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util: hide duplicated stack frames when using util.inspect #59447

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ea10a7b
util: hide duplicated stack frames when using util.inspect
BridgeAR Aug 11, 2025
5b60626
fixup! improve error message and collapse more lines properly
BridgeAR Aug 12, 2025
17a6673
fixup! improve worst case algorithm perf
BridgeAR Aug 13, 2025
d8a9686
fixup
BridgeAR Aug 18, 2025
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131 changes: 128 additions & 3 deletions lib/internal/util/inspect.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1305,13 +1305,122 @@ function identicalSequenceRange(a, b) {
len++;
}
if (len > 3) {
return { len, offset: i };
return [len, i];
}
}
}
}

return { len: 0, offset: 0 };
return [0, 0];
}

function getDuplicateErrorFrameRanges(frames) {
// Build a map: frame line -> sorted list of indices where it occurs
const result = [];
const lineToPositions = new Map();
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these are strings, is Map significantly faster than a null object or SafeMap (or even a $-key-salted normal object) such that this is the best choice?


for (let i = 0; i < frames.length; i++) {
const positions = lineToPositions.get(frames[i]);
if (positions === undefined) {
lineToPositions.set(frames[i], [i]);
} else {
positions.push(i);
}
}

const minimumDuplicateRange = 3;
// Not enough duplicate lines to consider collapsing
if (frames.length - lineToPositions.size <= minimumDuplicateRange) {
return result;
}

for (let i = 0; i < frames.length - minimumDuplicateRange; i++) {
const positions = lineToPositions.get(frames[i]);
// Find the next occurrence of the same line after i, if any
if (positions.length === 1 || positions.at(-1) === i) {
continue;
}

const current = positions.indexOf(i) + 1;
if (current === positions.length) {
continue;
}

// Theoretical maximum range, adjusted while iterating
let range = positions.at(-1) - i;
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Suggested change
let range = positions.at(-1) - i;
let range = positions[positions.length - 1] - i;

i assume this is faster, and avoids the need for a primordial

if (range < minimumDuplicateRange) {
continue;
}
let extraSteps;
if (current + 1 < positions.length) {
// Optimize initial step size by choosing the greatest common divisor (GCD)
// of all candidate distances to the same frame line. This tends to match
// the true repeating block size and minimizes fallback iterations.
let gcdRange = 0;
for (let j = current; j < positions.length; j++) {
let distance = positions[j] - i;
while (distance !== 0) {
const remainder = gcdRange % distance;
if (gcdRange !== 0) {
// Add other possible ranges as fallback
extraSteps ??= new Set();
extraSteps.add(gcdRange);
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similarly, this seems like it could be an object, with ObjectKeys at the end, and it'd be faster than spreading a Set into an array?

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I would be surprised if that's faster, while it will likely not be the major issue (this path is only triggered for some cases and for those, the set / object would contain normally never more than five entries).

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If it's not an issue, can we use SafeMap/SafeSet instead?

}
gcdRange = distance;
distance = remainder;
}
if (gcdRange === 1) break;
}
range = gcdRange;
if (extraSteps) {
extraSteps.delete(range);
extraSteps = [...extraSteps];
}
}
let maxRange = range;
let maxDuplicates = 0;

let duplicateRanges = 0;

for (let nextStart = i + range; /* ignored */ ; nextStart += range) {
let equalFrames = 0;
for (let j = 0; j < range; j++) {
if (frames[i + j] !== frames[nextStart + j]) {
break;
}
equalFrames++;
}
// Adjust the range to match different type of ranges.
if (equalFrames !== range) {
if (!extraSteps?.length) {
break;
}
// Memorize former range in case the smaller one would hide less.
if (duplicateRanges !== 0 && maxRange * maxDuplicates < range * duplicateRanges) {
maxRange = range;
maxDuplicates = duplicateRanges;
}
range = extraSteps.pop();
nextStart = i;
duplicateRanges = 0;
continue;
}
duplicateRanges++;
}

if (maxDuplicates !== 0 && maxRange * maxDuplicates >= range * duplicateRanges) {
range = maxRange;
duplicateRanges = maxDuplicates;
}

if (duplicateRanges * range >= 3) {
result.push([i + range, range, duplicateRanges]);
// Skip over the collapsed portion to avoid overlapping matches.
i += range * (duplicateRanges + 1) - 1;
}
}

return result;
}

function getStackString(ctx, error) {
Expand Down Expand Up @@ -1345,14 +1454,30 @@ function getStackFrames(ctx, err, stack) {
const causeStackStart = StringPrototypeIndexOf(causeStack, '\n at');
if (causeStackStart !== -1) {
const causeFrames = StringPrototypeSplit(StringPrototypeSlice(causeStack, causeStackStart + 1), '\n');
const { len, offset } = identicalSequenceRange(frames, causeFrames);
const { 0: len, 1: offset } = identicalSequenceRange(frames, causeFrames);
if (len > 0) {
const skipped = len - 2;
const msg = ` ... ${skipped} lines matching cause stack trace ...`;
frames.splice(offset + 1, skipped, ctx.stylize(msg, 'undefined'));
}
}
}

// Remove recursive repetitive stack frames in long stacks
if (frames.length > 10) {
const ranges = getDuplicateErrorFrameRanges(frames);

while (ranges.length) {
const { 0: offset, 1: length, 2: duplicateRanges } = ranges.pop();
const msg = ` ... collapsed ${length * duplicateRanges} duplicate lines ` +
'matching above ' +
(duplicateRanges > 1 ?
`${length} lines ${duplicateRanges} times...` :
'lines ...');
frames.splice(offset, length * duplicateRanges, ctx.stylize(msg, 'undefined'));
}
}

return frames;
}

Expand Down
169 changes: 169 additions & 0 deletions test/parallel/test-util-inspect.js
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Original file line number Diff line number Diff line change
Expand Up @@ -2920,6 +2920,175 @@ assert.strictEqual(
process.cwd = originalCWD;
}

{
// Use a fake stack to verify the expected colored outcome.
const err = new Error('Hide duplicate frames in long stack');
err.stack = [
'Error: Hide duplicate frames in long stack',
' at A.<anonymous> (/foo/node_modules/bar/baz.js:2:7)',
' at A.<anonymous> (/foo/node_modules/bar/baz.js:2:7)',
' at Module._compile (node:internal/modules/cjs/loader:827:30)',
' at Fancy (node:vm:697:32)',
' at tryModuleLoad (node:internal/modules/cjs/foo:629:12)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Fancy (node:vm:697:32)',
' at tryModuleLoad (node:internal/modules/cjs/foo:629:12)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)',
' at require (node:internal/modules/helpers:14:16)',
' at Array.forEach (<anonymous>)',
' at require (node:internal/modules/helpers:14:16)',
' at Array.forEach (<anonymous>)',
` at foobar/test/parallel/test-util-inspect.js:2760:12`,
` at Object.<anonymous> (foobar/node_modules/m/folder/file.js:2753:10)`,
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)',
' at require (node:internal/modules/helpers:14:16)',
' at Array.forEach (<anonymous>)',
` at foobar/test/parallel/test-util-inspect.js:2760:12`,
` at Object.<anonymous> (foobar/node_modules/m/folder/file.js:2753:10)`,
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)',
' at require (node:internal/modules/helpers:14:16)',
' at Array.forEach (<anonymous>)',
` at foobar/test/parallel/test-util-inspect.js:2760:12`,
` at Object.<anonymous> (foobar/node_modules/m/folder/file.js:2753:10)`,
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)',
' at require (node:internal/modules/helpers:14:16)',
' at Array.forEach (<anonymous>)',
` at foobar/test/parallel/test-util-inspect.js:2760:12`,
` at Object.<anonymous> (foobar/node_modules/m/folder/file.js:2753:10)`,
' at /test/test-util-inspect.js:2239:9',
' at getActual (node:assert:592:5)',
' at /test/test-util-inspect.js:2239:9',
' at getActual (node:assert:592:5)',
' at /test/test-util-inspect.js:2239:9',
' at getActual (node:assert:592:5)',
].join('\n');

assert.strictEqual(
util.inspect(err, { colors: true }),
'Error: Hide duplicate frames in long stack\n' +
' at A.<anonymous> (/foo/node_modules/\x1B[4mbar\x1B[24m/baz.js:2:7)\n' +
' at A.<anonymous> (/foo/node_modules/\x1B[4mbar\x1B[24m/baz.js:2:7)\n' +
'\x1B[90m at Module._compile (node:internal/modules/cjs/loader:827:30)\x1B[39m\n' +
'\x1B[90m at Fancy (node:vm:697:32)\x1B[39m\n' +
' at tryModuleLoad (node:internal/modules/cjs/foo:629:12)\n' +
'\x1B[90m at Function.Module._load (node:internal/modules/cjs/loader:621:3)\x1B[39m\n' +
'\x1B[90m ... collapsed 3 duplicate lines matching above lines ...\x1B[39m\n' +

'\x1B[90m at Function.Module._load (node:internal/modules/cjs/loader:621:3)\x1B[39m\n' +
'\x1B[90m ... collapsed 5 duplicate lines matching above 1 lines 5 times...\x1B[39m\n' +

' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)\n' +
'\x1B[90m at require (node:internal/modules/helpers:14:16)\x1B[39m\n' +
' at Array.forEach (<anonymous>)\n' +
'\x1B[90m at require (node:internal/modules/helpers:14:16)\x1B[39m\n' +
' at Array.forEach (<anonymous>)\n' +
' at foobar/test/parallel/test-util-inspect.js:2760:12\n' +
' at Object.<anonymous> (foobar/node_modules/\x1B[4mm\x1B[24m/folder/file.js:2753:10)\n' +
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)\n' +
'\x1B[90m ... collapsed 10 duplicate lines matching above 5 lines 2 times...\x1B[39m\n' +

'\x1B[90m at require (node:internal/modules/helpers:14:16)\x1B[39m\n' +
' at Array.forEach (<anonymous>)\n' +
' at foobar/test/parallel/test-util-inspect.js:2760:12\n' +
' at Object.<anonymous> (foobar/node_modules/\x1B[4mm\x1B[24m/folder/file.js:2753:10)\n' +
' at /test/test-util-inspect.js:2239:9\n' +
'\x1B[90m at getActual (node:assert:592:5)\x1B[39m\n' +
'\x1B[90m ... collapsed 4 duplicate lines matching above 2 lines 2 times...\x1B[39m',
);

// Use a fake stack to verify the expected colored outcome.
const err2 = new Error('Hide duplicate frames in long stack');
err2.stack = [
'Error: Hide duplicate frames in long stack',
' at A.<anonymous> (/foo/node_modules/bar/baz.js:2:7)',
' at A.<anonymous> (/foo/node_modules/bar/baz.js:2:7)',
' at Module._compile (node:internal/modules/cjs/loader:827:30)',

// 3
' at Fancy (node:vm:697:32)',
' at tryModuleLoad (node:internal/modules/cjs/foo:629:12)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Fancy (node:vm:697:32)',
' at tryModuleLoad (node:internal/modules/cjs/foo:629:12)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',

// 6 * 1
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',
' at Function.Module._load (node:internal/modules/cjs/loader:621:3)',

// 10
' at require (node:internal/modules/helpers:14:16)',
' at Array.forEach (<anonymous>)',
` at foobar/test/parallel/test-util-inspect.js:2760:12`,
` at Object.<anonymous> (foobar/node_modules/m/folder/file.js:2753:10)`,
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)',
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)',
' at require (node:internal/modules/helpers:14:16)',
' at Array.forEach (<anonymous>)',
` at foobar/test/parallel/test-util-inspect.js:2760:12`,
` at Object.<anonymous> (foobar/node_modules/m/folder/file.js:2753:10)`,

' at require (node:internal/modules/helpers:14:16)',
' at Array.forEach (<anonymous>)',
` at foobar/test/parallel/test-util-inspect.js:2760:12`,
` at Object.<anonymous> (foobar/node_modules/m/folder/file.js:2753:10)`,
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)',
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)',
' at require (node:internal/modules/helpers:14:16)',
' at Array.forEach (<anonymous>)',
` at foobar/test/parallel/test-util-inspect.js:2760:12`,
` at Object.<anonymous> (foobar/node_modules/m/folder/file.js:2753:10)`,

// 2 * 2
' at /test/test-util-inspect.js:2239:9',
' at getActual (node:assert:592:5)',
' at /test/test-util-inspect.js:2239:9',
' at getActual (node:assert:592:5)',
' at /test/test-util-inspect.js:2239:9',
' at getActual (node:assert:592:5)',
].join('\n');

assert.strictEqual(
util.inspect(err2, { colors: true }),
'Error: Hide duplicate frames in long stack\n' +
' at A.<anonymous> (/foo/node_modules/\x1B[4mbar\x1B[24m/baz.js:2:7)\n' +
' at A.<anonymous> (/foo/node_modules/\x1B[4mbar\x1B[24m/baz.js:2:7)\n' +
'\x1B[90m at Module._compile (node:internal/modules/cjs/loader:827:30)\x1B[39m\n' +
'\x1B[90m at Fancy (node:vm:697:32)\x1B[39m\n' +
' at tryModuleLoad (node:internal/modules/cjs/foo:629:12)\n' +
'\x1B[90m at Function.Module._load (node:internal/modules/cjs/loader:621:3)\x1B[39m\n' +
'\x1B[90m ... collapsed 3 duplicate lines matching above lines ...\x1B[39m\n' +
'\x1B[90m at Function.Module._load (node:internal/modules/cjs/loader:621:3)\x1B[39m\n' +
'\x1B[90m ... collapsed 6 duplicate lines matching above 1 lines 6 times...\x1B[39m\n' +
'\x1B[90m at require (node:internal/modules/helpers:14:16)\x1B[39m\n' +
' at Array.forEach (<anonymous>)\n' +
' at foobar/test/parallel/test-util-inspect.js:2760:12\n' +
' at Object.<anonymous> (foobar/node_modules/\x1B[4mm\x1B[24m/folder/file.js:2753:10)\n' +
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)\n' +
' at Module.require [as weird/name] (node:internal/aaaaa/loader:735:19)\n' +
'\x1B[90m at require (node:internal/modules/helpers:14:16)\x1B[39m\n' +
' at Array.forEach (<anonymous>)\n' +
' at foobar/test/parallel/test-util-inspect.js:2760:12\n' +
' at Object.<anonymous> (foobar/node_modules/\x1B[4mm\x1B[24m/folder/file.js:2753:10)\n' +
'\x1B[90m ... collapsed 10 duplicate lines matching above lines ...\x1B[39m\n' +
' at /test/test-util-inspect.js:2239:9\n' +
'\x1B[90m at getActual (node:assert:592:5)\x1B[39m\n' +
'\x1B[90m ... collapsed 4 duplicate lines matching above 2 lines 2 times...\x1B[39m',
);
}

{
// Cross platform checks.
const err = new Error('foo');
Expand Down
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