[FEATURE]: enable --just-types for --lang javascript

[vincesp]

Add --just-types support for --lang javascript to generate JSDoc-only output

Context (Input, Language)

Input Format:
JSON Schema

Output Language:
JavaScript (with JSDoc)

Description

quicktype already supports --just-types for TypeScript, which generates clean, type-only output without any runtime conversion or validation code. This is extremely useful when the goal is to use types purely for tooling (IntelliSense, static checking, documentation) and not for runtime parsing.

For JavaScript users who rely on JSDoc for typing (often combined with // @ts-check), there is currently no equivalent. Generating JavaScript output always produces a large amount of runtime conversion / validation code, even when only type information is desired.

Adding --just-types support to --lang javascript would enable quicktype to generate pure JSDoc typedefs without any executable code.

Current Behaviour / Output

When running:

quicktype --lang javascript --src schema.json

quicktype generates:

• runtime conversion functions
• validation helpers
• parsing / serialization logic
• no JSDoc comments

This output is overly complex and unsuitable for users who only want:

• JSDoc @typedef declarations
• zero runtime footprint
• simple, readable type definitions

Proposed Behaviour / Output

When running:

quicktype --lang javascript --just-types --src schema.json

quicktype should generate:

• only JSDoc type definitions
• no functions
• no runtime validation
• no parsing / serialization helpers

Example output:

/**
 * @typedef {Object} User
 * @property {string} name
 * @property {string} email
 */

This would mirror the existing --just-types behavior for TypeScript, but adapted to idiomatic JSDoc.

Solution

Extend the existing --just-types implementation so that:

• it is accepted for --lang javascript
• it suppresses all runtime code generation
• it emits only JSDoc @typedef blocks for all generated types

Internally, this could reuse the same model as TypeScript --just-types, but render JSDoc instead of TS interfaces.

Alternatives

• Generating TypeScript .d.ts files and importing them via JSDoc import() works, but:
  • introduces an extra language step
  • is less direct than native JSDoc output
• Post-processing quicktype’s JavaScript output to strip runtime code is brittle and error-prone
• Other tools (e.g. JSON Schema → TS → JSDoc) require multi-step pipelines

A native solution in quicktype would be simpler, cleaner, and more discoverable.

Context

This feature would be particularly valuable for:

• JavaScript codebases using JSDoc + @ts-check
• environments where TypeScript is intentionally avoided at runtime
• API / message-schema driven systems (e.g. messaging, pub/sub, configuration schemas)

It would make quicktype a best-in-class tool for JSON Schema → JSDoc workflows.