how to prevent adding duplicate keys to a javascript array

Question

I found a lot of related questions with answers talking about for   in loops and using hasOwnProperty but nothing I do works properly  All I want to do is check whether or not a key exists in an array and if not  add it   I start with an empty array then add keys as the page is scrubbed with jQuery   Initially  I hoped that something simple like the following would work   using generic names   if   array key      array key    value    No go  Followed it up with   for  var in array       if   array hasOwnProperty var         array key    value      Also tried   if  array hasOwnProperty key     false     array key    value    None of this has worked  Either nothing is pushed to the array or what I try is no better than simply declaring array key    value Why is something so simple so difficult to do  Any ideas to make this work

User · Accepted Answer

Generally speaking  this is better accomplished with an object instead since JavaScript doesn t really have associative arrays   var foo     bar  0      Then use in to check for a key   if       bar  in foo           foo  bar     42      As was rightly pointed out in the comments below  this method is useful only when your keys will be strings  or items that can be represented as strings  such as numbers

User · Answer

The logic is wrong  Consider this   x     a   b   c   x 0          a  x  0         a  0 in x      true  0  in x    true x hasOwnProperty 0       true x hasOwnProperty  0      true   There is no reason to loop to check for key  or indices for arrays  existence  Now  values are a different story     Happy coding

User · Answer

You can try this   var names     Mike   Matt   Nancy   Adam   Jenny   Nancy   Carl    var uniqueNames         each names  function i  el   if   inArray el  uniqueNames      -1  uniqueNames push el         Easiest way to find duplicate values in a JavaScript array

User · Answer

var a    1 2 3   b    4 1 5 2    b forEach function value     if  a indexOf value   -1  a push value        console log a       1  2  3  4  5    For more details read up on Array indexOf   If you want to rely on jQuery  instead use jQuery inArray     each b function value     if    inArray value a   -1  a push value         If all your values are simply and uniquely representable as strings  however  you should use an Object instead of an Array  for a potentially massive speed increase  as described in the answer by  JonathanSampson

User · Answer

function check  list       var foundRepeatingValue   false      var newList           for i 0 i lt list length i             var thisValue   list i           if i gt 0               if newList indexOf thisValue  gt -1                   foundRepeatingValue   true                  console log  getting repeated                    return true                         newList push thisValue         return false       nbsp   var list1     dse   dfg   dse    check list1     Output    getting repeated true

User · Answer

A better alternative is provided in ES6 using Sets  So  instead of declaring Arrays  it is recommended to use Sets if you need to have an array that shouldn t add duplicates   var array   new Set    array add 1   array add 2   array add 3    console log array      Prints  Set 3   1  2  3   array add 2      does not add any new element  console log array      Still Prints  Set 3   1  2  3

[javascript] how to prevent adding duplicate keys to a javascript array

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