How do I check in JavaScript if a value exists at a certain array index

Question

Will this work for testing whether a value at position index exists or not  or is there a better way  if arrayName index    quot  quot            do stuff

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OK  let s first see what would happens if an array value not exist in JavaScript  so if we have an array like below   const arr    1  2  3  4  5     and now we check if 6 is there at index 5 or not    arr 5     and we get  undefined     So that s basically give us the answer  the best way to check if undefined  so something like this   if  undefined      typeof arrayName index         array value is not there        It s better NOT doing this in this case   if  arrayName index         Don t check like this       Because imagine we have this array   const arr    0  1  2     and we do    if  arr 0         This get passed  because in JavaScript 0 is falsy     So as you see  even 0 is there  it doesn t get recognised  there are few other things which can do the same and make you application buggy  so be careful  I list them all down    undefined  if the value is not defined and it s undefined null  if it s null  for example if a DOM element not exists    empty string      0  number zero NaN  not a number false

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if typeof arr     object   amp  amp  arr instanceof Array       if  arr length         println  empty      else        printn  not Empty         else     println  Null      If you mean by  Null  -  Its elements are null or equals to      in this case   Check if the array is empty after filtering all  null  elements  if  arr clean   length  return  is null     Of course  Add Clean  method before     Array prototype clean function   return this filter function e  return  typeof  e     undefined   amp  amp  e   null  amp  amp  e

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I think this decision is appropriate for guys who prefer the declarative functional programming over the imperative OOP or the procedural  If your question is  Is there some values inside   a truthy or a falsy value   you can use  some method to validate the values inside       some el   gt  el     el      It isn t perfect but it doesn t require to apply any extra function containing the same logic  like function isEmpty arr           It still sounds better than  Is it zero length   when we do this    length resulting to 0 which is dangerous in some cases  Or even this    length  gt  0 saying  Is its length greater than zero     Advanced examples          some el   gt  el     el      false  null  some el   gt  el     el      true  1  3  some el   gt  el     el      true

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It depends on what you mean with  empty    When you attempt to get the value of a property on an object which has no property with that name  you will get the value undefined   That s what happens with sparse arrays  not all indices between 0 and array length-1 exist   So you could check if array index      undefined   However  the property index could exist with an undefined value  If you want to filter out this case  you can use the in operator or hasOwnProperty  as described in How do I check if an object has a property in JavaScript   index in array  array hasOwnProperty index     If you want consider an existing property with an undefined or null value to not exist  you can use the loose comparison array index     undefined or array index     null   If you know the array is not sparse  you could compare index with array length  But to be safe  you may want to ensure that index really is an array index  see Check if property name is array index

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You can use Loadsh library to do this more efficiently  like   if you have an array named  pets   for example   var pets     dog   undefined   cat   null    console log   isEmpty pets 1        true console log   isEmpty pets 3        true console log   isEmpty pets 4        false    map  pets   pet  index    gt    console log index            isEmpty pet          To check all array values for null or undefined values    x000D   x000D  var pets     dog   undefined   cat   null   x000D   x000D  console log   isEmpty pets 1        true x000D  console log   isEmpty pets 3        true x000D  console log   isEmpty pets 4        false x000D   x000D    map  pets   pet  index    gt    console log index            isEmpty pet        x000D   lt script src  https   cdnjs cloudflare com ajax libs underscore js 1 8 3 underscore-min js  gt  lt  script gt  x000D   x000D   x000D    Check more examples in http   underscorejs org

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I ran into this issue using laravel datatables  I was storing a JSON value called properties in an activity log and wanted to show a button based on this value being empty or not   Well  datatables was interpreting this as an array if it was empty  and an object if it was not  therefore  the following solution worked for me   render  function  data  type  full        if  full properties length     0               do stuff           An object does not have a length property

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I would like to point out something a few seem to have missed  namely it is possible to have an  empty  array position in the middle of your array  Consider the following   let arr    0  1  2  3  4  5   delete arr 3   console log arr           0  1  2  empty  4  5   console log arr 3        undefined   The natural way to check would then be to see whether the array member is undefined  I am unsure if other ways exists  if  arr index      undefined         member does not exist

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I would recommend creating a function like this   function isEmptyEl array  i       return   array i        You could call it like this   if  isEmptyEl arrayName  indexVal        console log  arrayName     indexVal      is empty        Forcing the developer to adhere to the isEmptyEl interface will catch input errors such as an undefined arrayName or indexVal variables    It s generally good practice to program defensively when programming in Javascript    You would get an error thrown like this if arrayName was not defined   Uncaught ReferenceError  arrayName is not defined     at  lt anonymous gt  2 15     at Object InjectedScript  evaluateOn   lt anonymous gt  895 140      at Object InjectedScript  evaluateAndWrap   lt anonymous gt  828 34      at Object InjectedScript evaluate   lt anonymous gt  694 21    Similar results for an undefined indexVal   You get an error if the array or index values do not exist   For valid input  you ll only get a true if arrayName indexVal  is any of the following    null undefined NaN empty string  0 false

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Using only  length is not safe and will cause an error in some browsers  Here is a better solution   if array  amp  amp  array length            not empty    else         empty     or  we can use   Object keys   array    length

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With Lodash  you can do   if   has req  documents           if  req documents length          forEach req documents  function document           records push document               else       if   has req  documents    is to check whether our request object has a property named documents and if it has that prop  the next if  req documents length  is to validate if it is not an empty array  so the other stuffs like forEach can be proceeded

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if  arrayName index            do stuff

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try this if array index  is null  if  array index     null

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Conceptually  arrays in JavaScript contain array length elements  starting with array 0  up until array array length - 1    An array element with index i is defined to be part of the array if i is between 0 and array length - 1 inclusive   If i is not in this range it s not in the array   So by concept  arrays are linear  starting with zero and going to a maximum  without any mechanism for having  gaps  inside that range where no entries exist   To find out if a value exists at a given position index  where index is 0 or a positive integer   you literally just use  if  i  gt   0  amp  amp  i  lt  array length         it is in array     Now  under the hood  JavaScript engines almost certainly won t allocate array space linearly and contiguously like this  as it wouldn t make much sense in a dynamic language and it would be inefficient for certain code   They re probably hash tables or some hybrid mixture of strategies  and undefined ranges of the array probably aren t allocated their own memory   Nonetheless  JavaScript the language wants to present arrays of array length n as having n members and they are named 0 to n - 1  and anything in this range is part of the array   What you probably want  however  is to know if a value in an array is actually something defined - that is  it s not undefined   Maybe you even want to know if it s defined and not null   It s possible to add members to an array without ever setting their value  for example  if you add array values by increasing the array length property  any new values will be undefined     To determine if a given value is something meaningful  or has been defined   That is  not undefined  or null   if  typeof array index       undefined       or  if  typeof array index       undefined   amp  amp  array index      null      Interestingly  because of JavaScript s comparison rules  my last example can be optimised down to this   if  array index     null         The    and    operators consider null equal to only null or undefined

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when you create empty array values ARE NOT undefined  they are EMPTY var arr   new Array 10       10    empty    10   but when you get item by index receive undefined arr 0      undefined  so you can t know by     comparing if they are undefined or empty we can make it by using JSON stringify  converting whole array it replaces empty values with null JSON stringify arr       quot  null null null null null null null null null null  quot  JSON stringify arr 0       but for single item it returns undefined  so real check for empty value  arr 0      null  amp  amp  JSON stringify arr map item   gt   item   slice 1  -1  split      0       null    compare item with null  should be not equal  map array removing false instead existing values  ensure stringified structure to be comma-separated without redundant commas  JSON stringify array remove brackets from string split into array by comma compare with null

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if arrayName length  gt  index  amp  amp  arrayName index      null          arrayName index  has a value

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To check if it has never been defined or if it was deleted   if typeof arrayName index     undefined           the index is not in the array     also works with associative arrays and arrays where you deleted some index  To check if it was never been defined  was deleted OR is a null or logical empty value  NaN  empty string  false    if typeof arrayName index     undefined   arrayName index           the index is not defined or the value an empty value

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Short and universal approach  If you want to check any array if it has falsy values  like false  undefined  null or empty strings  you can just use every   method like this   array every function element   return   element        returns true or false   For example     23   null  2   key   value    every function element   return   element        returns false    23       2   key   value    every function element   return   element        returns false    23   true  2   key   value    every function element   return   element        returns true   If you need to get a first index of falsy value  you can do it like this   let falsyIndex    if    23   true  2  null   key   value    every function element  index   falsyIndex   index  return   element         console log falsyIndex        logs 3   If you just need to check a falsy value of an array for a given index you can just do it like this   if    array index          array index  is a correct value   else        array index  is a falsy value

User · Answer

Can t we just do this   if arrayName length  gt  0            or   if arrayName length          this array is not empty   else       this array is empty

User · Answer

Real detection  in operator This question age is about 10 years and it is surprising that nobody mention about this yet - however some people see the problem when we use delete operator  e g here   This is also a little bit counter intuitive solution but the in operator which works in  object world  can also work with arrays  because we can look on array indexes like on  keys       In this way we can detect and distinct between undefined array value and value  index  removed by delete if index in arrayName          do stuff      x000D   x000D  let arr    0  1  2  3  null  undefined 6   delete arr 2      we delete element at index 2  if 2 in arr  console log  You will not see this because idx 2 was deleted    if 5 in arr  console log  This is element arr 5     arr 5          Whole array and indexes bigger than arr length  for let i 0  i lt  9  i        let val    i in arr    arr i     empty    let bound   i lt arr length          out of range     console log    i  value     val  bound       console log  Look on below aray on chrome console  not in SO snippet console     console log  typeof arr    typeof arr   console log arr   x000D   x000D   x000D   Chrome console reveals some info about snippet array with deleted index 2 - this index actually not exists at all        same way as key is removed from object   What is also interesting here array is viewd as key-value pairs  we even see  length  key   It is also interesting that typeof arr is Object        the delete and in operator works like for JS objects  also square brackets notation arr idx  and obj key  is similar  - so it looks like array is some special JS object in the core   To get similar effect without delete define array as follows  0  1   3  null  undefined  6     pay attention to double comma   quot    quot

[javascript] How do I check in JavaScript if a value exists at a certain array index?

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