Checking for duplicate strings in JavaScript array

Question

I have JS array with strings  for example   var strArray      q    w    w    e    i    u    r     I need to compare for duplicate strings inside array  and if duplicate string exists  there should be alert box pointing to that string   I was trying to compare it with for loop  but I don t know how to write code so that array checks it s own strings for duplicates  without already pre-determined string to compare

User · Answer

var strArray      quot q quot    quot w quot    quot w quot    quot e quot    quot i quot    quot u quot    quot r quot    quot q quot    var alreadySeen        strArray forEach function str      if  alreadySeen str       alert str     else     alreadySeen str    true       I added another duplicate in there from your original just to show it would find a non-consecutive duplicate  Updated version with arrow function  const strArray      quot q quot    quot w quot    quot w quot    quot e quot    quot i quot    quot u quot    quot r quot    quot q quot    const alreadySeen        strArray forEach str   gt  alreadySeen str    alert str    alreadySeen str    true

User · Answer

Using ES6 features function checkIfDuplicateExists w       return new Set w  size     w length     console log      checkIfDuplicateExists   quot a quot    quot b quot    quot c quot    quot a quot       true     console log      checkIfDuplicateExists   quot a quot    quot b quot    quot c quot       false

User · Answer

You could take a Set and filter the values who are alreday seen    x000D   x000D  var array     q    w    w    e    i    u    r    x000D      seen   array filter  s   gt  v   gt  s has v      s add v   new Set    x000D   x000D  console log seen   x000D   x000D   x000D

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Use object keys for good performance when you work with a big array  in that case  loop for each element and loop again to check duplicate will be very slowly    var strArray     q    w    w    e    i    u    r     var counting       strArray forEach function  str        counting str     counting str     0    1       if  Object keys counting  length     strArray length        console log  Has duplicates         var str      for  str in counting            if  counting hasOwnProperty str                 if  counting str   gt  1                    console log str     appears     counting str      times

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This is the simplest solution I guess   function diffArray arr1  arr2      return arr1      concat arr2       filter item   gt   arr1 includes item      arr2 includes item

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Using some function on arrays  If any item in the array has an index number from the beginning is not equals to index number from the end  then this item exists in the array more than once      vanilla js function hasDuplicates arr        return arr some  function item            return arr indexOf item      arr lastIndexOf item

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The findDuplicates function  below  compares index of all items in array with index of first occurrence of same item  If indexes are not same returns it as duplicate   x000D   x000D  let strArray      q    w    w    w    e    i    u    r    let findDuplicates   arr   gt  arr filter  item  index    gt  arr indexOf item     index   console log findDuplicates strArray      All duplicates console log     new Set findDuplicates strArray        Unique duplicates x000D   x000D   x000D

User · Answer

The following code uses a unique-filter  checks if every occurrence of an item is the first occurence  to compare the number of unique items in an array with the total number of items  if both are equal  the array only contains unique elements  otherwise there are some duplicates   var firstUnique    value  index  array    gt  array indexOf value      index  var numUnique   strArray filter firstUnique  length  var allUnique   strArray length     numUnique

User · Answer

I think it can t be simpler than this   x000D   x000D  const findDuplicates   arr   gt      new Set arr filter v   gt  arr indexOf v      arr lastIndexOf v       console log findDuplicates    q    w    w    e    i    u    r      x000D   x000D   x000D

User · Answer

Here s my solution if you are using typescript in a functional way  const hasDuplicates    lt T gt  arr  T     boolean   gt      if  arr length     0  return false   if  arr lastIndexOf arr 0       0  return true   return hasDuplicates arr slice 1

User · Answer

You can do this using a Set  You have to create a Set and put all the values in your Array  in that Set  Then  you check whether they have the same length or not  If not  you know there are duplicate values  because a Set can only have unique values  It is explained in the link below   https   medium com dailyjs how-to-remove-array-duplicates-in-es6-5daa8789641c

User · Answer

You could use reduce   const arr     q    w    w    e    i    u    r   arr reduce  acc  cur    gt       if acc cur         acc duplicates push cur      else       acc cur    true   anything could go here          duplicates          Result would look like this        Non Duplicate Values  duplicates    w       That way you can do whatever you want with the duplicate values

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var elems     f    a   b   f    c   d   e   f   c         elems sort         elems forEach function  value  index  arr            let first index   arr indexOf value           let last index   arr lastIndexOf value             if first index     last index             console log  Duplicate item in array     value              else            console log  unique items in array     value

User · Answer

x000D   x000D  function hasDuplicates arr        var counts            for  var i   0  i  lt   arr length  i              if  counts arr i       undefined                counts arr i     1            else               return true                      return false               var arr    1  1  2  3  4    if  hasDuplicates arr       alert  Error  you have duplicates values       x000D   x000D   x000D   Simple Javascript  if you don t know ES6  function hasDuplicates arr        var counts            for  var i   0  i  lt   arr length  i              if  counts arr i       undefined                counts arr i     1            else               return true                      return false               var arr    1  1  2  3  4    if  hasDuplicates arr       alert  Error  you have duplicates values

[javascript] Checking for duplicate strings in JavaScript array

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