chore: return "outputDiffLines" option

docs/config/index.md

@@ -400,6 +400,25 @@ Custom reporters for output. Reporters can be [a Reporter instance](https://gith
   - `'hanging-process'` - displays a list of hanging processes, if Vitest cannot exit process safely. This might be a heavy operation, enable it only if Vitest consistently cannot exit process
   - path of a custom reporter (e.g. `'./path/to/reporter.ts'`, `'@scope/reporter'`)
 
+ ### outputDiffLines
+
+ - **Type:** `number`
+ - **Default:** `15`
+ - **CLI:** `--outputDiffLines=<lines>`, `--output-diff-lines=<lines>`
+
+ Limit the number of single output diff lines up to `15`. Vitest counts all `+-` lines when determining when to stop. For example, you might see diff like this, if you set this property to `3`:
+
+ ```diff
+ - test: 1,
+ + test: 2,
+ - obj: '1',
+ ...
+ - test2: 1,
+ + test2: 1,
+ - obj2: '2',
+ ...
+ ```
+
 ### outputFile
 
 - **Type:** `string | Record<string, string>`

docs/guide/cli.md

@@ -69,6 +69,7 @@ export default {
 | `--silent` | Silent console output from tests |
 | `--isolate` | Isolate environment for each test file (default: `true`) |
 | `--reporter <name>` | Select reporter: `default`, `verbose`, `dot`, `junit`, `json`, or a path to a custom reporter |
+| `--outputDiffLines <lines>` | Limit number of output diff lines up to `<lines>`. |
 | `--outputFile <filename/-s>` | Write test results to a file when the `--reporter=json` or `--reporter=junit` option is also specified <br /> Via [cac's dot notation] you can specify individual outputs for multiple reporters |
 | `--coverage` | Enable coverage report |
 | `--run` | Do not watch |

packages/runner/src/run.ts

@@ -150,15 +150,15 @@ export async function runTest(test: Test, runner: VitestRunner) {
       test.result.state = 'pass'
     }
     catch (e) {
-      failTask(test.result, e)
+      failTask(test.result, e, runner)
     }
 
     try {
       await callSuiteHook(test.suite, test, 'afterEach', runner, [test.context, test.suite])
       await callCleanupHooks(beforeEachCleanups)
     }
     catch (e) {
-      failTask(test.result, e)
+      failTask(test.result, e, runner)
     }
 
     if (test.result.state === 'pass')
@@ -195,9 +195,9 @@ export async function runTest(test: Test, runner: VitestRunner) {
   updateTask(test, runner)
 }
 
-function failTask(result: TaskResult, err: unknown) {
+function failTask(result: TaskResult, err: unknown, runner: VitestRunner) {
   result.state = 'fail'
-  const error = processError(err)
+  const error = processError(err, runner.config)
   result.error = error
   result.errors ??= []
   result.errors.push(error)
@@ -268,15 +268,15 @@ export async function runSuite(suite: Suite, runner: VitestRunner) {
       }
     }
     catch (e) {
-      failTask(suite.result, e)
+      failTask(suite.result, e, runner)
     }
 
     try {
       await callSuiteHook(suite, suite, 'afterAll', runner, [suite])
       await callCleanupHooks(beforeAllCleanups)
     }
     catch (e) {
-      failTask(suite.result, e)
+      failTask(suite.result, e, runner)
     }
   }
 

packages/runner/src/types/runner.ts

@@ -12,6 +12,7 @@ export interface VitestRunnerConfig {
     seed?: number
     hooks: SequenceHooks
   }
+  outputDiffLines?: number
   maxConcurrency: number
   testTimeout: number
   hookTimeout: number

packages/runner/src/utils/error.ts

@@ -1,4 +1,5 @@
 import { deepClone, format, getOwnProperties, getType } from '@vitest/utils'
+import type { DiffOptions } from '@vitest/utils/diff'
 import { unifiedDiff } from '@vitest/utils/diff'
 
 export interface ParsedStack {
@@ -104,7 +105,7 @@ function normalizeErrorMessage(message: string) {
   return message.replace(/__vite_ssr_import_\d+__\./g, '')
 }
 
-export function processError(err: any) {
+export function processError(err: any, options: DiffOptions = {}) {
   if (!err || typeof err !== 'object')
     return err
   // stack is not serialized in worker communication
@@ -120,7 +121,7 @@ export function processError(err: any) {
   const { replacedActual, replacedExpected } = replaceAsymmetricMatcher(clonedActual, clonedExpected)
 
   if (err.showDiff || (err.showDiff === undefined && err.expected !== undefined && err.actual !== undefined))
-    err.diff = unifiedDiff(replacedActual, replacedExpected)
+    err.diff = unifiedDiff(replacedActual, replacedExpected, options)
 
   // some Error implementations don't allow rewriting message
   try {

packages/utils/src/diff.ts

@@ -3,6 +3,7 @@ import { diffDescriptors, getConcordanceTheme } from './descriptors'
 
 export interface DiffOptions {
   showLegend?: boolean
+  outputDiffLines?: number
 }
 
 /**
@@ -17,7 +18,7 @@ export function unifiedDiff(actual: unknown, expected: unknown, options: DiffOpt
   const theme = getConcordanceTheme()
   const diff = diffDescriptors(actual, expected, { theme })
 
-  const { showLegend = true } = options
+  const { showLegend = true, outputDiffLines = 15 } = options
 
   const counts = {
     '+': 0,
@@ -28,11 +29,16 @@ export function unifiedDiff(actual: unknown, expected: unknown, options: DiffOpt
   const minus = `  ${c.green('+')}`
 
   const lines = diff.split(/\r?\n/g)
-  lines.forEach((line) => {
-    if (line.startsWith(plus))
+  let firstErrorLine: number | null = null
+  lines.forEach((line, index) => {
+    if (line.startsWith(plus)) {
+      firstErrorLine ??= index
       counts['+']++
-    else if (line.startsWith(minus))
+    }
+    else if (line.startsWith(minus)) {
+      firstErrorLine ??= index
       counts['-']++
+    }
   })
   const isCompact = counts['+'] === 1 && counts['-'] === 1 && lines.length === 2
 
@@ -50,5 +56,18 @@ export function unifiedDiff(actual: unknown, expected: unknown, options: DiffOpt
     }
   }
 
+  if (firstErrorLine != null && outputDiffLines) {
+    const start = Math.max(0, firstErrorLine - 1)
+    const end = Math.min(lines.length, firstErrorLine + outputDiffLines)
+    const linesAfterCount = lines.length - end
+
+    const linesBefore = start ? `  ${c.gray(`... ${start} more line${start > 1 ? 's' : ''}\n`)}` : ''
+    const linesAfter = linesAfterCount ? `\n  ${c.gray(`... ${linesAfterCount} more line${linesAfterCount > 1 ? 's' : ''}\n`)}` : ''
+    const diffOutput = lines.slice(start, end).map(line => line.replace(/␊\s*$/, '')).join('\n')
+    const helperBunner = linesAfter && (counts['+'] + counts['-'] > outputDiffLines) ? `\n  Use ${c.gray('test.outputDiffLines')} to increase the number of lines shown.` : ''
+
+    return legend + linesBefore + diffOutput + linesAfter + helperBunner
+  }
+
   return legend + diff
 }

packages/vitest/src/types/config.ts

@@ -181,6 +181,12 @@ export interface InlineConfig {
    */
   reporters?: Arrayable<BuiltinReporters | 'html' | Reporter | Omit<string, BuiltinReporters>>
 
+  /**
+   * Maximum number of line to show in a single diff.
+   * @default 15
+   */
+  outputDiffLines?: number
+
   /**
    * Write test results to a file when the --reporter=json` or `--reporter=junit` option is also specified.
    * Also definable individually per reporter by using an object instead.