fix(pretty-format): fix output limit over counting (#9965)

hi-ogawa · web-flow · commit d3b7a40fa83d · 2026-03-25T18:48:32.000+01:00
diff --git a/packages/pretty-format/src/index.ts b/packages/pretty-format/src/index.ts
@@ -408,12 +408,15 @@ function printer(
     }
   }
 
-  // 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) {
+  // Per-depth output budget (inspired by Node's util.inspect).
+  // Each depth level tracks output independently, so nested results
+  // don't inflate a single counter (which would undercount by ~Nx for
+  // N levels of nesting). Nodes at the same depth produce disjoint spans
+  // in the output string, so each bucket accurately reflects output at
+  // that level. Total output is bounded by maxDepth × maxOutputLength.
+  config._outputLengthPerDepth[depth] ??= 0
+  config._outputLengthPerDepth[depth] += result.length
+  if (config._outputLengthPerDepth[depth] > config.maxOutputLength) {
     config.maxDepth = 0
   }
 
@@ -528,7 +531,7 @@ function getConfig(options?: OptionsReceived): Config {
     spacingInner: options?.min ? ' ' : '\n',
     spacingOuter: options?.min ? '' : '\n',
     maxOutputLength: options?.maxOutputLength ?? DEFAULT_OPTIONS.maxOutputLength,
-    outputLength: 0,
+    _outputLengthPerDepth: [],
   }
 }
 
diff --git a/packages/pretty-format/src/types.ts b/packages/pretty-format/src/types.ts
@@ -45,6 +45,13 @@ export interface PrettyFormatOptions {
   indent?: number
   maxDepth?: number
   maxWidth?: number
+  /**
+   * Approximate per-depth-level budget for output length.
+   * When the accumulated output at any single depth level exceeds this value,
+   * further nesting is collapsed. This is a heuristic safety valve, not a hard
+   * limit — total output can reach up to roughly `maxDepth × maxOutputLength`.
+   * @default 1_000_000
+   */
   maxOutputLength?: number
   min?: boolean
   printBasicPrototype?: boolean
@@ -73,7 +80,11 @@ export interface Config {
   spacingInner: string
   spacingOuter: string
   maxOutputLength: number
-  outputLength: number
+  /**
+   * Per-depth budget accumulator for {@link maxOutputLength}.
+   * @internal
+   */
+  _outputLengthPerDepth: number[]
 }
 
 export type Printer = (
diff --git a/test/core/test/pretty-format.test.ts b/test/core/test/pretty-format.test.ts
@@ -145,16 +145,30 @@ describe('maxOutputLength', () => {
           9729,
           36659,
           80789,
-          273009,
-          374009,
-          299009,
+          1056011,
+          1074009,
+          1088009,
         ]
       `)
 
     // 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`)
+    expect(format(createObjectGraph(10000).cats).length).toMatchInlineSnapshot(`1377439`)
+    expect(format(createObjectGraph(20000).cats).length).toMatchInlineSnapshot(`1497738`)
+  })
+
+  test('budget should not truncate output shorter than maxOutputLength', () => {
+    const data = Array.from({ length: 50 }, (_, i) => ({ a: { b: { c: i } } }))
+    const full = format(data, { maxOutputLength: Infinity })
+    const limited = format(data, { maxOutputLength: full.length })
+    // this invariant should hold for any input
+    expect(limited.length).toBe(full.length)
+    expect({ limited: limited.length, full: full.length }).toMatchInlineSnapshot(`
+      {
+        "full": 4349,
+        "limited": 4349,
+      }
+    `)
   })
 
   test('early elements expanded, later elements folded after budget trips', () => {
@@ -175,7 +189,9 @@ describe('maxOutputLength', () => {
         Object {
           "i": 3,
         },
-        [Object],
+        Object {
+          "i": 4,
+        },
         [Object],
         [Object],
         [Object],

Original file line number	Diff line number	Diff line change
`@@ -408,12 +408,15 @@ function printer(`
`408`	`408`	`}`
`409`	`409`	`}`
`410`	`410`
`411`		`- // Check string length budget:`
`412`		`- // accumulate output length and if exceeded,`
`413`		`- // force no further recursion by patching maxDepth.`
`414`		`- // Inspired by Node's util.inspect bail out approach.`
`415`		`- config.outputLength += result.length`
`416`		`- if (config.outputLength > config.maxOutputLength) {`
	`411`	`+ // Per-depth output budget (inspired by Node's util.inspect).`
	`412`	`+ // Each depth level tracks output independently, so nested results`
	`413`	`+ // don't inflate a single counter (which would undercount by ~Nx for`
	`414`	`+ // N levels of nesting). Nodes at the same depth produce disjoint spans`
	`415`	`+ // in the output string, so each bucket accurately reflects output at`
	`416`	`+ // that level. Total output is bounded by maxDepth × maxOutputLength.`
	`417`	`+ config._outputLengthPerDepth[depth] ??= 0`
	`418`	`+ config._outputLengthPerDepth[depth] += result.length`
	`419`	`+ if (config._outputLengthPerDepth[depth] > config.maxOutputLength) {`
`417`	`420`	`config.maxDepth = 0`
`418`	`421`	`}`
`419`	`422`
`@@ -528,7 +531,7 @@ function getConfig(options?: OptionsReceived): Config {`
`528`	`531`	`spacingInner: options?.min ? ' ' : '\n',`
`529`	`532`	`spacingOuter: options?.min ? '' : '\n',`
`530`	`533`	`maxOutputLength: options?.maxOutputLength ?? DEFAULT_OPTIONS.maxOutputLength,`
`531`		`- outputLength: 0,`
	`534`	`+ _outputLengthPerDepth: [],`
`532`	`535`	`}`
`533`	`536`	`}`
`534`	`537`