Deleting array elements in JavaScript - delete vs splice

Question

What is the difference between using the delete operator on the array element as opposed to using the Array splice method    For example   myArray     a    b    c    d     delete myArray 1       or myArray splice  1  1     Why even have the splice method if I can delete array elements like I can with objects

User · Answer

If the desired element to delete is in the middle  say we want to delete  c   which its index is 1   you can use   var arr     a   b   c    var indexToDelete   1  var newArray   arr slice 0 indexToDelete  combine arr slice indexToDelete 1  arr length

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splice will work with numeric indices   whereas delete can be used against other kind of indices    example   delete myArray  text1

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From  Core JavaScript 1 5 Reference   Operators   Special Operators   delete Operator       When you delete an array element  the   array length is not affected  For   example  if you delete a 3   a 4  is   still a 4  and a 3  is undefined  This   holds even if you delete the last   element of the array  delete   a a length-1

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Because delete only removes the object from the element in the array  the length of the array won t change  Splice removes the object and shortens the array   The following code will display  a    b    undefined    d   myArray     a    b    c    d    delete myArray 2    for  var count   0  count  lt  myArray length  count          alert myArray count        Whereas this will display  a    b    d   myArray     a    b    c    d    myArray splice 2 1    for  var count   0  count  lt  myArray length  count          alert myArray count

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From  Core JavaScript 1 5 Reference   Operators   Special Operators   delete Operator       When you delete an array element  the   array length is not affected  For   example  if you delete a 3   a 4  is   still a 4  and a 3  is undefined  This   holds even if you delete the last   element of the array  delete   a a length-1

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function deleteFromArray array  indexToDelete     var remain   new Array      for var i in array       if array i     indexToDelete         continue            remain push array i          return remain     myArray     a    b    c    d    deleteFromArray myArray   0        result   myArray     b    c    d

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If you have small array you can use filter   myArray     a    b    c    d    myArray   myArray filter x   gt  x      b

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They re different things that have different purposes   splice is array-specific and  when used for deleting  removes entries from the array and moves all the previous entries up to fill the gap   It can also be used to insert entries  or both at the same time   splice will change the length of the array  assuming it s not a no-op call  theArray splice x  0     delete is not array-specific  it s designed for use on objects  It removes a property  key value pair  from the object you use it on  It only applies to arrays because standard  e g   non-typed  arrays in JavaScript aren t really arrays at all   they re objects with special handling for certain properties  such as those whose names are  array indexes   which are defined as string names     whose numeric value i is in the range  0   i  lt  2 32-1   and length  When you use delete to remove an array entry  all it does is remove the entry  it doesn t move other entries following it up to fill the gap  and so the array becomes  sparse   has some entries missing entirely   It has no effect on length   A couple of the current answers to this question incorrectly state that using delete  sets the entry to undefined   That s not correct  It removes the entry  property  entirely  leaving a gap   Let s use some code to illustrate the differences    x000D   x000D  console log  Using  splice      x000D  var a     a    b    c    d    e    x000D  console log a length                 5 x000D  a splice 0  1   x000D  console log a length                 4 x000D  console log a 0                       b  x000D   x000D   x000D     x000D   x000D  console log  Using  delete     x000D  var a     a    b    c    d    e    x000D  console log a length                 5 x000D  delete a 0   x000D  console log a length                 still 5 x000D  console log a 0                      undefined x000D  console log  0  in a                 false x000D  console log a hasOwnProperty 0       false x000D   x000D   x000D     x000D   x000D  console log  Setting to  undefined     x000D  var a     a    b    c    d    e    x000D  console log a length                 5 x000D  a 0    undefined  x000D  console log a length                 still 5 x000D  console log a 0                      undefined x000D  console log  0  in a                 true x000D  console log a hasOwnProperty 0       true x000D   x000D   x000D         that s a post on my anemic little blog

User · Answer

delete Vs splice  when you delete an item from an array   x000D   x000D  var arr    1 2 3 4   delete arr 2     result  1  2  3   4  x000D  console log arr  x000D   x000D   x000D    when you splice   x000D   x000D  var arr    1 2 3 4   arr splice 1 1     result  1  3  4  x000D  console log arr   x000D   x000D   x000D    in case of delete the element is deleted but the index remains empty  while in case of splice element is deleted and the index of rest elements is reduced accordingly

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function remove array value array  value        var index   array indexOf value       if  index  gt   0            array splice index  1           reindex array array           function reindex array array       var result           for  var key in array            result push array key              return result      example   var example arr     apple    banana    lemon         length   3 remove array value example arr   banana      banana is deleted and array length   2

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As stated many times above  using splice   seems like a perfect fit  Documentation at Mozilla      The splice   method changes the content of an array by removing existing elements and or adding new elements   var myFish     angel    clown    mandarin    sturgeon     myFish splice 2  0   drum        myFish is   angel    clown    drum    mandarin    sturgeon    myFish splice 2  1       myFish is   angel    clown    mandarin    sturgeon         Syntax       array splice start  array splice start  deleteCount  array splice start  deleteCount  item1  item2                Parameters      start      Index at which to start changing the array  If greater than the length of the array  actual starting index will be set to the length of the array  If negative  will begin that many elements from the end       deleteCount      An integer indicating the number of old array elements to remove  If deleteCount is 0  no elements are removed  In this case  you should specify at least one new element  If deleteCount is greater than the number of elements left in the array starting at start  then all of the elements through the end of the array will be deleted       If deleteCount is omitted  deleteCount will be equal to  arr length - start        item1  item2           The elements to add to the array  beginning at the start index  If you don t specify any elements  splice   will only remove elements from the array       Return value      An array containing the deleted elements  If only one element is removed  an array of one element is returned  If no elements are removed  an empty array is returned

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delete will delete the object property  but will not reindex the array or update its length  This makes it appears as if it is undefined    gt  myArray     a    b    c    d       a    b    c    d    gt  delete myArray 0    true  gt  myArray 0    undefined   Note that it is not in fact set to the value undefined  rather the property is removed from the array  making it appear undefined  The Chrome dev tools make this distinction clear by printing empty when logging the array    gt  myArray 0    undefined  gt  myArray    empty   b    c    d     myArray splice start  deleteCount  actually removes the element  reindexes the array  and changes its length    gt  myArray     a    b    c    d       a    b    c    d    gt  myArray splice 0  2      a    b    gt  myArray     c    d

User · Answer

Currently there are two ways to do this    using splice    arrayObject splice index  1   using delete  delete arrayObject index     But I always suggest to use splice for array objects and delete for object attributes because delete does not update array length

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delete acts like a non real world situation  it just removes the item  but the array length stays the same   example from node terminal    gt  var arr     a   b   c   d     gt  delete arr 2  true  gt  arr    a    b      d    e      Here is a function to remove an item of an array by index  using slice    it takes the arr as the first arg  and the index of the member you want to delete as the second argument  As you can see  it actually deletes the member of the array  and will reduce the array length by 1  function arr arrIndex       return arr slice 0 arrIndex  concat arr slice arrIndex   1        What the function above does is take all the members up to the index  and all the members after the index   and concatenates them together  and returns the result   Here is an example using the function above as a node module  seeing the terminal will be useful    gt  var arr     a   b   c   d    gt  arr    a    b    c    d     gt  arr length 4   gt  var arrayRemoveIndex   require    lib array remove index     gt  var newArray   arrayRemoveIndex arr arr indexOf  c     gt  newArray    a    b    d       c ya later  gt  newArray length 3   please note that this will not work one array with dupes in it  because indexOf  c   will just get the first occurance  and only splice out and remove the first  c  it finds

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Easiest way is probably   var myArray     a    b    c    d    delete myArray 1        a   undefined   c    d    Then use lodash compact method to remove false  null  0      undefined and NaN myArray     compact myArray     a    c    d      Hope this helps   Reference  https   lodash com docs compact

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Array remove   Method  John Resig  creator of jQuery created a very handy Array remove method that I always use it in my projects      Array Remove - By John Resig  MIT Licensed  Array prototype remove   function from  to      var rest   this slice  to    from    1    this length     this length   from  lt  0   this length   from   from    return this push apply this  rest        and here s some examples of how it could be used      Remove the second item from the array array remove 1      Remove the second-to-last item from the array array remove -2      Remove the second and third items from the array array remove 1 2      Remove the last and second-to-last items from the array array remove -2 -1     John s website

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you can use something like this   x000D   x000D  var my array    1 2 3 4 5 6   x000D  delete my array 4   x000D  console log my array filter function a  return typeof a      undefined           1 2 3 4 6  x000D   x000D   x000D

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delete acts like a non real world situation  it just removes the item  but the array length stays the same   example from node terminal    gt  var arr     a   b   c   d     gt  delete arr 2  true  gt  arr    a    b      d    e      Here is a function to remove an item of an array by index  using slice    it takes the arr as the first arg  and the index of the member you want to delete as the second argument  As you can see  it actually deletes the member of the array  and will reduce the array length by 1  function arr arrIndex       return arr slice 0 arrIndex  concat arr slice arrIndex   1        What the function above does is take all the members up to the index  and all the members after the index   and concatenates them together  and returns the result   Here is an example using the function above as a node module  seeing the terminal will be useful    gt  var arr     a   b   c   d    gt  arr    a    b    c    d     gt  arr length 4   gt  var arrayRemoveIndex   require    lib array remove index     gt  var newArray   arrayRemoveIndex arr arr indexOf  c     gt  newArray    a    b    d       c ya later  gt  newArray length 3   please note that this will not work one array with dupes in it  because indexOf  c   will just get the first occurance  and only splice out and remove the first  c  it finds

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The difference can be seen by logging the length of each array after the delete operator and splice   method are applied  For example   delete operator  var trees     redwood    bay    cedar    oak    maple    delete trees 3    console log trees        redwood    bay    cedar   empty   maple   console log trees length      5   The delete operator removes the element from the array  but the  placeholder  of the element still exists  oak has been removed but it still takes space in the array  Because of this  the length of the array remains 5   splice   method  var trees     redwood    bay    cedar    oak    maple    trees splice 3 1    console log trees        redwood    bay    cedar    maple   console log trees length      4   The splice   method completely removes the target value and the  placeholder  as well  oak has been removed as well as the space it used to occupy in the array  The length of the array is now 4

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splice will work with numeric indices   whereas delete can be used against other kind of indices    example   delete myArray  text1

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As stated many times above  using splice   seems like a perfect fit  Documentation at Mozilla      The splice   method changes the content of an array by removing existing elements and or adding new elements   var myFish     angel    clown    mandarin    sturgeon     myFish splice 2  0   drum        myFish is   angel    clown    drum    mandarin    sturgeon    myFish splice 2  1       myFish is   angel    clown    mandarin    sturgeon         Syntax       array splice start  array splice start  deleteCount  array splice start  deleteCount  item1  item2                Parameters      start      Index at which to start changing the array  If greater than the length of the array  actual starting index will be set to the length of the array  If negative  will begin that many elements from the end       deleteCount      An integer indicating the number of old array elements to remove  If deleteCount is 0  no elements are removed  In this case  you should specify at least one new element  If deleteCount is greater than the number of elements left in the array starting at start  then all of the elements through the end of the array will be deleted       If deleteCount is omitted  deleteCount will be equal to  arr length - start        item1  item2           The elements to add to the array  beginning at the start index  If you don t specify any elements  splice   will only remove elements from the array       Return value      An array containing the deleted elements  If only one element is removed  an array of one element is returned  If no elements are removed  an empty array is returned

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Because delete only removes the object from the element in the array  the length of the array won t change  Splice removes the object and shortens the array   The following code will display  a    b    undefined    d   myArray     a    b    c    d    delete myArray 2    for  var count   0  count  lt  myArray length  count          alert myArray count        Whereas this will display  a    b    d   myArray     a    b    c    d    myArray splice 2 1    for  var count   0  count  lt  myArray length  count          alert myArray count

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For those who wants to use Lodash can use  myArray     without myArray  itemToRemove   Or as I use in Angular2  import   without   from  lodash       myArray   without myArray  itemToRemove

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delete  delete will delete the object property  but will not reindex   the array or update its length  This makes it appears as if it is   undefined       splice  actually removes the element  reindexes the array  and changes   its length    Delete element from last          arrName pop      Delete element from first     arrName shift      Delete from middle  arrName splice starting index number of element you wnt to delete    Ex  arrName splice 1 1     Delete one element from last  arrName splice -1     Delete by using array index number   delete arrName 1

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If you want to iterate a large array and selectively delete elements  it would be expensive to call splice   for every delete because splice   would have to re-index subsequent elements every time  Because arrays are associative in Javascript  it would be more efficient to delete the individual elements then re-index the array afterwards    You can do it by building a new array  e g  function reindexArray  array            var result               for  var key in array                   result push  array key             return result       But I don t think you can modify the key values in the original array  which would be more efficient - it looks like you might have to create a new array   Note that you don t need to check for the  undefined  entries as they don t actually exist and the for loop doesn t return them  It s an artifact of the array printing that displays them as undefined  They don t appear to exist in memory   It would be nice if you could use something like slice   which would be quicker  but it does not re-index  Anyone know of a better way     Actually  you can probably do it in place as follows which is probably more efficient  performance-wise   reindexArray   function  array         var index   0                              The index where the element should be     for  var key in array                      Iterate the array               if  parseInt  key       index          If the element is out of sequence                       array index    array key           Move it to the correct  earlier position in the array               index                            Update the index                      array splice  index        Remove any remaining elements  These will be duplicates of earlier items

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I stumbled onto this question while trying to understand how to remove every occurrence of an element from an Array  Here s a comparison of splice and delete for removing every  c  from the items Array   var items     a    b    c    d    a    b    c    d     while  items indexOf  c       -1      items splice items indexOf  c    1      console log items        a    b    d    a    b    d    items     a    b    c    d    a    b    c    d     while  items indexOf  c       -1      delete items items indexOf  c        console log items        a    b   undefined   d    a    b   undefined   d

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IndexOf accepts also a reference type  Suppose the following scenario    x000D   x000D  var arr     item  1    item  2    item  3    x000D  var found   find 2  3     pseudo code  will return   item  2    item 3   x000D  var l   found length  x000D   x000D  while l--    x000D     var index   arr indexOf found l   x000D        arr splice index  1   x000D       x000D      x000D  console log arr length     1 x000D   x000D   x000D    Differently   var item2   findUnique 2     will return  item  2  var l   arr length  var found   false    while  found  amp  amp  l--      found   arr l      item2     console log l  arr l      l is index  arr l  is the item you look for

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OK  imagine we have this array below   const arr    1  2  3  4  5     Let s do delete first   delete arr 1     and this is the result    1  empty  3  4  5     empty  and let s get it   arr 1     undefined   So means just the value deleted and it s undefined now  so length is the same  also it will return true     Let s reset our array and do it with splice this time   arr splice 1  1     and this is the result this time    1  3  4  5     As you see the array length changed and arr 1  is 3 now     Also this will return the deleted item in an Array which is  3  in this case

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Performance There are already many nice answer about functional differences - so here I want to focus on performance  Today  2020 06 25  I perform tests for Chrome 83 0  Safari 13 1 and Firefox 77 0 for solutions mention in question and additionally from chosen answers Conclusions  the splice  B  solution is fast for small and big arrays the delete  A  solution is fastest for big and medium fast for small arrays the filter  E  solution is fastest on Chrome and Firefox for small arrays  but slowest on Safari  and slow for big arrays  solution D is quite slow solution C not works for big arrays in Chrome and Safari  x000D   x000D  function C arr  idx      var rest   arr slice idx   1    arr length     arr length   idx  lt  0   arr length   idx   idx    arr push apply arr  rest     return arr         Crash test  let arr        abcdefghij  repeat 100000       1M elements  try    C arr 1    catch e   console error e message   x000D   x000D   x000D     Details I perform following tests for solutions A B C D E  my   for small array  4 elements  - you can run test HERE for big array  1M elements  - you can run test HERE   x000D   x000D  function A arr  idx      delete arr idx     return arr     function B arr  idx      arr splice idx 1     return arr     function C arr  idx      var rest   arr slice idx   1    arr length     arr length   idx  lt  0   arr length   idx   idx    arr push apply arr  rest     return arr     function D arr idx       return arr slice 0 idx  concat arr slice idx   1       function E arr idx      return arr filter  a i    gt  i     idx      myArray     a    b    c    d      A B C D E  map f   gt  console log    f name    JSON stringify f     myArray  1        x000D  This snippet only presents used solutions x000D   x000D   x000D   Example results for Chrome

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OK  imagine we have this array below   const arr    1  2  3  4  5     Let s do delete first   delete arr 1     and this is the result    1  empty  3  4  5     empty  and let s get it   arr 1     undefined   So means just the value deleted and it s undefined now  so length is the same  also it will return true     Let s reset our array and do it with splice this time   arr splice 1  1     and this is the result this time    1  3  4  5     As you see the array length changed and arr 1  is 3 now     Also this will return the deleted item in an Array which is  3  in this case

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Others have already properly compared delete with splice   Another interesting comparison is delete versus undefined  a deleted array item uses less memory than one that is just set to undefined   For example  this code will not finish   let y   1  let ary       console log  Fatal Error Coming Soon    while  y  lt  4294967295        ary push y       ary y    undefined      y    1    console ary length     It produces this error   FATAL ERROR  CALL AND RETRY LAST Allocation failed - JavaScript heap out of memory    So  as you can see undefined actually takes up heap memory   However  if you also delete the ary-item  instead of just setting it to undefined   the code will slowly finish   let x   1  let ary       console log  This will take a while  but it will eventually finish successfully     while  x  lt  4294967295        ary push x       ary x    undefined      delete ary x       x    1    console log  Success  array-length    ary length        These are extreme examples  but they make a point about delete that I haven t seen anyone mention anywhere

User · Answer

Performance There are already many nice answer about functional differences - so here I want to focus on performance  Today  2020 06 25  I perform tests for Chrome 83 0  Safari 13 1 and Firefox 77 0 for solutions mention in question and additionally from chosen answers Conclusions  the splice  B  solution is fast for small and big arrays the delete  A  solution is fastest for big and medium fast for small arrays the filter  E  solution is fastest on Chrome and Firefox for small arrays  but slowest on Safari  and slow for big arrays  solution D is quite slow solution C not works for big arrays in Chrome and Safari  x000D   x000D  function C arr  idx      var rest   arr slice idx   1    arr length     arr length   idx  lt  0   arr length   idx   idx    arr push apply arr  rest     return arr         Crash test  let arr        abcdefghij  repeat 100000       1M elements  try    C arr 1    catch e   console error e message   x000D   x000D   x000D     Details I perform following tests for solutions A B C D E  my   for small array  4 elements  - you can run test HERE for big array  1M elements  - you can run test HERE   x000D   x000D  function A arr  idx      delete arr idx     return arr     function B arr  idx      arr splice idx 1     return arr     function C arr  idx      var rest   arr slice idx   1    arr length     arr length   idx  lt  0   arr length   idx   idx    arr push apply arr  rest     return arr     function D arr idx       return arr slice 0 idx  concat arr slice idx   1       function E arr idx      return arr filter  a i    gt  i     idx      myArray     a    b    c    d      A B C D E  map f   gt  console log    f name    JSON stringify f     myArray  1        x000D  This snippet only presents used solutions x000D   x000D   x000D   Example results for Chrome

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Array remove   Method  John Resig  creator of jQuery created a very handy Array remove method that I always use it in my projects      Array Remove - By John Resig  MIT Licensed  Array prototype remove   function from  to      var rest   this slice  to    from    1    this length     this length   from  lt  0   this length   from   from    return this push apply this  rest        and here s some examples of how it could be used      Remove the second item from the array array remove 1      Remove the second-to-last item from the array array remove -2      Remove the second and third items from the array array remove 1 2      Remove the last and second-to-last items from the array array remove -2 -1     John s website

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It s probably also worth mentioning that splice only works on arrays   Object properties can t be relied on to follow a consistent order    To remove the key-value pair from an object  delete is actually what you want    delete myObj propName           or  delete myObj  propName        Equivalent

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For those who wants to use Lodash can use  myArray     without myArray  itemToRemove   Or as I use in Angular2  import   without   from  lodash       myArray   without myArray  itemToRemove

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Others have already properly compared delete with splice   Another interesting comparison is delete versus undefined  a deleted array item uses less memory than one that is just set to undefined   For example  this code will not finish   let y   1  let ary       console log  Fatal Error Coming Soon    while  y  lt  4294967295        ary push y       ary y    undefined      y    1    console ary length     It produces this error   FATAL ERROR  CALL AND RETRY LAST Allocation failed - JavaScript heap out of memory    So  as you can see undefined actually takes up heap memory   However  if you also delete the ary-item  instead of just setting it to undefined   the code will slowly finish   let x   1  let ary       console log  This will take a while  but it will eventually finish successfully     while  x  lt  4294967295        ary push x       ary x    undefined      delete ary x       x    1    console log  Success  array-length    ary length        These are extreme examples  but they make a point about delete that I haven t seen anyone mention anywhere

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Currently there are two ways to do this    using splice    arrayObject splice index  1   using delete  delete arrayObject index     But I always suggest to use splice for array objects and delete for object attributes because delete does not update array length

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delete will delete the object property  but will not reindex the array or update its length  This makes it appears as if it is undefined    gt  myArray     a    b    c    d       a    b    c    d    gt  delete myArray 0    true  gt  myArray 0    undefined   Note that it is not in fact set to the value undefined  rather the property is removed from the array  making it appear undefined  The Chrome dev tools make this distinction clear by printing empty when logging the array    gt  myArray 0    undefined  gt  myArray    empty   b    c    d     myArray splice start  deleteCount  actually removes the element  reindexes the array  and changes its length    gt  myArray     a    b    c    d       a    b    c    d    gt  myArray splice 0  2      a    b    gt  myArray     c    d

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delete will delete the object property  but will not reindex the array or update its length  This makes it appears as if it is undefined    gt  myArray     a    b    c    d       a    b    c    d    gt  delete myArray 0    true  gt  myArray 0    undefined   Note that it is not in fact set to the value undefined  rather the property is removed from the array  making it appear undefined  The Chrome dev tools make this distinction clear by printing empty when logging the array    gt  myArray 0    undefined  gt  myArray    empty   b    c    d     myArray splice start  deleteCount  actually removes the element  reindexes the array  and changes its length    gt  myArray     a    b    c    d       a    b    c    d    gt  myArray splice 0  2      a    b    gt  myArray     c    d

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If you have small array you can use filter   myArray     a    b    c    d    myArray   myArray filter x   gt  x      b

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Because delete only removes the object from the element in the array  the length of the array won t change  Splice removes the object and shortens the array   The following code will display  a    b    undefined    d   myArray     a    b    c    d    delete myArray 2    for  var count   0  count  lt  myArray length  count          alert myArray count        Whereas this will display  a    b    d   myArray     a    b    c    d    myArray splice 2 1    for  var count   0  count  lt  myArray length  count          alert myArray count

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you can use something like this   x000D   x000D  var my array    1 2 3 4 5 6   x000D  delete my array 4   x000D  console log my array filter function a  return typeof a      undefined           1 2 3 4 6  x000D   x000D   x000D

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Why not just filter  I think it is the most clear way to consider the arrays in js   myArray   myArray filter function item       return item anProperty    whoShouldBeDeleted

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It s probably also worth mentioning that splice only works on arrays   Object properties can t be relied on to follow a consistent order    To remove the key-value pair from an object  delete is actually what you want    delete myObj propName           or  delete myObj  propName        Equivalent

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From  Core JavaScript 1 5 Reference   Operators   Special Operators   delete Operator       When you delete an array element  the   array length is not affected  For   example  if you delete a 3   a 4  is   still a 4  and a 3  is undefined  This   holds even if you delete the last   element of the array  delete   a a length-1

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delete  delete will delete the object property  but will not reindex   the array or update its length  This makes it appears as if it is   undefined       splice  actually removes the element  reindexes the array  and changes   its length    Delete element from last          arrName pop      Delete element from first     arrName shift      Delete from middle  arrName splice starting index number of element you wnt to delete    Ex  arrName splice 1 1     Delete one element from last  arrName splice -1     Delete by using array index number   delete arrName 1

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They re different things that have different purposes   splice is array-specific and  when used for deleting  removes entries from the array and moves all the previous entries up to fill the gap   It can also be used to insert entries  or both at the same time   splice will change the length of the array  assuming it s not a no-op call  theArray splice x  0     delete is not array-specific  it s designed for use on objects  It removes a property  key value pair  from the object you use it on  It only applies to arrays because standard  e g   non-typed  arrays in JavaScript aren t really arrays at all   they re objects with special handling for certain properties  such as those whose names are  array indexes   which are defined as string names     whose numeric value i is in the range  0   i  lt  2 32-1   and length  When you use delete to remove an array entry  all it does is remove the entry  it doesn t move other entries following it up to fill the gap  and so the array becomes  sparse   has some entries missing entirely   It has no effect on length   A couple of the current answers to this question incorrectly state that using delete  sets the entry to undefined   That s not correct  It removes the entry  property  entirely  leaving a gap   Let s use some code to illustrate the differences    x000D   x000D  console log  Using  splice      x000D  var a     a    b    c    d    e    x000D  console log a length                 5 x000D  a splice 0  1   x000D  console log a length                 4 x000D  console log a 0                       b  x000D   x000D   x000D     x000D   x000D  console log  Using  delete     x000D  var a     a    b    c    d    e    x000D  console log a length                 5 x000D  delete a 0   x000D  console log a length                 still 5 x000D  console log a 0                      undefined x000D  console log  0  in a                 false x000D  console log a hasOwnProperty 0       false x000D   x000D   x000D     x000D   x000D  console log  Setting to  undefined     x000D  var a     a    b    c    d    e    x000D  console log a length                 5 x000D  a 0    undefined  x000D  console log a length                 still 5 x000D  console log a 0                      undefined x000D  console log  0  in a                 true x000D  console log a hasOwnProperty 0       true x000D   x000D   x000D         that s a post on my anemic little blog

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Because delete only removes the object from the element in the array  the length of the array won t change  Splice removes the object and shortens the array   The following code will display  a    b    undefined    d   myArray     a    b    c    d    delete myArray 2    for  var count   0  count  lt  myArray length  count          alert myArray count        Whereas this will display  a    b    d   myArray     a    b    c    d    myArray splice 2 1    for  var count   0  count  lt  myArray length  count          alert myArray count

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If you want to iterate a large array and selectively delete elements  it would be expensive to call splice   for every delete because splice   would have to re-index subsequent elements every time  Because arrays are associative in Javascript  it would be more efficient to delete the individual elements then re-index the array afterwards    You can do it by building a new array  e g  function reindexArray  array            var result               for  var key in array                   result push  array key             return result       But I don t think you can modify the key values in the original array  which would be more efficient - it looks like you might have to create a new array   Note that you don t need to check for the  undefined  entries as they don t actually exist and the for loop doesn t return them  It s an artifact of the array printing that displays them as undefined  They don t appear to exist in memory   It would be nice if you could use something like slice   which would be quicker  but it does not re-index  Anyone know of a better way     Actually  you can probably do it in place as follows which is probably more efficient  performance-wise   reindexArray   function  array         var index   0                              The index where the element should be     for  var key in array                      Iterate the array               if  parseInt  key       index          If the element is out of sequence                       array index    array key           Move it to the correct  earlier position in the array               index                            Update the index                      array splice  index        Remove any remaining elements  These will be duplicates of earlier items

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delete Vs splice  when you delete an item from an array   x000D   x000D  var arr    1 2 3 4   delete arr 2     result  1  2  3   4  x000D  console log arr  x000D   x000D   x000D    when you splice   x000D   x000D  var arr    1 2 3 4   arr splice 1 1     result  1  3  4  x000D  console log arr   x000D   x000D   x000D    in case of delete the element is deleted but the index remains empty  while in case of splice element is deleted and the index of rest elements is reduced accordingly

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function remove array value array  value        var index   array indexOf value       if  index  gt   0            array splice index  1           reindex array array           function reindex array array       var result           for  var key in array            result push array key              return result      example   var example arr     apple    banana    lemon         length   3 remove array value example arr   banana      banana is deleted and array length   2

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IndexOf accepts also a reference type  Suppose the following scenario    x000D   x000D  var arr     item  1    item  2    item  3    x000D  var found   find 2  3     pseudo code  will return   item  2    item 3   x000D  var l   found length  x000D   x000D  while l--    x000D     var index   arr indexOf found l   x000D        arr splice index  1   x000D       x000D      x000D  console log arr length     1 x000D   x000D   x000D    Differently   var item2   findUnique 2     will return  item  2  var l   arr length  var found   false    while  found  amp  amp  l--      found   arr l      item2     console log l  arr l      l is index  arr l  is the item you look for

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delete will delete the object property  but will not reindex the array or update its length  This makes it appears as if it is undefined    gt  myArray     a    b    c    d       a    b    c    d    gt  delete myArray 0    true  gt  myArray 0    undefined   Note that it is not in fact set to the value undefined  rather the property is removed from the array  making it appear undefined  The Chrome dev tools make this distinction clear by printing empty when logging the array    gt  myArray 0    undefined  gt  myArray    empty   b    c    d     myArray splice start  deleteCount  actually removes the element  reindexes the array  and changes its length    gt  myArray     a    b    c    d       a    b    c    d    gt  myArray splice 0  2      a    b    gt  myArray     c    d

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function deleteFromArray array  indexToDelete     var remain   new Array      for var i in array       if array i     indexToDelete         continue            remain push array i          return remain     myArray     a    b    c    d    deleteFromArray myArray   0        result   myArray     b    c    d

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The difference can be seen by logging the length of each array after the delete operator and splice   method are applied  For example   delete operator  var trees     redwood    bay    cedar    oak    maple    delete trees 3    console log trees        redwood    bay    cedar   empty   maple   console log trees length      5   The delete operator removes the element from the array  but the  placeholder  of the element still exists  oak has been removed but it still takes space in the array  Because of this  the length of the array remains 5   splice   method  var trees     redwood    bay    cedar    oak    maple    trees splice 3 1    console log trees        redwood    bay    cedar    maple   console log trees length      4   The splice   method completely removes the target value and the  placeholder  as well  oak has been removed as well as the space it used to occupy in the array  The length of the array is now 4

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I stumbled onto this question while trying to understand how to remove every occurrence of an element from an Array  Here s a comparison of splice and delete for removing every  c  from the items Array   var items     a    b    c    d    a    b    c    d     while  items indexOf  c       -1      items splice items indexOf  c    1      console log items        a    b    d    a    b    d    items     a    b    c    d    a    b    c    d     while  items indexOf  c       -1      delete items items indexOf  c        console log items        a    b   undefined   d    a    b   undefined   d

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From  Core JavaScript 1 5 Reference   Operators   Special Operators   delete Operator       When you delete an array element  the   array length is not affected  For   example  if you delete a 3   a 4  is   still a 4  and a 3  is undefined  This   holds even if you delete the last   element of the array  delete   a a length-1

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Why not just filter  I think it is the most clear way to consider the arrays in js   myArray   myArray filter function item       return item anProperty    whoShouldBeDeleted

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If the desired element to delete is in the middle  say we want to delete  c   which its index is 1   you can use   var arr     a   b   c    var indexToDelete   1  var newArray   arr slice 0 indexToDelete  combine arr slice indexToDelete 1  arr length

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Easiest way is probably   var myArray     a    b    c    d    delete myArray 1        a   undefined   c    d    Then use lodash compact method to remove false  null  0      undefined and NaN myArray     compact myArray     a    c    d      Hope this helps   Reference  https   lodash com docs compact

[javascript] Deleting array elements in JavaScript - delete vs splice

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