Rule proposal: no-void-return-type

[remcohaszing]

Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• My proposal specifically checks TypeScript syntax, or it proposes a check that requires type information to be accurate.
• My proposal is not a "formatting rule"; meaning it does not just enforce how code is formatted (whitespace, brace placement, etc).
• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

The void type in TypeScript shouldn’t exist, because it represents something that doesn’t exist in JavaScript. microsoft/TypeScript#42709 explains it in more depth.

There is are the @typescript-eslint/no-invalid-void-type and @typescript-eslint/no-meaningless-void-operator rules. I would like to propose the @typescript-eslint/no-void-return-type rule to disallow returning void, Promise<void>, and PromiseLike<void>.

Fail Cases

// This function returns undefined
function a(): void {}

// This function returns undefined
function b() {}

// This probably types a function that returns undefined
declare function c(): void

// This function returns a promise that resolves to undefined
async function d(): Promise<void> {}

// This function returns a promise that resolves to undefined
declare function e(): Promise<void>

// This function returns a promise-like that resolves to undefined
declare function e(): PromiseLike<void>

// If void is used as anything but a return value, it’s invalid anyway
type G = string | void

Pass Cases

// This function returns undefined
function a(): undefined {}

// This function returns undefined
function b(): undefined {}

// This probably types a function that returns undefined
declare function c(): undefined

// This function returns a promise that resolves to undefined
async function d(): Promise<undefined> {}

// This function returns a promise that resolves to undefined
declare function e(): Promise<undefined>

// This function returns a promise-like that resolves to undefined
declare function e(): PromiseLike<undefined>

Additional Info

TypeScript 5.1 loosened the rules for void/undefined, making it viable to stop using void.

This rule could be auto-fixable by replacing a return type of void with undefined, or adding an explicit return type if none of the branches return an explicit value.

It would also just report any usage of void.

[bradzacher]

I don't personally agree with this, but regardless we don't look to making this change to ban features without really good reasons or significant value we can add.

These sorts of rules that ban features are always quite opinionated and thus are very divisive - it's not something we really want to get into now-a-days.

https://typescript-eslint.io/linting/troubleshooting/#how-can-i-ban-specific-language-feature

example playground

[remcohaszing]

Thanks for the tip to use no-restricted-syntax. It doesn’t catch implicit void return types, but it’s sufficient for my use case.