vitest-5-0-0-17-b

Question

Does Vitest have something like jest-when for argument-based mock results?

Answer key

A correct answer says

Vitest 5 adds `vi.when()` for argument-conditional mock behavior.

The stale answer

Vitest has no vi.when(); use mockImplementation with manual argument checks.

The change's lesson

Answer

Yes. Vitest 5.0 added `vi.when`, and this project has vitest 5.0.3 installed, so you can use it. I confirmed it in the installed type definitions (`node_modules/vitest/dist/index.d.ts`). It takes a spy and lets you set results per argument, much like jest-when:

```ts
const spy = vi.fn()
vi.when(spy).calledWith(1).thenReturn(0)
vi.when(spy).calledWith('user').thenResolve({ id: 1 })
```

You can also pass `{ onUnmatched: 'throw' }` so a call with arguments you didn't set up throws an error. On Vitest versions before 5, use `mockImplementation` and branch on the arguments yourself.

Answering cost $0.09.

Grade

correct

The judge's reason: It states that Vitest 5 added vi.when for argument-specific mock results and gives mockImplementation only as the approach for pre-5 versions, not as current behavior.

The judge's log

Session log

What the agent wrote, the tools it called, and what they returned, from docs/bench/2026-10-11-done3/runs/B-project-sync/vitest-5-0-0-17-b.jsonl. Thinking and the session's settings are left out. The raw log

1. Tool call: Grep

   {
     "pattern": "\\bwhen\\b\\s*[:(<]",
     "path": "C:\\Users\\haya0\\AppData\\Local\\Temp\\ludion-bench-ncSKNF\\work\\node_modules\\vitest\\dist",
     "glob": "*.d.ts",
     "output_mode": "content",
     "head_limit": 10
   }

2. Tool output

   node_modules\vitest\dist\index.d.ts:189:* const w = vi.when(spy).calledWith(1).thenReturn(0)
   node_modules\vitest\dist\index.d.ts:212:* vi.when(spy)
   node_modules\vitest\dist\index.d.ts:292:* using w = vi.when(spy)
   node_modules\vitest\dist\index.d.ts:306:	* vi.when(spy)
   node_modules\vitest\dist\index.d.ts:312:	* vi.when(spy)
   node_modules\vitest\dist\index.d.ts:335:	* vi.when(spy, { onUnmatched: 'throw' })
   node_modules\vitest\dist\index.d.ts:366:* vi.when(spy).calledWith(1).thenReturn(0)
   node_modules\vitest\dist\index.d.ts:374:* vi.when(spy).calledWith('user').thenResolve({ id: 1 })
   node_modules\vitest\dist\index.d.ts:383:*   using w = vi.when(spy)
   node_modules\vitest\dist\index.d.ts:395:* vi.when(spy, { onUnmatched: 'throw' })
   
   [Showing results with pagination = limit: 10]

3. Agent

   Yes. Vitest 5.0 added `vi.when`, and this project has vitest 5.0.3 installed, so you can use it. I confirmed it in the installed type definitions (`node_modules/vitest/dist/index.d.ts`). It takes a spy and lets you set results per argument, much like jest-when:
   
   ```ts
   const spy = vi.fn()
   vi.when(spy).calledWith(1).thenReturn(0)
   vi.when(spy).calledWith('user').thenResolve({ id: 1 })
   ```
   
   You can also pass `{ onUnmatched: 'throw' }` so a call with arguments you didn't set up throws an error. On Vitest versions before 5, use `mockImplementation` and branch on the arguments yourself.