Javascript array value is undefined how do I test for that

Question

I am trying to test to see whether a Javascript variable is undefined   You will see that I am not expecting the value of predQuery preId  to be  undefined  if I don t first get an alert saying  its unbelievable   But I often do  so I am guessing that my statement   predQuery preId    undefined      is not matching the undefined elements properly   if  predQuery length  lt  preId      predQuery preId           predQuery preId    undefined        alert  its unbelievable       alert predQuery preId       queryPreds variables    preId     queryObjs variables    objId     predQuery preId    variables    else       alert predQuery preId       var predIndex   predQuery preId      queryPreds predIndex    preId     queryObjs predIndex    objId      I can add more code if needed

User · Answer

try  typeof predQuery preId     undefined  or more generally  typeof yourArray yourIndex     undefined  You re comparing  undefined  to undefined  which returns false

User · Answer

Check for  if  predQuery preId      undefined    Use the strict equal to operator  See comparison operators

User · Answer

predQuery preId    undefined    You re testing against the string  undefined   you ve confused this test with the typeof test which would return a string  You probably mean to be testing against the special value undefined   predQuery preId    undefined   Note the strict-equality operator to avoid the generally-unwanted match null  undefined   However there are two ways you can get an undefined value  either preId isn t a member of predQuery  or it is a member but has a value set to the special undefined value  Often  you only want to check whether it s present or not  in that case the in operator is more appropriate     preId in predQuery

User · Answer

There are more  many  ways to Rome       gt considering predQuery preId  is undefined  predQuery preId      undefined      gt  true undefined     predQuery preId      gt  true predQuery preId      it  s unbelievable       gt  it s unbelievable var isdef   predQuery preId    predQuery preId    null     gt  isdef   null   cheers

User · Answer

array index      undefined  compares the value of the array index to the string  undefined   You re probably looking for typeof array index      undefined   which compares the type

User · Answer

You are checking it the array index contains a string  undefined   you should either use the typeof operator   typeof predQuery preId      undefined    Or use the undefined global property   predQuery preId      undefined   The first way is safer  because the undefined global property is writable  and it can be changed to any other value

User · Answer

This code works very well  function isUndefined array  index        return   String array index       undefined      Yes     No

[javascript] Javascript array value is undefined ... how do I test for that

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