    This does assume that there are no common elements between the keys and values

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With the Underscore js library   var hash       foo  1    bar  2        invert hash   1        gt   foo

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Here is my solution first   For example  I suppose that we have an object that contains three value pairs    function findKey object  value         for  let key in object          if  object key      value  return key       return  key is not found      const object     id 1   apple   id 2   pear   id 3   peach      console log findKey object   pear       expected output  id 2   We can simply write a findKey array  value  that takes two parameters which are an object and the value of the key you are looking for  As such  this method is reusable and you do not need to manually iterate the object every time by only passing two parameters for this function

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As said  iteration is needed  For instance  in modern browser you could have   var key   Object keys obj  filter function key   return obj key      value   0     Where value contains the value you re looking for  Said that  I would probably use a loop   Otherwise you could use a proper  hashmap  object - there are several implementation in JS around - or implement by your own   UPDATE 2018  Six years passed  but I still get some vote here  so I feel like a more modern solution     for modern browser environment     should be mentioned in the answer itself and not just in the comments   const key   Object keys obj  find key   gt  obj key      value     Of course it can be also a function   const getKeyByValue    obj  value    gt           Object keys obj  find key   gt  obj key      value

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The lodash way https   lodash com docs findKey   x000D   x000D  var users     x000D     barney       age   36   active   true    x000D     fred         age   40   active   false    x000D     pebbles      age   1    active   true   x000D     x000D   x000D    findKey users     age   1   active   true     x000D        pebbles  x000D   x000D   x000D

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Keep your prototype clean    function val2key val array       for  var key in array            if array key     val               return key                   return false      Example   var map     first    1   second    2   var key   val2key 2 map     returns  second

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function getKeyByValue object  value      return Object keys object  find key   gt  object key      value       ES6  no prototype mutations or external libraries   Example    x000D   x000D  function getKeyByValue object  value    x000D    return Object keys object  find key   gt  object key      value   x000D    x000D   x000D   x000D  const map     first     1    second     2    x000D  console log getKeyByValue map  2     x000D   x000D   x000D

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Here s a Lodash solution to this that works for flat key    value object  rather than a nested object  The accepted answer s suggestion to use   findKey works for objects with nested objects  but it doesn t work in this common circumstance   This approach inverts the object  swapping keys for values  and then finds the key by looking up the value on the new  inverted  object  If the key isn t found then false is returned  which I prefer over undefined  but you could easily swap this out in the third parameter of the   get method in getKey      x000D   x000D     Get an object s key by value x000D  var getKey   function  obj  value     x000D   var inverse     invert  obj    x000D   return   get  inverse  value  false    x000D     x000D   x000D     US states used as an example x000D  var states     x000D    AL    Alabama   x000D    AK    Alaska   x000D    AS    American Samoa   x000D    AZ    Arizona   x000D    AR    Arkansas   x000D    CA    California   x000D    CO    Colorado   x000D    CT    Connecticut   x000D    DE    Delaware   x000D    DC    District Of Columbia   x000D    FM    Federated States Of Micronesia   x000D    FL    Florida   x000D    GA    Georgia   x000D    GU    Guam   x000D    HI    Hawaii   x000D    ID    Idaho   x000D    IL    Illinois   x000D    IN    Indiana   x000D    IA    Iowa   x000D    KS    Kansas   x000D    KY    Kentucky   x000D    LA    Louisiana   x000D    ME    Maine   x000D    MH    Marshall Islands   x000D    MD    Maryland   x000D    MA    Massachusetts   x000D    MI    Michigan   x000D    MN    Minnesota   x000D    MS    Mississippi   x000D    MO    Missouri   x000D    MT    Montana   x000D    NE    Nebraska   x000D    NV    Nevada   x000D    NH    New Hampshire   x000D    NJ    New Jersey   x000D    NM    New Mexico   x000D    NY    New York   x000D    NC    North Carolina   x000D    ND    North Dakota   x000D    MP    Northern Mariana Islands   x000D    OH    Ohio   x000D    OK    Oklahoma   x000D    OR    Oregon   x000D    PW    Palau   x000D    PA    Pennsylvania   x000D    PR    Puerto Rico   x000D    RI    Rhode Island   x000D    SC    South Carolina   x000D    SD    South Dakota   x000D    TN    Tennessee   x000D    TX    Texas   x000D    UT    Utah   x000D    VT    Vermont   x000D    VI    Virgin Islands   x000D    VA    Virginia   x000D    WA    Washington   x000D    WV    West Virginia   x000D    WI    Wisconsin   x000D    WY    Wyoming  x000D     x000D   x000D  console log   The key for  Massachusetts  is      getKey  states   Massachusetts             x000D   lt script src  https   cdnjs cloudflare com ajax libs lodash js 4 17 4 lodash min js  gt  lt  script gt  x000D   x000D   x000D

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If you are working with Underscore or Lodash library  you can use the   findKey function   var users        barney       age   36   active   true       fred         age   40   active   false       pebbles      age   1    active   true         findKey users  function o    return o age  lt  40           gt   barney   iteration order is not guaranteed      The    matches  iteratee shorthand    findKey users     age   1   active   true          gt   pebbles      The    matchesProperty  iteratee shorthand    findKey users    active   false         gt   fred      The    property  iteratee shorthand    findKey users   active         gt   barney

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I created the bimap library  https   github com alethes bimap  which implements a powerful  flexible and efficient JavaScript bidirectional map interface  It has no dependencies and is usable both on the server-side  in Node js  you can install it with npm install bimap  and in the browser  by linking to lib bimap js    Basic operations are really simple   var bimap   new BiMap  bimap push  k    v    bimap key  k        gt   v  bimap val  v        gt   k   bimap push  UK     London    Manchester     bimap key  UK         gt    London    Manchester   bimap val  London         gt   UK  bimap val  Manchester         gt   UK    Retrieval of the key-value mapping is equally fast in both directions  There are no costly object array traversals under the hood so the average access time remains constant regardless of the size of the data

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didn t see the following    x000D   x000D  const obj     x000D    id  1  x000D    name   Den  x000D     x000D   x000D  function getKeyByValue obj  value    x000D    return Object entries obj  find     name     gt  value     name   x000D    x000D   x000D  const   key     getKeyByValue obj   Den    x000D  console log key  x000D     x000D   x000D   x000D

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This is a small extension to the Underscorejs method  and uses Lodash instead   var getKeyByValue   function searchValue      return   findKey hash  function hashValue        return searchValue     hashValue            FindKey will search and return the first key which matches the value  If you want the last match instead  use FindLastKey instead

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Keep it simple   You don t need to filter the object through sophisticated methods or libs  Javascript has a built in function called Object values   Example   let myObj    jhon   age  20  job   Developer    marie   age  20  job    Developer      function giveMeTheObjectData object  property       return Object values object property       giveMeTheObjectData myObj   marie         gt  returns marie       This will return the object property data   References  https   developer mozilla org pt-BR docs Web JavaScript Reference Global Objects Object values

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Underscore js solution  let samplLst     id 1 title Lorem   id 2 title Ipsum   let sampleKey     findLastIndex samplLst   id 2      result would be 1 console log samplLst sampleKey     output -  id 2 title Ipsum

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function extractKeyValue obj  value        return Object keys obj  Object values obj  indexOf value        Made for closure compiler to extract key name which will be unknown after compilation  More sexy version but using future Object entries function   function objectKeyByValue  obj  val      return Object entries obj  find i   gt  i 1      val

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ES6  One Liners  let key   Object keys obj  find k  gt obj k    value       Return all keys with the value   let keys   Object keys obj  filter k  gt obj k    value

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No standard method available  You need to iterate and you can create a simple helper   Object prototype getKeyByValue   function  value         for  var prop in this             if  this hasOwnProperty  prop                    if  this  prop       value                    return prop                     var test        key1  42     key2   foo      test getKeyByValue  42        returns  key1    One word of caution  Even if the above works  its generally a bad idea to extend any host or native object s  prototype  I did it here because it fits the issue very well  Anyway  you should probably use this function outside the  prototype and pass the object into it instead

[javascript] How to get a key in a JavaScript object by its value?

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