feat: add chai config

docs/config/index.md

@@ -1386,3 +1386,33 @@ Path to custom tsconfig, relative to the project root.
 - **Default**: `300`
 
 The number of milliseconds after which a test is considered slow and reported as such in the results.
+
+### chaiConfig
+
+- **Type:** `{ includeStack?, showDiff?, truncateThreshold? }`
+- **Default:** `{ includeStack: false, showDiff: true, truncateThreshold: 40 }`
+
+Equivalent to [Chai config](https://github.com/chaijs/chai/blob/4.x.x/lib/chai/config.js). 
+
+#### chaiConfig.includeStack
+
+- **Type:** `boolean`
+- **Default:** `false`
+
+Influences whether stack trace is included in Assertion error message. Default of false suppresses stack trace in the error message.
+
+#### chaiConfig.showDiff
+
+- **Type:** `boolean`
+- **Default:** `true`
+
+Influences whether or not the `showDiff` flag should be included in the thrown AssertionErrors. `false` will always be `false`; `true` will be true when the assertion has requested a diff to be shown.
+
+#### chaiConfig.truncateThreshold
+
+- **Type:** `number`
+- **Default:** `40`
+
+Sets length threshold for actual and expected values in assertion errors. If this threshold is exceeded, for example for large data structures, the value is replaced with something like `[ Array(3) ]` or `{ Object (prop1, prop2) }`. Set it to `0` if you want to disable truncating altogether.
+
+This config option affects truncating values in `test.each` titles and inside the assertion error message.

packages/vitest/src/integrations/chai/index.ts

@@ -78,3 +78,7 @@ Object.defineProperty(globalThis, GLOBAL_EXPECT, {
 
 export { assert, should } from 'chai'
 export { chai, globalExpect as expect }
+export const setupChaiConfig = (config: ChaiConfig) => {
+  Object.assign(chai.config, config)
+}
+export type ChaiConfig = Omit<Partial<typeof chai.config>, 'useProxy' | 'proxyExcludedKeys'>

packages/vitest/src/runtime/entry.ts

@@ -6,6 +6,7 @@ import { getWorkerState, resetModules } from '../utils'
 import { vi } from '../integrations/vi'
 import { distDir } from '../paths'
 import { startCoverageInsideWorker, stopCoverageInsideWorker, takeCoverageInsideWorker } from '../integrations/coverage'
+import { setupChaiConfig } from '../integrations/chai'
 import { setupGlobalEnv, withEnv } from './setup.node'
 import { rpc } from './rpc'
 import type { VitestExecutor } from './execute'
@@ -71,6 +72,9 @@ export async function run(files: string[], config: ResolvedConfig, environment:
 
   const workerState = getWorkerState()
 
+  if (config.chaiConfig)
+    setupChaiConfig(config.chaiConfig)
+
   const runner = await getTestRunner(config, executor)
 
   // @ts-expect-error untyped global

packages/vitest/src/types/config.ts

@@ -4,6 +4,7 @@ import type { FakeTimerInstallOpts } from '@sinonjs/fake-timers'
 import type { SequenceHooks, SequenceSetupFiles } from '@vitest/runner'
 import type { BuiltinReporters } from '../node/reporters'
 import type { TestSequencerConstructor } from '../node/sequencers/types'
+import type { ChaiConfig } from '../integrations/chai'
 import type { CoverageOptions, ResolvedCoverageOptions } from './coverage'
 import type { JSDOMOptions } from './jsdom-options'
 import type { Reporter } from './reporter'
@@ -583,6 +584,12 @@ export interface InlineConfig {
    * Requires `singleThread: true` OR `threads: false`.
    */
   inspectBrk?: boolean
+
+  /**
+   * Modify default Chai config. Vitest uses Chai for `expect` and `assert` matches.
+   * https://github.com/chaijs/chai/blob/4.x.x/lib/chai/config.js
+  */
+  chaiConfig?: ChaiConfig
 }
 
 export interface TypecheckConfig {