javascript - Why is !== "" is not equal to !== null - Stack Overflow

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Recently in JavaScript i have picked up the habit of using

if(data !== "")

to check if the data is null || undefined || blank string.

When i try to use

if(data !== null)
   //work here

Resharper throws a horrible error at me saying that the

"qualifier can be null or undefined"

I have added a jFiddle just to clarify : JsFiddle

My question is: Is this a ReSharper error or is there something behind this ?

Recently in JavaScript i have picked up the habit of using

if(data !== "")

to check if the data is null || undefined || blank string.

When i try to use

if(data !== null)
   //work here

Resharper throws a horrible error at me saying that the

"qualifier can be null or undefined"

I have added a jFiddle just to clarify : JsFiddle

My question is: Is this a ReSharper error or is there something behind this ?

• javascript
• resharper

Share Improve this question Follow edited Oct 22, 2017 at 2:01 101 8,96966 gold badges4646 silver badges7171 bronze badges asked Jun 25, 2014 at 16:20 PogrindisPogrindis 8,09155 gold badges3333 silver badges4444 bronze badges 8

• 3 If you want to check for an falsey value, why not just use if (data) {} – jfriend00 Commented Jun 25, 2014 at 16:22
• the if(data) will not work correctly for older versions of IE, if data is undefined IE7 i believe will error up – Pogrindis Commented Jun 25, 2014 at 16:24
• 1 @jfriend00 there's a slight difference for empty arrays [] != "" -> false but !![] -> true. – p.s.w.g Commented Jun 25, 2014 at 16:25
• 1 @p.s.w.g - If you want to test for an empty array, then test for an empty array. An empty array isn't a falsey value and testing for !== "" is no way to test for an empty array either as that doesn't distinguish between an empty array and a full array so I really don't get your point. I wasn't saying that if (data) is equivalent to what the OP was using, just that it's probably a better way of testing a truthy/falsey value which is what it sounds like the OP actually wanted. – jfriend00 Commented Jun 25, 2014 at 16:28
• Perhaps the question needs more context.. data in this case is an object, is there no universal nullcheck for all objects, variable type and arrays ? – Pogrindis Commented Jun 25, 2014 at 16:30

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4 Answers 4

Sorted by: Reset to default 14

First of all, your habit is wrong. Using:

if(data !== "")

Will only check for an empty string. If data is undefined or null, the if block will still execute because !== checks for equality without performing any type conversions.

Secondly, ReSharper does not have an issue. It's trying to tell you that you may be making a mistake. if(data !== null) will only check against null. undefined and an empty string will still return true and cause the block to execute. ReSharper is warning you that you may be making a mistake (because rarely do you ever need to check for just null).

Just remember that undefined !== null !== "". You could attempt several of the shortcuts that are being mentioned but if you really want your code to be thorough, just check all three. If you're worried about code being too long, move the check to a utility method:

function hasValue(var data) {
    return (data !== undefined) && (data !== null) && (data !== "");
}

In short, a var is null when it's not pointing anywhere. In the other hand, a var equal to "" is a defined var pointing to a variable which contains an empty string. That's essentially different.

[EDIT] As @jfriend00 correctly points out, null is a specific value in javascript. The difference would be then that null is a different value than "", which is an empty string and therefor not null.

The correct value for a var which is not initialized is undefined.

Null and undefined can be treated in the same way

if (typeof data === "undefined") will return true if the data is undefined (or null), so

if (typeof data !== "undefined")

will return true if the data has been defined (so is not null), then you can check if it is an empty string,

if (data.length == 0)

will return true if the string is empty.

Is this a ReSharper error or is there something behind this ?

The error might be thrown because of a linter in resharper.

To check if the data is null || undefined || blank string, you could coherce the data value to a boolean (implicitly done by js):

if(!data) //implicit coercion

or do it explicitly:

if(!Boolean(data)){ //same as above
//if data is a falsy value (false, 0, null , undefined, NaN, '' or ""), run
}

You can check the list of truthy and falsy values here

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