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 < 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.
This question is related to
javascript
arrays
undefined
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)
There are more (many) ways to Rome:
//=>considering predQuery[preId] is undefined:
predQuery[preId] === undefined; //=> true
undefined === predQuery[preId] //=> true
predQuery[preId] || 'it\'s unbelievable!' //=> it's unbelievable
var isdef = predQuery[preId] ? predQuery[preId] : null //=> isdef = null
cheers!
try: typeof(predQuery[preId])=='undefined'
or more generally: typeof(yourArray[yourIndex])=='undefined'
You're comparing "undefined" to undefined, which returns false =)
This code works very well
function isUndefined(array, index) {
return ((String(array[index]) == "undefined") ? "Yes" : "No");
}
Check for
if (predQuery[preId] === undefined)
Use the strict equal to operator. See comparison operators
Source: Stackoverflow.com