Check if all values of array are equal

Question

I need to find arrays where all values are equal  What s the fastest way to do this  Should I loop through it and just compare values     a    a    a    a      true   a    a    b    a      false

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Yes  you can check it also using filter as below  very simple  checking every values are the same as the first one     ES6 function sameValues arr      return arr filter  v i a   gt v   a 0   length     arr length       also can be done using every method on the array     ES6 function sameValues arr      return arr every  v i a   gt v   a 0         and you can check your arrays like below   sameValues   a    a    a    a        true sameValues   a    a    b    a        false   Or you can add it to native Array functionalities in JavaScript if you reuse it a lot     ES6 Array prototype sameValues   Array prototype sameValues    function     this every  v i a   gt v   a 0        and you can check your arrays like below     a    a    a    a   sameValues       true   a    a    b    a   sameValues       false

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function isUniform array         for  var i 1  i lt  array length  i          if  array i      array 0     return false           for  var i 1  i lt  array length  i          if  array i      array 0     return true             For the first loop  whenever it detects uneven  returns  false  The first loop runs  and if it returns false  we have  false  When it s not return false  it means there will be true  so we do the second loop  And of course we will have  true  from the second loop  because the first loop found it s NOT false

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And for performance comparison I also did a benchmark    function allAreEqual array       if  array length  return true         I also made sure it works with  false  false  array     return array reduce function a  b  return  a     b  a   b         array 0     function same a        if   a length  return true      return  a filter function  e            return e     a 0          length     function allTheSame array        var first   array 0       return array every function element            return element     first             function useSome array       return  array some function value  index  array           return value     array 0               Results    allAreEqual x 47 565 ops sec   0 16   100 runs sampled  same x 42 529 ops sec   1 74   92 runs sampled  allTheSame x 66 437 ops sec   0 45   102 runs sampled  useSome x 70 102 ops sec   0 27   100 runs sampled    So apparently using builtin array some   is the fastest method of the ones sampled

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You can use Array every if supported   var equals   array every function value  index  array       return value     array 0         Alternatives approach of a loop could be something like sort  var temp   array slice 0  sort    var equals   temp 0      temp temp length - 1     Or  if the items are like the question  something dirty like   var equals   array join     split array 0   join     length     0    Also works

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Update new solution  check index   let a     a    a    b    a     let a     a    a    a    a     let check    list    gt  list every item   gt  list indexOf item      0    check a      false   check b      true    Updated with ES6  Use list every is the fastest way     let a     a    a    b    a     let check    list    gt  list every item   gt  item     list 0      old version         var listTrue     a    a    a    a          var listFalse     a    a    a    ab           function areWeTheSame list              var sample   list 0            return  list every  item    gt  item     sample

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this might work   you can use the comment out code as well that also woks well with the given scenerio    x000D   x000D  function isUniform    x000D   var arrayToMatch    1 1 1 1 1   x000D   var temp   arrayToMatch 0   x000D   console log temp   x000D       return arrayToMatch every function check   x000D      return check    temp  x000D           x000D  var bool  x000D     arrayToMatch forEach function check   x000D      bool  check    temp   x000D        x000D    console log bool   x000D    x000D  isUniform    x000D   x000D   x000D

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Simple one line solution  just compare it to an array filled with the first entry   if arr join         Array arr length  fill arr 0   join

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Well  this is really not very complicated  I have strong suspicion you didn t even try  What you do is that you pick the first value  save it in the variable  and then  within a for loop  compare all subsequent values with the first one  I intentionally didn t share any code  Find how for is used and how variables are compared

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I think the simplest way to do this is to create a loop to compare the each value to the next   As long as there is a break in the  chain  then it would return false   If the first is equal to the second  the second equal to the third and so on  then we can conclude that all elements of the array are equal to each other   given an array data    then you can use   for x 0 x lt data length - 1 x         if  data x     data x 1            isEqual   false                      alert  All elements are equal is     isEqual

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You can convert array to a Set and check its size  In case of primitive array entries  i e  number  string   const isArrayWithEqualEntries   array   gt  new Set array  size     1   In case of array of objects with some field to be tested for equivalence  say id   const mapper     id     gt  id const isArrayWithEqualEntries   array   gt  new Set array map mapper   size     1

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var listTrue     a    a    a    a    var listFalse     a    a    a    ab     function areWeTheSame list         var sample   list 0       return   list some function item            return   item    sample

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You can turn the Array into a Set  If the size of the Set is equal to 1  then all elements of the Array are equal    function allEqual arr      return new Set arr  size    1     allEqual   a    a    a    a        true allEqual   a    a    b    a        false

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You could use a for loop   function isEqual arr      var first   arr 0     for  let i   1  i  lt  arr length  i          if  first     arr i           return false              return true

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Create a string by joining the array  Create string by repetition of the first character of the given array  match both strings    x000D   x000D   function checkArray array   x000D    return array join        array 0  repeat array length    x000D     x000D   x000D   console log  array   a a a a       checkArray   a    a    a    a      x000D   console log  array   a a b a       checkArray   a    a    b    a      x000D   x000D   x000D    And you are DONE

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You can get this one-liner to do what you want using Array prototype every  Object is  and ES6 arrow functions   const all   arr   gt  arr every x   gt  Object is arr 0   x

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Another interesting way when you use ES6 arrow function syntax    x     a    a    a    a    x filter e  gt e   x 0   0      true  x     a    a    b    a    x filter e  gt e   x 0   0     false  x       x filter e  gt e   x 0   0      true   And when you don t want to reuse the variable for array  x        a    a    a    a   filter  e i a   gt e   a 0   0        true   IMO previous poster who used array every      has the cleanest solution

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Its Simple  Create a function and pass a parameter  In that function copy the first index into a new variable  Then Create a for loop and loop through the array  Inside a loop create an while loop with a condition checking whether the new created variable is equal to all the elements in the loop  if its equal return true after the for loop completes else return false inside the while loop    function isUniform arra       var k arra 0       for  var i   0  i  lt  arra length  i              while k   arra i                return false                      return true

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Another way with delimited size and organized list   array1    1 2 3   array2    1 2 3    function isEqual         return array1 toString    array2 toString

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If you re already using underscore js  then here s another option using   uniq   function allEqual arr        return   uniq arr  length     1        uniq returns a duplicate-free version of the array  If all the values are the same  then the length will be 1   As mentioned in the comments  given that you may expect an empty array to return true  then you should also check for that case   function allEqual arr        return arr length     0      uniq arr  length     1

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arr length  amp  amp  arr reduce function a  b  return  a     b  a false        arr 0

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In JavaScript 1 6  you can use Array every   function AllTheSame array        var first   array 0       return array every function element            return element     first              You probably need some sanity checks  e g  when the array has no elements   Also  this won t work when all elements are NaN since NaN     NaN  but that shouldn t be an issue    right

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const allEqual   arr   gt  arr every  v   gt  v     arr 0    allEqual   1 1 1 1        true   Or one-liner    1 1 1 1  every   val  i  arr    gt  val     arr 0         true   Array prototype every  from MDN        The every   method tests whether all elements in the array pass the test implemented by the provided function

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The accepted answer worked great but I wanted to add a tiny bit  It didn t work for me to use     because I was comparing arrays of arrays of objects  however throughout my app I ve been using the fast-deep-equal package which I highly recommend  With that  my code looks like this    let areAllEqual   arrs every  val  i  arr    gt  equal val  arr 0        and my data looks like this                        ID   28         AuthorID   121         VisitTypeID   2                     ID   115         AuthorID   121         VisitTypeID   1                     ID   121         AuthorID   121         VisitTypeID   1                             ID   121         AuthorID   121         VisitTypeID   1                             ID   5         AuthorID   121         VisitTypeID   1                     ID   121         AuthorID   121         VisitTypeID   1

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Underscore s   isEqual object  other  function seems to work well for arrays   The order of items in the array matter when it checks for equality   See http   underscorejs org  isEqual

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In PHP  there is a solution very simple  one line method     count array count values  array      1   For example     arr1     a    a    a    a     arr2     a    a    b    a      print  count array count values  arr1      1    identical     not identical       identical print  count array count values  arr2      1    identical     not identical       not identical   That s all

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Shortest answer using underscore lodash  function elementsEqual arr        return    without arr  arr 0   length     spec   elementsEqual null     throws error elementsEqual        true elementsEqual        true elementsEqual  1      true elementsEqual  1 2      false elementsEqual NaN     true   edit   Or even shorter  inspired by Tom s answer   function elementsEqual2 arr        return   uniq arr  length  lt   1      spec   elementsEqual2 null     true  beware  it s different than above  elementsEqual2        true elementsEqual2        true elementsEqual2  1      true elementsEqual2  1 2      false elementsEqual2 NaN     true

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Now you can make use of sets to do that easily    x000D   x000D  let a    a    a    a    a       true x000D  let b    a    a    b    a      false x000D   x000D  console log new Set a  size     1   x000D  console log new Set b  size     1   x000D   x000D   x000D

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Edit  Be a Red ninja      array reduce function a  b   return  a     b    a   NaN        Results    var array     a    a    a     gt  result   true  var array     a    b    a     gt  result   false  var array     false         gt  result   false  var array     false   false    gt  result   false  var array     false    false     gt  result   true  var array    NaN  NaN    gt  result   false       Warning   var array        gt  result  TypeError thrown   This is because we do not pass an initialValue  So  you may wish to check array length first

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You can use this   function same a        if   a length  return true      return  a filter function  e            return e     a 0          length      The function first checks whether the array is empty  If it is it s values are equals   Otherwise it filter the array and takes all elements which are different from the first one  If there are no such values    the array contains only equal elements otherwise it doesn t

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This works  You create a method on Array by using prototype   if  Array prototype allValuesSame     undefined      Array prototype allValuesSame   function         for  let i   1  i  lt  this length  i            if  this i      this 0             return false                    return true          Call this in this way   let a     a    a    a    let b   a allValuesSame       true a     a    b    a    b   a allValuesSame           false

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every   function check if all elements of an array     const checkArr   a   gt  a every  val   gt  val     a 0        checkArr   a   a   a        true

[javascript] Check if all values of array are equal

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