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Optional local variables #41901
Description
Suggestion
TypeScript already supports optional parameters and fields with a question mark.
Why not extend this feature to local variables as well, to allow them having an undefined value, e.g.
Use Cases/Examples
let optional?: number = 123
optional = undefined // allowed
in this case specifying number type is also optional:
let optional? = 123
This would complete the usage of question mark ? as nullablity/unefineability operator, similar to Kotlin.
Currently, defining an optional variable requires writing a lot of noise, e.g.
let optional: number|undefined = undefined // too verbose
Checklist
My suggestion meets these guidelines:
- This wouldn't be a breaking change in existing TypeScript/JavaScript code
- This wouldn't change the runtime behavior of existing JavaScript code
- This could be implemented without emitting different JS based on the types of the expressions
- This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
- This feature would agree with the rest of TypeScript's Design Goals.
Activity
MartinJohns commentedon Dec 9, 2020
But the variable in your example is not optional. It's always defined. You're confusing optional with "may be undefined".
angryziber commentedon Dec 9, 2020
Martin Johns (@MartinJohns) I am not confusing - that's exactly what I want. No matter how you name it, it would extend the already existing syntax and make it useful for local variables as well. You already can use it for class variables aka members/properties, so why not local as well
MartinJohns commentedon Dec 9, 2020
It would complicate the implementation of #13195, if it ever is to be added. Optional properties and arguments can be missing, not only
undefined, which is a relevant difference.RyanCavanaugh commentedon Dec 10, 2020
I don't think it makes sense to put the
?on the binding name; it's confusing. You wouldn't writelet array[]; the| undefinedis part of the type, not the variable. So the logical thing would benumber?, which we discussed to death when addingstrictNullChecks(since it might meannumber | null,number | undefined, ornumber | null | undefined) and decided to just not do since different people would rightly declare in a split plurality that it's "obvious" which one of those it should mean and be angry when 66% of them were surprised.hannah23280 commentedon Dec 11, 2023
I also support let name: string? to be equivalent to let name:string|undefined|null.
So as of current latest version, is this actually implemented?
MartinJohns commentedon Dec 11, 2023
Including
nullwould just be plain confusing, because optional properties and parameters do not includenull.No, and it (most likely) never will be implemented. The feature has been declined.
hannah23280 commentedon Dec 11, 2023
Wonder why is this a bad thing, such that it is not implemented. verbosity can be greatly reduced if implemented. I mean if using question mark to represent undefined|null can cause confusion, then other characters can be considered.
malthe commentedon Feb 2, 2024
I think this is a great idea.
It's obvious that
resultat this point isundefined. I don't think anyone in their right mind would think that it'snullfor example.Now, Martin Johns (@MartinJohns) argued that supporting this would complicate #13195 – meanwhile, on Jun 1, 2021 that issue was actually resolved.
Does this mean that in principle, TypeScript could support this syntax?
I totally agree with Anton Keks (@angryziber) on this:
And there's good code to be written that uses this syntax. In the example above, we can imagine that there is no result yet, neither success or failure.