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assert: avoid expensive diff for large values #62798

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assert: avoid expensive diff for large values #62798

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assert: avoid expensive diff for large values
semimikoh Apr 18, 2026
8f55a99
assert: address large diff review feedback
semimikoh Apr 20, 2026
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16 changes: 16 additions & 0 deletions lib/internal/assert/assertion_error.js
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Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,8 @@ const kReadableOperator = {

const kMaxShortStringLength = 12;
const kMaxLongStringLength = 512;
const kMaxDiffLineCount = 1000;
const kMaxDiffLinesToShow = 50;

const kMethodsWithCustomMessageDiff = new SafeSet()
.add('deepStrictEqual')
Expand Down Expand Up @@ -182,6 +184,13 @@ function isSimpleDiff(actual, inspectedActual, expected, inspectedExpected) {
return typeof actual !== 'object' || actual === null || typeof expected !== 'object' || expected === null;
}

function getTruncatedDiffValue(lines) {
if (lines.length > kMaxDiffLinesToShow) {
return `${ArrayPrototypeJoin(ArrayPrototypeSlice(lines, 0, kMaxDiffLinesToShow), '\n')}\n...`;
}
return ArrayPrototypeJoin(lines, '\n');
}

function createErrDiff(actual, expected, operator, customMessage, diffType = 'simple') {
operator = checkOperator(actual, expected, operator);

Expand Down Expand Up @@ -213,6 +222,13 @@ function createErrDiff(actual, expected, operator, customMessage, diffType = 'si
message = ArrayPrototypeJoin(inspectedSplitActual, '\n');
}
header = '';
} else if (
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@thisalihassan thisalihassan Apr 18, 2026

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This can completely hide the real diff when users actually need it.

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@semimikoh semimikoh Apr 20, 2026 •
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Narrowed the skip path to large root-level mismatches only. Large values with the same root representation still use the normal line diff, and I added a regression test for that case.

diffType !== 'full' &&
inspectedSplitActual.length + inspectedSplitExpected.length > kMaxDiffLineCount
) {
message = `\n${colors.green}+${colors.white} ${getTruncatedDiffValue(inspectedSplitActual)}\n` +
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@thisalihassan thisalihassan Apr 18, 2026

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Hardcoded colors bypasses the partialDeepStrictEqual coloring convention

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@semimikoh semimikoh Apr 20, 2026

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Updated the truncated diff path to follow the partialDeepStrictEqual coloring convention.

`${colors.red}-${colors.white} ${getTruncatedDiffValue(inspectedSplitExpected)}`;
skipped = true;
} else {
const checkCommaDisparity = actual != null && typeof actual === 'object';
const diff = myersDiff(inspectedSplitActual, inspectedSplitExpected, checkCommaDisparity);
Expand Down
13 changes: 13 additions & 0 deletions test/parallel/test-assert-deep.js
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Expand Up @@ -1180,6 +1180,19 @@ test('Strict equal with identical objects that are not identical ' +
);
});

test('Strict equal skips line diff for very large objects', () => {
const buffer = Buffer.alloc(1_000);

assert.throws(
() => assert.strictEqual(buffer, [buffer]),
{
code: 'ERR_ASSERTION',
name: 'AssertionError',
message: /Skipped lines[\s\S]*Buffer\(1000\)/
}
);
});

test('Basic valueOf check', () => {
const a = new String(1);
a.valueOf = undefined;
Expand Down
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