Javascript array value is undefined ... how do I test for that
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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.
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.
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)
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