[typescript-fetch] Fix oneOf with mixed array types causing TypeScript compile error

[jasperpatterson]

When a oneOf schema contains both complex array types (e.g. Array<SomeObject>) and primitive array types (e.g. Array<string>), the generated TypeScript code fails to compile:

error TS2339: Property 'every' does not exist on type 'never'.

The generated ToJSON/FromJSON functions handle complex array members inside an if (Array.isArray(value)) { ... return value; } block that always returns for arrays. TypeScript's control flow analysis then narrows the type to exclude arrays, making subsequent Array.isArray() checks for primitive array members unreachable — the type becomes never.

Example schema that triggers the bug:

ExampleType:
  oneOf:
    - type: array
      items:
        $ref: "#/components/schemas/SomeArrayType"
    - type: array
      items:
        type: string
    - type: boolean
    - type: number
    - type: string

Fix

• Remove the early return from inside the oneOfArrays Array.isArray() block so TypeScript doesn't narrow the type after it
• Add a separate array catch-all (if (Array.isArray()) { return value; }) after all primitive array checks, preserving the fallback behavior
• Fix a syntax bug (missing closing paren) in the isDateType array ToJSON template

Before

// Always returns for arrays → TypeScript narrows to non-array after this
if (Array.isArray(value)) {
    if (value.every(item => typeof item === 'object')) { /* complex types */ }
    return value;
}
// Unreachable — type is 'never' here
if (Array.isArray(value)) {
    if (value.every(item => typeof item === 'string')) { return value; }
}

After

// Doesn't always return → no narrowing
if (Array.isArray(value)) {
    if (value.every(item => typeof item === 'object')) { /* complex types */ }
}
// Reachable
if (Array.isArray(value)) {
    if (value.every(item => typeof item === 'string')) { return value; }
}
// Catch-all for unrecognized arrays
if (Array.isArray(value)) {
    return value;
}

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---

Summary by cubic

Fixes a TypeScript compile error (TS2339: 'every' on 'never') in generated typescript-fetch clients when a oneOf mixes complex and primitive array types. Adjusts control flow so primitive array checks are reachable and arrays fall back safely.

• Bug Fixes
  • Remove early return from Array.isArray(...) in FromJSON/ToJSON for oneOf arrays; add a final array catch-all after primitive array checks.
  • Fix a missing closing parenthesis in the array isDateType ToJSON template.
  • Regenerated sample TestArrayResponse.ts to reflect the new control flow.

^{Written for commit ac1d613. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

[wing328]

cc @TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02) @davidgamero (2022/03) @mkusaka (2022/04) @joscha (2024/10) @dennisameling (2026/02)

[macjohnny]

thanks for the fix!

[macjohnny]

@jasperpatterson can you merge in the latest master branch?

[jasperpatterson]

@macjohnny done