return-undefined false positive

[cartant]

Bug Report

• TSLint version: 5.3.0
• TypeScript version: 2.3.3
• Running TSLint via: CLI

TypeScript code being linted

export function t(p: (a: any) => any): void {}
t((a) => { return undefined; });

with tslint.json configuration:

{
  "rules": {
    "return-undefined": { "severity": "error" }
  }
}

Actual behavior

ERROR: ...: `void` function should use `return;`, not `return undefined;`.

Expected behavior

No error, as the function parameter is typed as returning any - not void.

Prior to updating to the latest version of TSLint, the configuration did not see an error effected. No error is effected if 5.2.0 is used.

[adidahiya]

looks like a direct result of #2731. notice the test baseline that got changed: https://github.com/palantir/tslint/pull/2731/files#diff-bd90f0718fe7a8725e9db572ef333339R36

@cartant can you provide a bit more context around the code your'e trying to lint here? should it really be typed any?

... either way, this behavior is kind of surprising, so we should probably revert that part of the rule change @andy-hanson

[cartant]

@adidahiya Sure. The typings aren't mine and any is reasonably appropriate for the usage.

It's a transactionUpdate parameter in the Firebase JavaScript client:

transaction (
  transactionUpdate : (a : any ) => any ,
  onComplete ? : (
    a : Error | null ,
    b : boolean ,
    c : firebase.database.DataSnapshot | null
  ) => any ,
  applyLocally ? : boolean
) : firebase.Promise < any > ;

I need to implement that interface in a mock.

[cartant]

@adidahiya And the code that fails the linting is the test for the mock:

return mockRef
    .transaction((value) => {

        return undefined;
    })
    .then(({ committed, snapshot }) => {

Firebase transactions return undefined if nothing is updated.

[cartant]

Seeing as though this is a breaking change, is it going to be considered a bug? Or should I implement the trivial workaround that'll be required if this is indeed how you want the rule to work?

[adidahiya]

I consider it a bug. Will label it as such.