I see that noUncheckedIndexedAccess is false. Did you want to use Record<string, T|undefined>?

2023-09-27

It is common in typescript to write Record<string,T> for whatever type T you store in an object.

But, are you sure you're actually going to get an T back? What if some random user provided string is used to index the Record, and it produces undefined? You might think, as a typescripter "do I really have to add undefined to the signature, shouldn't Typescript check this for me"? Well..

#Introducing: the noUncheckedIndexedAccess tsconfig.json setting

There is actually a built-in Typescript setting called noUncheckedIndexedAccess

This setting makes the return type of accessing any Record or Array into an T|undefined.

The catch?

• This setting is turned off by default
• It is probably too "annoying" (or produces too many "false positives") for most projects to warrant it being turned on
• Hence, very few people use this setting

Therefore: you really do have to ask yourself. Do you feel lucky punk? Sorry wrong line. I mean, do you want to use a Record<string,T> or a Record<string,T|undefined>?

Enjoy! https://www.typescriptlang.org/tsconfig#noUncheckedIndexedAccess

Discussion on why this is not on by default: https://github.com/microsoft/TypeScript/issues/49169

#Footnote 1: Should noUncheckedIndexedAccess be split into two settings?

I feel like using 'untrusted input' or 'runtime values' to access a Record is more common than using 'untrusted input' to access an array index, so it would be sort of nice if noUncheckedIndexedAccess was split into two separate settings...one for Records, one for Arrays. A common complaint is that the 'array access' part of this setting is a little too strict, and requires non-null assertions

#Footnote 2: Consider using a Map instead

The Map::get method returns T|undefined, so it forces you to consider the undefined case always. I find Maps slightly more cumbersome to use than plain old objects, but has this benefit amongst others!

#Footnote 3: Small downside to Record<string, T|undefined>

Maybe you are certain there aren't actually undefined values stored in your object, and you just used Record<string, T|undefined> to be able to say obj['randomthing'] returns undefined.

Well, now calling e.g. Object.values(obj) will return (T|undefined)[] (similar for Object.entries) which is slightly annoying as a user, why may want it to just be T[] but they can non-null assert or filter out potential undefined's as a workaround.

#Footnote 4: Small upside to Record<string, T|undefined>

If you read this article and said, ya, I'm gonna use noUncheckedIndexedAccess in the tsconfig.json of my library, and you feel all safe and cozy using Record<string, T>, well your users may still have trouble: if the consumers of your library don't use the noUncheckedIndexedAccess tsconfig.json setting, then they will not get the benefit of that undefined condition unless you explicitly mark your exported types as Record<string, T|undefined>