Is array[index] === undefined the always same as typeof array[index] === 'undefined'?

Question

I need to see whether something there is an entry for an array index in Javascript and this answer say to uses (Essentially I changed it from === to !==):

if(typeof arrayName[index] !== 'undefined')

IIUC this is the same as `arrayName[index] !== 'undefined'?

I experimented with it and it works, but I want to make sure I'm not missing any edge cases?

Update

To clearify WRT to some answers given (Ran this on node 9.11.2):

    let undefined = "Hello";
    console.log(undefined);
    let arrayName = [];
    if(arrayName[0] !== undefined) {
        console.log("Test passes");
        console.log("undefined is: ", undefined);
        console.log("arrayName[0] is: ", arrayName[0]);
    }

This prints:

Hello
Test passes
undefined is:  Hello
arrayName[0] is:  undefined

So it seems the answer is "No undefined could sometimes be redefind ..." ... and it's better to stick with typeof array[index] === 'undefined', but as some have indicated, undefined cannot be redefined globally, so it should be fairly safe to use the shorter version.

Just stick with void 0 if you're worried about undefined being redefined somewhere. — camillobruni
– camillobruni, Commented Jul 5, 2018 at 17:18

Jonas Wilms · Accepted Answer · 2018-07-04 20:56:32Z

4

Yes there is a very obscure edgecase:

let undefined = "confuse me!";

that means that

 "confuse me!" === undefined

might be true and

typeof undefined === "undefined"

might be false, but if

 typeof arrayName[index] === "undefined"

is true then its definetly not defined, however it might not be undefined :)

But as this is terrible, you don't really have to consider that.

edited Jul 4, 2018 at 20:56

answered Jul 4, 2018 at 20:43

Jonas Wilms

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8 Comments

Ole Over a year ago

OK - So Javascript allows us to use Javascript undefined as a variable name, hence the recommendation is to use 'undefined' explicitly ...

Jonas Wilms Over a year ago

@ole no, that means that "confuse me!" === undefined might be true and typeof undefined === "undefined" might be false, but if typeof arrayName[index] === "undefined" then its definetly undefined

Hikmat G. Over a year ago

but this throws error 'undefined' has already been declared

jhpratt Over a year ago

@HikmatGurbanli Just tried it in Node, I'm surprised to say that's correct. Older versions that was certainly not the case.

Jonas Wilms Over a year ago

@hikmat then wrap it in a block.

|

Marcos Casagrande · Accepted Answer · 2018-07-04 20:54:20Z

1

if(typeof arrayName[index] !== 'undefined')

is the same as

if(arrayName[index] !== undefined) // without quotes

But not as you stated:

IIUC this is the same as `arrayName[index] !== 'undefined'?

if(arrayName[index] !== 'undefined') // this is not correct

How dangerous is it in JavaScript, really, to assume undefined is not overwritten?

edited Jul 4, 2018 at 20:54

answered Jul 4, 2018 at 20:45

Marcos Casagrande

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7 Comments

charlietfl Over a year ago

The first two are not the same. typeof returns string always

Marcos Casagrande Over a year ago

Yeah, my bad, copy pasted and forgot to remove typeof, edited.

DrewT Over a year ago

This is technically incorrect, they are not equivalent based on the syntax of quotes. My answer explains why.

Marcos Casagrande Over a year ago

You should read my answer again, I'm not using quotes on my second statement.

Marcos Casagrande Over a year ago

you're not correct, they're equivalent. Tell me one case where (typeof arrayName[index] !== 'undefined') !== (arrayName[index] !== undefined), you won't find unless the edge case describe by @Jonas, but you won't have that in a real world scneario

|

Brendan Gannon · Accepted Answer · 2018-07-04 20:56:55Z

1

Undefined is a global variable, and as such its value can be reassigned. So it is possible that the variable undefined could not be undefined. But typeof an undefined value will always return 'undefined', so it is technically safer. In practice, most people wouldn't reassign undefined, so either works.

answered Jul 4, 2018 at 20:56

Brendan Gannon

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