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

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
• 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.

[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.

[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);
}