How to find first element of array matching a boolean condition in JavaScript

Question

I m wondering if there s a known  built-in elegant way to find the first element of a JS array  matching a given condition  A C  equivalent would be List Find   So far I ve been using a two-function combo like this      Returns the first element of an array that satisfies given predicate Array prototype findFirst   function  predicateCallback        if  typeof predicateCallback      function             return undefined             for  var i   0  i  lt  arr length  i              if  i in this  amp  amp  predicateCallback this i    return this i              return undefined         Check if element is not undefined  amp  amp  not null isNotNullNorUndefined   function  o        return  typeof  o       undefined   amp  amp  o     null        And then I can use   var result   someArray findFirst isNotNullNorUndefined     But since there are so many functional-style array methods in ECMAScript  perhaps there s something out there already like this  I imagine lots of people have to implement stuff like this all the time

User · Answer

foundElement   myArray myArray findIndex element   gt    condition here

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As of ECMAScript 6  you can use Array prototype find for this  This is implemented and working in Firefox  25 0   Chrome  45 0   Edge  12   and Safari  7 1   but not in Internet Explorer or a bunch of other old or uncommon platforms  For example  x below is 106   x000D   x000D  const x    100 101 102 103 104 105 106 107 108 109  find function  el        return el  gt  105      console log x   x000D   x000D   x000D   If you want to use this right now but need support for IE or other unsupporting browsers  you can use a shim  I recommend the es6-shim  MDN also offers a shim if for some reason you don t want to put the whole es6-shim into your project  For maximum compatibility you want the es6-shim  because unlike the MDN version it detects buggy native implementations of find and overwrites them  see the comment that begins  quot Work around bugs in Array find and Array findIndex quot  and the lines immediately following it

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If you re using underscore js you can use its find and indexOf functions to get exactly what you want   var index     indexOf your array    find your array  function  d        return d     true         Documentation    http   underscorejs org  find http   underscorejs org  indexOf

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As of ES 2015  Array prototype find   provides for this exact functionality   For browsers that do not support this feature  the Mozilla Developer Network has provided a polyfill  pasted below    if   Array prototype find      Array prototype find   function predicate        if  this     null          throw new TypeError  Array prototype find called on null or undefined              if  typeof predicate      function           throw new TypeError  predicate must be a function              var list   Object this       var length   list length  gt  gt  gt  0      var thisArg   arguments 1       var value       for  var i   0  i  lt  length  i            value   list i         if  predicate call thisArg  value  i  list             return value                    return undefined

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Summary   For finding the first element in an array which matches a boolean condition we can use the ES6 find   find   is located on Array prototype so it can be used on every array  find   takes a callback where a boolean condition is tested  The function returns the value  not the index    Example   x000D   x000D  const array    4  33  8  56  23    const found   array find element   gt      return element  gt  50       console log found         56 x000D   x000D   x000D

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It should be clear by now that JavaScript offers no such solution natively  here are the closest two derivatives  the most useful first    Array prototype some fn  offers the desired behaviour of stopping when a condition is met  but returns only whether an element is present  it s not hard to apply some trickery  such as the solution offered by Bergi s answer  Array prototype filter fn  0  makes for a great one-liner but is the least efficient  because you throw away N - 1 elements just to get what you need    Traditional search methods in JavaScript are characterized by returning the index of the found element instead of the element itself or -1  This avoids having to choose a return value from the domain of all possible types  an index can only be a number and negative values are invalid   Both solutions above don t support offset searching either  so I ve decided to write this    function ns      ns search   function array  callback  offset        var size   array length       offset   offset    0      if  offset  gt   size    offset  lt   -size          return -1        else if  offset  lt  0          offset   size - offset             while  offset  lt  size          if  callback array offset   offset  array             return offset                  offset            return -1         this     search  1  2  NaN  4   Number isNaN      2 search  1  2  3  4   Number isNaN      -1 search  1  NaN  3  NaN   Number isNaN  2      3

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Use findIndex as other previously written  Here s the full example  function find arr  predicate        foundIndex   arr findIndex predicate       return foundIndex     -1   arr foundIndex    null     And usage is following  we want to find first element in array which has property id     1   var firstElement   find arr  e   gt  e id     1

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I have got inspiration from multiple sources on the internet to derive into the solution below  Wanted to take into account both some default value and to provide a way to compare each entry for a generic approach which this solves   Usage   giving value  Second    var defaultItemValue     id  -1  name   Undefined     var containers  Container        id  1  name   First       id  2  name   Second      GetContainer 2  name    Implementation   class Container       id  number      name  string     public GetContainer containerId  number   Container     var comparator    item  Container   boolean   gt          return item id    containerId             return this Get lt Container gt  this containers  comparator  this defaultItemValue        private Get lt T gt  array  T    comparator   item  T    gt  boolean  defaultValue  T   T     var found  T   null    array some function element  index        if  comparator element           found   element        return true                 if   found        found   defaultValue         return found

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Array prototype find   does just that  more info  https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Array find

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There is no built-in function in Javascript to perform this search   If you are using jQuery you could do a jQuery inArray element array

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A less elegant way that will throw all the right error messages  based on Array prototype filter  but will stop iterating on the first result is  function findFirst arr  test  context        var Result   function  v  i   this value   v  this index   i        try           Array prototype filter call arr  function  v  i  a                if  test v  i  a   throw new Result v  i              context         catch  e            if  e instanceof Result  return e          throw e            Then examples are  findFirst  -2  -1  0  1  2  3   function  e   return e  gt  1  amp  amp  e   2        Result  value  3  index  5  findFirst  0  1  2  3   0                    bad function param    TypeError  number is not a function findFirst 0  function     return true        bad arr param    undefined findFirst  1   function  e   return 0        no match    undefined   It works by ending filter by using throw

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Since ES6 there is the native find method for arrays  this stops enumerating the array once it finds the first match and returns the value  const result   someArray find isNotNullNorUndefined     Old answer  I have to post an answer to stop these filter suggestions  -   since there are so many functional-style array methods in ECMAScript  perhaps there s something out there already like this   You can use the some Array method to iterate the array until a condition is met  and then stop   Unfortunately it will only return whether the condition was met once  not by which element  or at what index  it was met  So we have to amend it a little  function find arr  test  ctx        var result   null      arr some function el  i            return test call ctx  el  i  arr      result   el   true    false              return result      var result   find someArray  isNotNullNorUndefined

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What about using filter and getting the first index from the resulting array   var result   someArray filter isNotNullNorUndefined  0

[javascript] How to find first element of array matching a boolean condition in JavaScript?

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