undefined[] converted to any[] in js

[awerlogus]

TypeScript Version: 4.2.0-dev.20201222

Search Terms: undefined any array

Code
In .ts everything is ok:

const a = [1, 2, undefined, 3]

const isUndef = (v: unknown): v is undefined => v === undefined

// OK: undefined[]
const b = a.filter(isUndef)

.js:

const a = [1, 2, undefined, 3]

/** @type {(v: unknown) => v is undefined} */
const isUndef = v => v === undefined

// Expected: undefined[]
// Actually: any[]
const b = a.filter(isUndef)

Expected behavior:
Variable b has type undefined[]

Actual behavior:
Filter method returns undefined[], but variable b has any[] type

[MartinJohns]

Interestingly the playground gets it right with strictNullChecks disabled (when it shouldn't), but gets it wrong with strictNullChecks enabled (when it should get it right).

Playground with strictNullChecks enabled
Playground with strictNullChecks disabled

[awerlogus]

@DanielRosenwasser, reopen this. I found related broken example.

Version 4.3.0-dev.20210228

/** @type {() => undefined} */
function a () { return undefined }

// Actually: any
// Expected: undefined
const b = a()

[DanielRosenwasser]

In the future it's easier for us to just create a new issue and reference the old one.