Javascript Array Value Is Undefined ... How Do I Test For That

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

Solution 1:

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.

Solution 2:

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.

Solution 3:

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)

Solution 4:

There are more (many) ways to Rome:

//=>considering predQuery[preId] is undefined:
predQuery[preId] === undefined; //=> trueundefined === predQuery[preId] //=> true
predQuery[preId] || 'it\'s unbelievable!'//=> it's unbelievablevar isdef = predQuery[preId] ? predQuery[preId] : null//=> isdef = null

cheers!

Solution 5:

Check for

if (predQuery[preId] === undefined)

Use the strict equal to operator. See comparison operators