PR #24803 - OpenAPITools/openapi-generator

Bug

A query parameter whose schema is an object, with style/explode left at their defaults (form, true), must go on the wire as one parameter per entry, keyed by the property name alone. The typescript-fetch client bracketed it instead.

parameters:
  - in: query
    name: filter
    schema:
      type: object

called with {"category": "books", "createdDate:gte": "2023-01-01"}:

	on the wire
expected	`category=books&createdDate%3Agte=2023-01-01`
typescript-fetch before	`filter[category]=books&filter[createdDate%3Agte]=2023-01-01`

Series

One of six per-language PRs for the same bug: #24797 go, #24802 python, #24803 typescript-fetch, #24867 kotlin, #24868 dart, #24869 ruby. No shared main/ code; each adds the same fixture, 3_0/exploded-object-query-param.yaml.

Fix

apis.mustache gates the explode loop on isMap, not isContainer. A bare type: object is isMap but not isContainer (only an additionalProperties map is), so it was assigned whole and the runtime bracketed it.
TypeScriptFetchClientCodegen copies isDeepObject. ExtendedCodegenParameter's copy constructor dropped it, so it was always false in the templates. A deepObject parameter is explode: true as well, and now stays assigned whole so the runtime brackets it.
as any on the indexing. A free-form object is typed object, and object[key] is TS7053 under strict.
A null entry is left out. The runtime sent {k: null} as k=null and {k: undefined} as k=undefined.
An entry is defined, not assigned. queryParameters['__proto__'] = v hits the prototype setter and drops the entry; Object.defineProperty keeps it (__proto__=x), as the runtime already does under deepObject.
Side effect: a deepObject parameter typed as a map was flattened on master (key=value); it now gets its brackets.

Verified

A client generated from the fixture, recording the URL it hands to fetch:

input	on the wire
`filter: {category: 'books', 'createdDate:gte': '2023-01-01'}`	`category=books&createdDate%3Agte=2023-01-01`
`typedFilter: {category: 'books'}`	`category=books`
`filter: {k: null, u: undefined, a: 'b', l: ['x', 2]}`	`a=b&l=x&l=2` (before the null skip: `k=null&u=undefined&a=b&l=x&l=2`)
`deepFilter: {category: 'books'}`	`deepFilter%5Bcategory%5D=books` (as on master)
`flatFilter: {category: 'books'}`	`flatFilter%5Bcategory%5D=books` (as on master)

TypeScriptFetchClientCodegenTest#testExplodedObjectQueryParameter fails without the fix.

Known gaps

Models. A $refed object model is isModel, so it still goes on the wire whole. Its interface properties aren't the wire names (_float for float, a Date for dateTime), so a fix has to go through {{dataType}}ToJSON first.
explode: false is still bracketed; the spec wants flatFilter=category,books.
Name collisions. An entry goes into the same queryParameters object as the declared parameters, so an entry and a declared parameter with the same name overwrite each other, depending on emit order. Keeping both would mean merging at every query assignment in apis.mustache, not just this loop.
A null item of a list entry still goes out as null.

PR checklist

Read the contribution guidelines.
Built the project and updated samples (./bin/generate-samples.sh bin/configs/typescript-fetch*.yaml): 3 FakeApi.ts files, the cast and the null skip on the petstore's language map.
Technical committee: @TiFu @macjohnny @topce @akehir @amakhrov @davidgamero @joscha

Summary by cubic

Fixes the typescript-fetch client so object query parameters with default form/explode go on the wire as one parameter per entry, keyed by the property name alone (category=books), instead of bracketed under the parameter name (filter[category]=books). deepObject and explode: false objects keep their previous wire format.

Bug Fixes

The explode branch was gated on isContainer, which free-form objects never set; it now keys on isMap, with an as any cast so strict TypeScript still compiles.
Null or undefined entries are now left off the wire instead of being sent as k=null / k=undefined.
ExtendedCodegenParameter's copy constructor dropped style-related flags, breaking deepObject parameters; it now copies isDeepObject, isFormStyle, isMatrix, isAllowEmptyValue, isSpaceDelimited, and isPipeDelimited. A duplicate this.isExplode = cp.isExplode; line from the latest commit should be removed.
Adds a test fixture covering the four style/explode combinations.

Known gap

A $refed object model as a query parameter still goes on the wire as a whole; fixing it requires routing through the model's ToJSON for wire-name translation, which is out of scope.

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