Typescript: Revisiting _ underscore for unused parameters: create `unused` type

[aryzing]

Search Terms

_, underscore, noUnusedParameters, --noUnusedParameters, tslint, ts-lint, #9458, palantir/tslint#3442

Suggestion

Create an unused type for function parameters that are not used.

Use Cases

Can be used to clearly state unused parameters, while still playing nice with --noUnusedParameters and linters enforcing variable name conventions.

May also help in cases such as

• #14154 - destructuring will fail at runtime if argument is not an object
• #18420 - type Foo<usused> = { } ?

Examples

1. Unused parameter

function typicalNodeCallback(err: unused, data: any) {
  // do stuff with `data`
}

2. Using unused parameter is an error

function expressMiddleware(req: unused, res: any) {
  const body = req.body // ERROR!
}

Checklist

My suggestion meets these guidelines:

• [ ] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [x] This wouldn't change the runtime behavior of existing JavaScript code
• [x] This could be implemented without emitting different JS based on the types of the expressions
• [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• [x] This feature would agree with the rest of TypeScript's Design Goals.

[ajafff]

I don't think adding a type solves anything. It actually loses type information: Sometimes you need to declare the type of a parameter even if you don't use is. For example in methods of a base class that are meant to be overridden.

[aryzing]

Haven't run across that scenario that often, perhaps an example might help? In any case, off the top of my head I can think of something like

myMethod(param: unused | MyType) {
//
}

If it's unused, then unused applies. Otherwise, WhateverType applies.

[ajafff]

perhaps an example might help?

interface Options {}
class Base {
  constructor(options: Options) {
    this.init(options);
  }
  init(_options: Options) {
    // can be overridden by a subclass
    // the parameter needs to be there so the method in the subclass knows that parameter is available
  }
}

If it's unused, then unused applies. Otherwise, WhateverType applies.

That could be very confusing if that unused "type" magically disappears. I guess a type annotation is not the right place to declare something as unused.
And then we're back to searching a sigil to declare something as unused. How about prefixing with an underscore?

[aryzing]

It's hard to talk about what's right or wrong in something that we're making up as we go. Instead it might be of more value to the community to consider things such as convenience, playing well with other tools, and separation of concerns between syntax and compiler logic.

As far as ✨ magic ✨ goes, subjective as it may be, seems no more magical than a leading underscore.

Moreover, the issue for this feature was opened and merged in less than 24 hours, leaving little room for community feedback and a proper discussion during its inception, while the merge request itself has 50% of :-1: . Once merged, several different points and propositions were made regarding issues we could face down the line, which we now are.

In any case, the objective of this issue is not to push the specific syntax I've proposed above, but rather to ensure that we find a lasting and viable solution to unused parameters. In terms of playing well with other tools and separation of concerns, I do believe my proposal is a step in the right direction. Convenience, again, is subjective, so we may yet find a better solution!

[MartinJohns]

[x] This wouldn't be a breaking change in existing TypeScript/JavaScript code

As unused is not a reserved keyword currently, this most definitely would be a breaking change.

Can be used to clearly state unused parameters, while still playing nice with --noUnusedParameters and linters enforcing variable name conventions.

I don't see this as a reason enough to add another keyword/type. Having a _ prefix on the variable name works nice with --noUnusedParameters. And if your linter complains, then you should configure your linter to accept this very valid use case. And it being represented as a type will likely open a can of worms.

The way you have your third-party tools configured should not affect the development of a language.

[aryzing]

would be a breaking change.

Thanks, updated original post :) In any case, the impact would be very low. For example, in the entire DefinitelyTyped repo there are only three instances of unused being used.

Having a _ prefix on the variable name works nice with --noUnusedParameters.

Perhaps I should have expressed myself better: it only works nice with --noUnusedParameters. The point I was trying to make is that it would be better if we found a more universal solution, one confined within TypeScript's additions to the language and that leaves no trace when typings are removed.

As per point 9 of the Design Goals, Use a consistent, fully erasable, structural type system. If the type system introduces an underscore "idiom" in order for it to work properly when one of its options is activated (--noUnusedParameters) and just leaves it there when the types are stripped, then I'd say that does not conform to the project goals: the type system is not fully erasable.

Moreover, coming back to the ✨magic✨ discussion we were having earlier, the underscore would probably go against point 7 of Non Goals: Introduce behaviour that is likely to surprise users. Instead have due consideration for patterns adopted by other commonly-used languages. While the pattern may be common, having variable names linked to compiler options for the type system is certainly surprising.

if your linter complains,

The way you have your third-party tools configured should not affect the development of a language.

And that is by no means the angle I'm going for here. I appreciate that you care about my particular dev environment, but rest assured, it is working quite fine. The idea behind this issue is not to discuss the particulars of how this feature interacts with a particular dev environment or tool, but rather to recognize that there is room for improvement.

Seeing that this feature was introduced in less than a day and with a non-negligible amount of controversy, we have more than enough grounds for at least revisiting it. I'm not saying that my particular solution be the one we go for, it's just a starting point.

[AnyhowStep]

The name of a function parameter is not part of its type, or signature.

declare function foo (a : number) : void;
declare function bar (_a : number) : void;

The above are the same type, and those types do get erased. Parameter names just aren't part of a function's type.

[weswigham]

The idea behind this issue is not to discuss the particulars of how this feature interacts with a particular dev environment or tool, but rather to recognize that there is room for improvement.

Any "improvement" can only be recognized in some context... that's why you keep getting asked. Nobody wants a change for change's sake - every change we make is targeted at satisfying particular use-cases or issues. I don't see us adding some new type variant just for tracking the suppression of the noUnusedParameter warning - it doesn't even behave like a real type. Types flow from point A to point B in a program along with values assigned that type - this marker would do... nothing... other than suppress an error... which is something we already have the ability to do. You can use the convention-based suppression (_ name prefix) or the comment-based suppression (@ts-ignore).

Unless you can present an actual reason why the other options available fail in some context and why adding behavior to TS is the best way to improve this scenario in that context, I don't see us doing anything like this in the near future. Plus, enumerating that context lets us explore likely more fruitful alternatives to alleviate whatever inconvenience prompted this.

[Qix-]

At the very least, allow us to use _ more than once, or some way to indicate that a parameter is unused, similar to C's (void) foo;.

I run into this all the time - where I'm adhering to some interface or abstract method and I just _don't need_ one of the parameters that is being passed down.

Linters for Javascript have the logic that reports any unused parameters _after_ the last used parameter. For example, if function (a, b, c, d, e) uses a and c, then only d and e are reported as unused since b is required to be there in order to have proper parameter ordering.

It's not clear how to work around this in Typescript. While I agree that unused as a type isn't the best way, having some means to signify that a parameter is unused would be nice.

[pronebird]

@aryzing in Electron, event handles are the case where the Event itself can be useless if you don't wish to send something back. I believe same applies to DOM events in many cases.

const handleEvent = (_ipcEvent: Electron.Event, result: string) => { // tslint error
 /// code
}

[RyanCavanaugh]

?

type unused = unknown;

const x = (_event: unused, result: string) { 

[Qix-]

After revisiting this I still think allowing multiple underscores is a good thing. It cuts down on noise and immediately indicates intent to ignore while keeping types intact, which is important for a variety of reasons.

[InnanYuan]

My solution: prefix unused parameter name with "_unused" (and optionally with camelCasing).

function handleEvent(_unusedEvent: Event, result: string) { ... }

Two rationales behind this:

1. Follow "explicit is better than implicit" rule from Python Zen.
2. No changes needed in existing language or compiler features. Existing "_" prefixing solution is enough.

[mjackson]

At the very least, a function parameter should not be considered to be "unused" if a parameter that comes after it is used. In that case, the "unused" parameter is actually serving a purpose by taking up a slot in the argument list.

function fun(a, b) {
  // The `a` parameter should not be considered "unused"
  console.log(b);
}

[infacto]

Why not just detect that b is used and don't claim a as unused. (Without any other changes line underscore or special unused type.) Because that's just logical. This should be automatically detected by TypeScript (which is already done. But don't claim it. Just inform on hover.) A "unused" type is not a good idea in my opinion. And underscore may be left by humans. So also not the best solution. But if I see that correctly, it's currently not a warning or error. So it's ok, isn't? It's just grayed-out (not underlined / marked) in vscode with a information about unused variable on hover. Which is a correct message. But neither the compiler nor the linter must fail here. It works correctly at this moment. So no need for changes. Or did I miss something?
At most maybe just don't consider the parameter a as unused if b is used. No message about, not grayed-out (maybe a vscode issue). But nothing critical I think.

[aryzing]

Seems @mjackson hit the nail on the head -- I take my suggestion back. If the compiler can be tweaked to work as described above, that may be all we need.