fix(pretty-format): limit output for large object (#9949)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hi-ogawa

packages/pretty-format/src/index.ts

@@ -380,29 +380,44 @@ function printer(
   refs: Refs,
   hasCalledToJSON?: boolean,
 ): string {
+  let result: string
+
   const plugin = findPlugin(config.plugins, val)
   if (plugin !== null) {
-    return printPlugin(plugin, val, config, indentation, depth, refs)
+    result = printPlugin(plugin, val, config, indentation, depth, refs)
+  }
+  else {
+    const basicResult = printBasicValue(
+      val,
+      config.printFunctionName,
+      config.escapeRegex,
+      config.escapeString,
+    )
+    if (basicResult !== null) {
+      result = basicResult
+    }
+    else {
+      result = printComplexValue(
+        val,
+        config,
+        indentation,
+        depth,
+        refs,
+        hasCalledToJSON,
+      )
+    }
   }
 
-  const basicResult = printBasicValue(
-    val,
-    config.printFunctionName,
-    config.escapeRegex,
-    config.escapeString,
-  )
-  if (basicResult !== null) {
-    return basicResult
+  // Check string length budget:
+  // accumulate output length and if exceeded,
+  // force no further recursion by patching maxDepth.
+  // Inspired by Node's util.inspect bail out approach.
+  config.outputLength += result.length
+  if (config.outputLength > config.maxOutputLength) {
+    config.maxDepth = 0
   }
 
-  return printComplexValue(
-    val,
-    config,
-    indentation,
-    depth,
-    refs,
-    hasCalledToJSON,
-  )
+  return result
 }
 
 const DEFAULT_THEME: Theme = {
@@ -425,6 +440,9 @@ export const DEFAULT_OPTIONS: Options = {
   highlight: false,
   indent: 2,
   maxDepth: Number.POSITIVE_INFINITY,
+  // Practical default hard-limit to avoid too long string being generated
+  // (Node's limit is buffer.constants.MAX_STRING_LENGTH ~ 512MB)
+  maxOutputLength: 1_000_000,
   maxWidth: Number.POSITIVE_INFINITY,
   min: false,
   plugins: [],
@@ -509,6 +527,8 @@ function getConfig(options?: OptionsReceived): Config {
     printShadowRoot: options?.printShadowRoot ?? true,
     spacingInner: options?.min ? ' ' : '\n',
     spacingOuter: options?.min ? '' : '\n',
+    maxOutputLength: options?.maxOutputLength ?? DEFAULT_OPTIONS.maxOutputLength,
+    outputLength: 0,
   }
 }
 

packages/pretty-format/src/types.ts

@@ -45,6 +45,7 @@ export interface PrettyFormatOptions {
   indent?: number
   maxDepth?: number
   maxWidth?: number
+  maxOutputLength?: number
   min?: boolean
   printBasicPrototype?: boolean
   printFunctionName?: boolean
@@ -71,6 +72,8 @@ export interface Config {
   printShadowRoot: boolean
   spacingInner: string
   spacingOuter: string
+  maxOutputLength: number
+  outputLength: number
 }
 
 export type Printer = (

pnpm-lock.yaml

@@ -22,6 +22,7 @@
     "@test/vite-external": "link:./projects/vite-external",
     "@vitest/expect": "workspace:*",
     "@vitest/mocker": "workspace:*",
+    "@vitest/pretty-format": "workspace:*",
     "@vitest/runner": "workspace:*",
     "@vitest/test-dep-cjs": "file:./deps/dep-cjs",
     "@vitest/test-dep-nested-cjs": "file:./deps/dep-nested-cjs",

test/core/package.json

@@ -0,0 +1,187 @@
+import { format } from '@vitest/pretty-format'
+import { describe, expect, test } from 'vitest'
+
+describe('maxOutputLength', () => {
+  function createObjectGraph(n: number) {
+    // owner
+    //  |-> cats
+    //       |-> cat0 -> owner
+    //       |-> cat1 -> owner
+    //       |-> cat2
+    //       |-> ...
+    //  |-> dogs
+    //       |-> dog0
+    //       |-> dog1
+    //       |-> dog2
+    //       |-> ...
+    interface Owner {
+      dogs: Pet[]
+      cats: Pet[]
+    }
+    interface Pet {
+      name: string
+      owner: Owner
+    }
+    const owner: Owner = { dogs: [], cats: [] }
+    for (let i = 0; i < n; i++) {
+      owner.dogs.push({ name: `dog${i}`, owner })
+    }
+    for (let i = 0; i < n; i++) {
+      owner.cats.push({ name: `cat${i}`, owner })
+    }
+    return owner
+  }
+
+  test('quadratic growth example depending on format root', () => {
+    const owner = createObjectGraph(3)
+
+    // when starting from owner, each pet is expanded once, so no amplification, just linear growth.
+    expect(format(owner)).toMatchInlineSnapshot(`
+      "Object {
+        "cats": Array [
+          Object {
+            "name": "cat0",
+            "owner": [Circular],
+          },
+          Object {
+            "name": "cat1",
+            "owner": [Circular],
+          },
+          Object {
+            "name": "cat2",
+            "owner": [Circular],
+          },
+        ],
+        "dogs": Array [
+          Object {
+            "name": "dog0",
+            "owner": [Circular],
+          },
+          Object {
+            "name": "dog1",
+            "owner": [Circular],
+          },
+          Object {
+            "name": "dog2",
+            "owner": [Circular],
+          },
+        ],
+      }"
+    `)
+
+    // when starting from owner.cats, each cat re-expands the full dogs list via owner.
+    // this exhibits quadratic growth, which is what the budget is designed to prevent.
+    expect(format(owner.cats)).toMatchInlineSnapshot(`
+      "Array [
+        Object {
+          "name": "cat0",
+          "owner": Object {
+            "cats": [Circular],
+            "dogs": Array [
+              Object {
+                "name": "dog0",
+                "owner": [Circular],
+              },
+              Object {
+                "name": "dog1",
+                "owner": [Circular],
+              },
+              Object {
+                "name": "dog2",
+                "owner": [Circular],
+              },
+            ],
+          },
+        },
+        Object {
+          "name": "cat1",
+          "owner": Object {
+            "cats": [Circular],
+            "dogs": Array [
+              Object {
+                "name": "dog0",
+                "owner": [Circular],
+              },
+              Object {
+                "name": "dog1",
+                "owner": [Circular],
+              },
+              Object {
+                "name": "dog2",
+                "owner": [Circular],
+              },
+            ],
+          },
+        },
+        Object {
+          "name": "cat2",
+          "owner": Object {
+            "cats": [Circular],
+            "dogs": Array [
+              Object {
+                "name": "dog0",
+                "owner": [Circular],
+              },
+              Object {
+                "name": "dog1",
+                "owner": [Circular],
+              },
+              Object {
+                "name": "dog2",
+                "owner": [Circular],
+              },
+            ],
+          },
+        },
+      ]"
+    `)
+  })
+
+  test('budget prevents blowup on large graphs', () => {
+    // quickly hit the kill switch due to quadratic growth
+    expect([10, 20, 30, 1000, 2000, 3000].map(n => format(createObjectGraph(n).cats).length))
+      .toMatchInlineSnapshot(`
+        [
+          9729,
+          36659,
+          80789,
+          273009,
+          374009,
+          299009,
+        ]
+      `)
+
+    // depending on object/array shape, output can exceed the limit 1mb
+    // but the output size is proportional to the amount of objects and the size of array.
+    expect(format(createObjectGraph(10000).cats).length).toMatchInlineSnapshot(`936779`)
+    expect(format(createObjectGraph(20000).cats).length).toMatchInlineSnapshot(`1236779`)
+  })
+
+  test('early elements expanded, later elements folded after budget trips', () => {
+    // First few objects are fully expanded, but once budget is exceeded,
+    // maxDepth = 0 means no more expansion.
+    const arr = Array.from({ length: 10 }, (_, i) => ({ i }))
+    expect(format(arr, { maxOutputLength: 100 })).toMatchInlineSnapshot(`
+      "Array [
+        Object {
+          "i": 0,
+        },
+        Object {
+          "i": 1,
+        },
+        Object {
+          "i": 2,
+        },
+        Object {
+          "i": 3,
+        },
+        [Object],
+        [Object],
+        [Object],
+        [Object],
+        [Object],
+        [Object],
+      ]"
+    `)
+  })
+})