I'm using ESLint to check my javascript code and I found an warning for "Unexpected use of undefined"

The statement is

if (data.items === undefined) {...}

Data.items is equal to a JSON object normally, but under some conditions it could just be undefined and I want to take a separate action if that is the case.

To get rid of this warning, is it appropriate to use

if (data.items === false) {...}

Are these two statements equivalent under these circumstances?

I'm using ESLint to check my javascript code and I found an warning for "Unexpected use of undefined"

The statement is

if (data.items === undefined) {...}

Data.items is equal to a JSON object normally, but under some conditions it could just be undefined and I want to take a separate action if that is the case.

To get rid of this warning, is it appropriate to use

if (data.items === false) {...}

Are these two statements equivalent under these circumstances?

• javascript
• angularjs
• eslint

• 7 No, false is different than undefined. In fact, false === undefined will give you false. – Derek 朕會功夫 Commented Feb 18, 2016 at 16:37
• It seems you have this rule enabled: eslint/docs/rules/no-undefined . – Felix Kling Commented Feb 18, 2016 at 16:41
• What is the appropriate way to check for an undefined value then? – LBaelish Commented Feb 18, 2016 at 16:45
• 1 See my answer for a couple of options. If you actually want to use undefined though, you can just disable the rule. – Felix Kling Commented Feb 18, 2016 at 16:49

4 Answers 4

No, they are not equivalent. For them to be equivalent, undefined === false would have to be true. However, === performs strict equality parison, meaning that values of different data types are never equal. false is a value of the Boolean data type, undefined is the value of the Undefined data type. Hence undefined === true is false.

If you want to test for the value undefined explicitly without referencing undefined, you can use typeof:

if (typeof data.items === 'undefined')

If you want to test for the existence of the property items, you can use the in operator or .hasOwnPropery:

if (!data.hasOwnProperty('items'))
if (!('items' in data))

If know that the possible values of data.items are either undefined or an object, you can simply let JavaScript's type conversion work for you and use

if (!data.items)

If data.items is an object (including arrays) !data.items will be false. If it is undefined, it will be true.

People often remend doing this for checking the existence of a value:

if (typeof data.items === "undefined" || data.items === null)

It is worth noting that it is exactly the same as a perfectly valid:

if (data.items == null)

And the only reason that it is problematic are the linter rules created for beginners and imposed on everyone.

What the == null does is checking for null or undefined. Of course your linter will scream at you under the assumption that you cannot be trusted to understand the behavior of == and remember two values that the == null matches - namely null and undefined, the two most mon values that you often pare your data with.

When the linters started plaining, we had to bloat our code by changing all:

if (x == null)

to:

if (x === null || x === undefined)

but apparently it wasn't verbose enough and now we have to write:

if (x === null || typeof x === 'undefined')

I'm sure that some day people will figure out that it is still too short and an even more correct way will get invented. Until then this is the currently blessed way to test for existence of a value.

By the way, I don't remend using:

if (!x)

because it will match not only null and undefined but also false, 0 and the empty string. In fact I am surprised that the linters still allow this construct and haven't forced us to use this yet:

if (x === false || x === 0 || x === '' || x === null || typeof x === 'undefined')

I'm sure it's just a matter of time, though.

While if statements will evaluate the individual values (false, null, and undefined) as falsy, it doesn't hold them to be exactly the same. null and undefined can be == evaluated as true, but false is not equivalent, and even with null and undefined, === will return false.

TL;DR- No, they aren't equivalent.

Both are falsy values, but are not the same. In addition to what Felix Kling has mentioned, if you want to check the existence of a variable, I think a fairly safe way to do is how coffeescript does it:

if(typeof data.items === "undefined" || data.items === null)

Also, the way you are currently checking with undefined would try to equate it with an object called undefined. Now modern browsers have made this object immutable and set its initial value to primitive undefined, but I believe there was a time when it was mutable. In that case, if someone were to change the value of undefined (the object) your check would fail! :D