TS7018 error in generated typescript

[bagley2014]

In the generated typescript there's a function m that gives a compiler error.

function m(additional: any) {
    return { props: [], additional };
}

The error is error TS7018: Object literal's property 'props' implicitly has an 'any[]' type.

Someone else may have had a similar issue (#1437) but their post is fairly vague. I created bagley2014/quicktype-bug-repro to make it easy for others to reproduce the error. npm install, npm run quicktype , and npm run build should repro the build error.

[schani]

Triage sweep note (systematic issue-backlog triage, assisted by Claude): confirmed reproducible on current master (v23.3.0, 68ecb284).

Any TS output that needs the m() "map/additional-properties" runtime helper (e.g. an object with only an untyped/dictionary-like nested object) emits:

function m(additional: any) {
    return { props: [], additional };
}

Compiling with tsc --noImplicitAny --noEmit (and strictNullChecks: false, matching the reporter's tsconfig.json) gives:

error TS7018: Object literal's property 'props' implicitly has an 'any[]' type.

Verified with typescript@5.8.3 against quicktype's own output for a minimal sample.

Root cause: packages/quicktype-core/src/language/JavaScript/JavaScriptRenderer.ts:467-468, the m() helper template emits a bare [] for props, which TypeScript can't widen to never[]/any[] under noImplicitAny unless strictNullChecks is also on (under full --strict it infers never[] and compiles). The sibling o() helper's props parameter is typed, but m()'s literal isn't annotated.

Assessment

• Severity: medium — narrow but common compiler config (noImplicitAny: true, strictNullChecks: false), and it's a hard compile error, not a warning.
• Difficulty: easy — annotate the literal, e.g. props: [] as any[] (TS-mode only), mirroring the existing anyAnnotation substitution mechanism already used elsewhere in this renderer.

Reproduction:

$ cat sample.json
{"name": "hello", "values": {"a": 1}}

$ quicktype -s json sample.json -o out.ts
$ tsc --noImplicitAny --noEmit out.ts
out.ts(176,14): error TS7018: Object literal's property 'props' implicitly has an 'any[]' type.