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Aug 29, 2025
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feat: return mock from then methods #27

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191 changes: 82 additions & 109 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -69,13 +69,13 @@ You should call `vi.resetAllMocks()` in your suite's `afterEach` hook to remove

[vitest's mock functions]: https://vitest.dev/api/mock.html
[stubs]: https://en.wikipedia.org/wiki/Test_stub
[when]: #whenspy-tfunc-stubwrappertfunc
[called-with]: #calledwithargs-targs-stubtargs-treturn
[then-return]: #thenreturnvalue-treturn
[then-resolve]: #thenresolvevalue-treturn
[then-throw]: #thenthrowerror-unknown
[then-reject]: #thenrejecterror-unknown
[then-do]: #thendocallback-args-targs--treturn
[when]: #whenmock-tfunc-options-whenoptions-stubwrappertfunc
[called-with]: #calledwithargs-parameterstfunc-stubtfunc
[then-return]: #thenreturnvalue-treturn---mocktfunc
[then-resolve]: #thenresolvevalue-treturn---mocktfunc
[then-throw]: #thenthrowerror-unknown---mocktfunc
[then-reject]: #thenrejecterror-unknown---mocktfunc
[then-do]: #thendocallback-args-targs--treturn---mocktfunc

### Why not vanilla Vitest mocks?

Expand Down Expand Up @@ -184,19 +184,19 @@ export const calculateQuestion = async (answer: number): Promise<string> => {

## API

### `when(spy: TFunc, options?: WhenOptions): StubWrapper<TFunc>`
### `when(mock: TFunc, options?: WhenOptions): StubWrapper<TFunc>`

Configures a `vi.fn()` or `vi.spyOn()` mock function to act as a vitest-when stub. Adds an implementation to the function that initially no-ops, and returns an API to configure behaviors for given arguments using [`.calledWith(...)`][called-with]

```ts
import { vi } from 'vitest'
import { when } from 'vitest-when'

const spy = vi.fn()
const mock = vi.fn()

when(spy)
when(mock)

expect(spy()).toBe(undefined)
expect(mock()).toBe(undefined)
```

#### Options
Expand All @@ -209,36 +209,32 @@ import type { WhenOptions } from 'vitest-when'
| ------- | ------- | ------- | -------------------------------------------------- |
| `times` | N/A | integer | Only trigger configured behavior a number of times |

### `.calledWith(...args: TArgs): Stub<TArgs, TReturn>`
### `.calledWith(...args: Parameters<TFunc>): Stub<TFunc>`

Create a stub that matches a given set of arguments which you can configure with different behaviors using methods like [`.thenReturn(...)`][then-return].

```ts
const spy = vi.fn()
const mock = when(vi.fn()).calledWith('hello').thenReturn('world')

when(spy).calledWith('hello').thenReturn('world')

expect(spy('hello')).toEqual('world')
expect(mock('hello')).toEqual('world')
```

When a call to a mock uses arguments that match those given to `calledWith`, a configured behavior will be triggered. All arguments must match, but you can use Vitest's [asymmetric matchers][] to loosen the stubbing:

```ts
const spy = vi.fn()

when(spy).calledWith(expect.any(String)).thenReturn('world')
const mock = when(vi.fn()).calledWith(expect.any(String)).thenReturn('world')

expect(spy('hello')).toEqual('world')
expect(spy('anything')).toEqual('world')
expect(mock('hello')).toEqual('world')
expect(mock('anything')).toEqual('world')
```

If `calledWith` is used multiple times, the last configured stubbing will be used.

```ts
when(spy).calledWith('hello').thenReturn('world')
expect(spy('hello')).toEqual('world')
when(spy).calledWith('hello').thenReturn('goodbye')
expect(spy('hello')).toEqual('goodbye')
when(mock).calledWith('hello').thenReturn('world')
expect(mock('hello')).toEqual('world')
when(mock).calledWith('hello').thenReturn('goodbye')
expect(mock('hello')).toEqual('goodbye')
```

[asymmetric matchers]: https://vitest.dev/api/expect.html#expect-anything
Expand Down Expand Up @@ -269,184 +265,165 @@ when<() => null>(overloaded).calledWith().thenReturn(null)
By default, if arguments do not match, a vitest-when stub will no-op and return `undefined`. You can customize this fallback by configuring your own unconditional behavior on the mock using Vitest's built-in [mock API][].

```ts
const spy = vi.fn().mockReturnValue('you messed up!')

when(spy).calledWith('hello').thenReturn('world')
const mock = when(vi.fn(() => 'you messed up!')))
.calledWith('hello')
.thenReturn('world')

spy('hello') // "world"
spy('jello') // "you messed up!"
mock('hello') // "world"
mock('jello') // "you messed up!"
```

[mock API]: https://vitest.dev/api/mock.html

### `.thenReturn(value: TReturn)`
### `.thenReturn(value: TReturn) -> Mock<TFunc>`

When the stubbing is satisfied, return `value`

```ts
const spy = vi.fn()

when(spy).calledWith('hello').thenReturn('world')
const mock = when(vi.fn()).calledWith('hello').thenReturn('world')

expect(spy('hello')).toEqual('world')
expect(mock('hello')).toEqual('world')
```

To only return a value once, use the `times` option.

```ts
import { when } from 'vitest-when'

const spy = vi.fn()

when(spy, { times: 1 }).calledWith('hello').thenReturn('world')
const mock = when(vi.fn(), { times: 1 }).calledWith('hello').thenReturn('world')

expect(spy('hello')).toEqual('world')
expect(spy('hello')).toEqual(undefined)
expect(mock('hello')).toEqual('world')
expect(mock('hello')).toEqual(undefined)
```

You may pass several values to `thenReturn` to return different values in succession. If you do not specify `times`, the last value will be latched. Otherwise, each value will be returned the specified number of times.

```ts
const spy = vi.fn()
const mock = when(vi.fn()).calledWith('hello').thenReturn('hi', 'sup?')

when(spy).calledWith('hello').thenReturn('hi', 'sup?')

expect(spy('hello')).toEqual('hi')
expect(spy('hello')).toEqual('sup?')
expect(spy('hello')).toEqual('sup?')
expect(mock('hello')).toEqual('hi')
expect(mock('hello')).toEqual('sup?')
expect(mock('hello')).toEqual('sup?')
```

### `.thenResolve(value: TReturn)`
### `.thenResolve(value: TReturn) -> Mock<TFunc>`

When the stubbing is satisfied, resolve a `Promise` with `value`

```ts
const spy = vi.fn()

when(spy).calledWith('hello').thenResolve('world')
const mock = when(vi.fn()).calledWith('hello').thenResolve('world')

expect(await spy('hello')).toEqual('world')
await expect(mock('hello')).resolves.toEqual('world')
```

To only resolve a value once, use the `times` option.

```ts
import { when } from 'vitest-when'

const spy = vi.fn()

when(spy, { times: 1 }).calledWith('hello').thenResolve('world')
const mock = when(vi.fn(), { times: 1 })
.calledWith('hello')
.thenResolve('world')

expect(await spy('hello')).toEqual('world')
expect(spy('hello')).toEqual(undefined)
await expect(mock('hello')).resolves.toEqual('world')
expect(mock('hello')).toEqual(undefined)
```

You may pass several values to `thenResolve` to resolve different values in succession. If you do not specify `times`, the last value will be latched. Otherwise, each value will be resolved the specified number of times.

```ts
const spy = vi.fn()

when(spy).calledWith('hello').thenResolve('hi', 'sup?')
const mock = when(vi.fn()).calledWith('hello').thenResolve('hi', 'sup?')

expect(await spy('hello')).toEqual('hi')
expect(await spy('hello')).toEqual('sup?')
expect(await spy('hello')).toEqual('sup?')
await expect(mock('hello')).resolves.toEqual('hi')
await expect(mock('hello')).resolves.toEqual('sup?')
await expect(mock('hello')).resolves.toEqual('sup?')
```

### `.thenThrow(error: unknown)`
### `.thenThrow(error: unknown) -> Mock<TFunc>`

When the stubbing is satisfied, throw `error`.

```ts
const spy = vi.fn()
const mock = when(vi.fn()).calledWith('hello').thenThrow(new Error('oh no'))

when(spy).calledWith('hello').thenThrow(new Error('oh no'))

expect(() => spy('hello')).toThrow('oh no')
expect(() => mock('hello')).toThrow('oh no')
```

To only throw an error only once, use the `times` option.

```ts
import { when } from 'vitest-when'

const spy = vi.fn()

when(spy, { times: 1 }).calledWith('hello').thenThrow(new Error('oh no'))
const mock = when(vi.fn(), { times: 1 })
.calledWith('hello')
.thenThrow(new Error('oh no'))

expect(() => spy('hello')).toThrow('oh no')
expect(spy('hello')).toEqual(undefined)
expect(() => mock('hello')).toThrow('oh no')
expect(mock('hello')).toEqual(undefined)
```

You may pass several values to `thenThrow` to throw different errors in succession. If you do not specify `times`, the last value will be latched. Otherwise, each error will be thrown the specified number of times.

```ts
const spy = vi.fn()

when(spy)
const mock = when(vi.fn())
.calledWith('hello')
.thenThrow(new Error('oh no'), new Error('this is bad'))

expect(() => spy('hello')).toThrow('oh no')
expect(() => spy('hello')).toThrow('this is bad')
expect(() => spy('hello')).toThrow('this is bad')
expect(() => mock('hello')).toThrow('oh no')
expect(() => mock('hello')).toThrow('this is bad')
expect(() => mock('hello')).toThrow('this is bad')
```

### `.thenReject(error: unknown)`
### `.thenReject(error: unknown) -> Mock<TFunc>`

When the stubbing is satisfied, reject a `Promise` with `error`.

```ts
const spy = vi.fn()

when(spy).calledWith('hello').thenReject(new Error('oh no'))
const mock = when(vi.fn()).calledWith('hello').thenReject(new Error('oh no'))

await expect(spy('hello')).rejects.toThrow('oh no')
await expect(mock('hello')).rejects.toThrow('oh no')
```

To only throw an error only once, use the `times` option.

```ts
import { times, when } from 'vitest-when'

const spy = vi.fn()

when(spy, { times: 1 }).calledWith('hello').thenReject(new Error('oh no'))
const mock = when(vi.fn(), { times: 1 })
.calledWith('hello')
.thenReject(new Error('oh no'))

await expect(spy('hello')).rejects.toThrow('oh no')
expect(spy('hello')).toEqual(undefined)
await expect(mock('hello')).rejects.toThrow('oh no')
expect(mock('hello')).toEqual(undefined)
```

You may pass several values to `thenReject` to throw different errors in succession. If you do not specify `times`, the last value will be latched. Otherwise, each rejection will be triggered the specified number of times.

```ts
const spy = vi.fn()

when(spy)
const mock = when(vi.fn())
.calledWith('hello')
.thenReject(new Error('oh no'), new Error('this is bad'))

await expect(spy('hello')).rejects.toThrow('oh no')
await expect(spy('hello')).rejects.toThrow('this is bad')
await expect(spy('hello')).rejects.toThrow('this is bad')
await expect(mock('hello')).rejects.toThrow('oh no')
await expect(mock('hello')).rejects.toThrow('this is bad')
await expect(mock('hello')).rejects.toThrow('this is bad')
```

### `.thenDo(callback: (...args: TArgs) => TReturn)`
### `.thenDo(callback: (...args: TArgs) => TReturn) -> Mock<TFunc>`

When the stubbing is satisfied, run `callback` to trigger a side-effect and return its result (if any). `thenDo` is a relatively powerful tool for stubbing complex behaviors, so if you find yourself using `thenDo` often, consider refactoring your code to use more simple interactions! Your future self will thank you.

```ts
const spy = vi.fn()
let called = false

when(spy)
const mock = when(vi.fn())
.calledWith('hello')
.thenDo(() => {
called = true
return 'world'
})

expect(spy('hello')).toEqual('world')
expect(mock('hello')).toEqual('world')
expect(called).toEqual(true)
```

Expand All @@ -455,33 +432,29 @@ To only run the callback once, use the `times` option.
```ts
import { times, when } from 'vitest-when'

const spy = vi.fn()

when(spy, { times: 1 })
const mock = when(vi.fn(), { times: 1 })
.calledWith('hello')
.thenDo(() => 'world')

expect(spy('hello')).toEqual('world')
expect(spy('hello')).toEqual(undefined)
expect(mock('hello')).toEqual('world')
expect(mock('hello')).toEqual(undefined)
```

You may pass several callbacks to `thenDo` to trigger different side-effects in succession. If you do not specify `times`, the last callback will be latched. Otherwise, each callback will be triggered the specified number of times.

```ts
const spy = vi.fn()

when(spy)
const mock = when(vi.fn())
.calledWith('hello')
.thenDo(
() => 'world',
() => 'solar system',
)

expect(spy('hello')).toEqual('world')
expect(spy('hello')).toEqual('solar system')
expect(mock('hello')).toEqual('world')
expect(mock('hello')).toEqual('solar system')
```

### `debug(spy: TFunc, options?: DebugOptions): DebugResult`
### `debug(mock: TFunc, options?: DebugOptions): DebugResult`

Logs and returns information about a mock's stubbing and usage. Useful if a test with mocks is failing and you can't figure out why.

Expand Down
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