Check if an array item is set in JS

Question

I ve got an array      var assoc pagine   new Array        assoc pagine  home   0      assoc pagine  about   1      assoc pagine  work   2    I tried       if  assoc pagine var      undefined       but it doesn t seem to work  I m using jquery  I don t know if it can help  Thanks

User · Answer

This worked for me  if  assoc pagine var     undefined      instead this  if  assoc pagine var      undefined

User · Answer

TLDR  The best I can come up with is this   Depending on your use case  there are a number of ways to optimize this function    function arrayIndexExists array  index       if   typeof index      number   amp  amp  index     parseInt index  toString              index   parseInt index         else           return false   to avoid checking typeof again           return typeof index      number   amp  amp  index   1   0  amp  amp  index  gt   0  amp  amp  array hasOwnKey index       The other answer s examples get close and will work for some  probably most  purposes  but are technically quite incorrect for reasons I explain below   Javascript arrays only use  numerical  keys  When you set an  associative key  on an array  you are actually setting a property on that array object  not an element of that array  For example  this means that the  associative key  will not be iterated over when using Array forEach   and will not be included when calculating Array length   The exception for this is strings like  0  will resolve to an element of the array  but strings like   0  won t    Additionally  checking array element or object property that doesn t exist does evaluate as undefined  but that doesn t actually tell you that the array element or object property hasn t been set yet  For example  undefined is also the result you get by calling a function that doesn t terminate with a return statement  This could lead to some strange errors and difficulty debugging code   This can be confusing  but can be explored very easily using your browser s javascript console   I used chrome  each comment indicates the evaluated value of the line before it     var foo   new Array    foo       foo length    0 foo  bar      bar      bar  foo       foo length    0 foo bar     bar    This shows that associative keys are not used to access elements in the array  but for properties of the object   foo 0    0    0 foo     0  foo length    1 foo 2    undefined   undefined typeof foo 2     undefined  foo length   3   This shows that checking typeof doesn t allow you to see if an element has been set   var foo   new Array      undefined foo       foo 0    0    0 foo  0     0 foo   0     undefined   This shows the exception I mentioned above and why you can t just use parseInt     If you want to use associative arrays  you are better off using simple objects as other answers have recommended

User · Answer

if  assoc pagine indexOf  home    gt  -1          we have home element in the assoc pagine array     Mozilla indexOf

User · Answer

function isset key   ret   false  array example forEach function entry      if  entry    key        ret   true          return ret     alert  isset  key search

User · Answer

The most effective way   if  array indexOf element   gt  -1       alert  Bingooo       W3Schools

User · Answer

var assoc pagine   new Array    assoc pagine  home   0    Don t use an Array for this  Arrays are for numerically-indexed lists  Just use a plain Object        What you are thinking of with the  undefined  string is probably this   if  typeof assoc pagine key     undefined     This is  more or less  the same as saying  if  assoc pagine key    undefined    However  either way this is a bit ugly  You re dereferencing a key that may not exist  which would be an error in any more sensible language   and relying on JavaScript s weird hack of giving you the special undefined value for non-existent properties   This also doesn t quite tell you if the property really wasn t there  or if it was there but explicitly set to the undefined value   This is a more explicit  readable and IMO all-round better approach   if  key in assoc pagine

User · Answer

This is not an Array   Better declare it like this   var assoc pagine       assoc pagine  home   0  assoc pagine  about   1  assoc pagine  work   2    or  var assoc pagine                      home 0                   about 1                   work 2                     To check if an object contains some label you simply do something like this   if  work  in assoc pagine         do your thing

User · Answer

Use the in keyword to test if a attribute is defined in a object  if  assoc var in assoc pagine    OR  if   home  in assoc pagine      There are quite a few issues here    Firstly  is var supposed to a variable has the value  home    work  or  about   Or did you mean to inspect actual property called  var    If var is supposed to be a variable that has a string value  please note that var is a reserved word in JavaScript and you will need to use another name  such as assoc var   var assoc var    home   assoc pagine assoc var     equals 0 in your example   If you meant to inspect the property called  var   then you simple need to put it inside of quotes   assoc pagine  var     Then  undefined is not the same as  undefined   You will need typeof to get the string representation of the objects type    This is a breakdown of all the steps    var assoc var    home    var value   assoc pagine assoc var      0 var typeofValue   typeof value      number    So to fix your problem  if  typeof assoc pagine assoc var      undefined        update  As other answers have indicated  using a array is not the best sollution for this problem  Consider using a Object instead   var assoc pagine   new Object    assoc pagine  home   0  assoc pagine  about   1  assoc pagine  work   2

User · Answer

var is a statement    so it s a reserved word    So just call it another way  And that s a better way of doing it      is better than      if typeof array name       undefined         alert  Has var      else       alert  Doesn t have var

[javascript] Check if an array item is set in JS

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