Javascript array value is undefined … how do I test for that [duplicate]

问题

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.

回答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.

回答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.

回答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)

回答4:

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!

回答5:

Check for

if (predQuery[preId] === undefined)

Use the strict equal to operator. See comparison operators

回答6:

try: typeof(predQuery[preId])=='undefined'
or more generally: typeof(yourArray[yourIndex])=='undefined'
You're comparing "undefined" to undefined, which returns false =)

回答7:

This code works very well

function isUndefined(array, index) {
    return ((String(array[index]) == "undefined") ? "Yes" : "No");
}

来源：https://stackoverflow.com/questions/2673147/javascript-array-value-is-undefined-how-do-i-test-for-that

标签

javascript

arrays

undefined