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assert: improve deepEqual perf for large input #12849

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assert: improve deepEqual perf for large input #12849

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21 changes: 15 additions & 6 deletions lib/assert.js
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Original file line number Diff line number Diff line change
Expand Up @@ -285,15 +285,24 @@ function _deepEqual(actual, expected, strict, memos) {
// Note: this accounts for both named and indexed properties on Arrays.

// Use memos to handle cycles.
memos = memos || { actual: [], expected: [] };
const actualIndex = memos.actual.indexOf(actual);
if (actualIndex !== -1) {
if (actualIndex === memos.expected.indexOf(expected)) {
if (!memos) {
memos = {
actual: { map: new Map(), position: 0 },
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How about a WeakMap?

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@joyeecheung That thought occurred to me and I couldn’t really find a difference … would there be one? Is that going to be more performant?

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@addaleax I think the GC pressure would be smaller after we leave the function because the references don't have be traversed?

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@joyeecheung ok, updated :)

expected: { map: new Map(), position: 0 }
};
}

const actualPosition = memos.actual.map.get(actual);
if (actualPosition !== undefined) {
if (actualPosition === memos.expected.map.get(expected)) {
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Changing from Array.prototype.indexOf (which uses Strict Equality Comparison) to Map.prototype.get (which uses SameValueZero) can have subtle nuances that might result in incompatibilities, especially around NaN. Not sure if we need to care about it though.

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I don’t think that actually affects the logic here – if it matters at all, it should only affect cases where non-primitives compare deep-equal to primitives, but that doesn’t seem to happen anyway (e.g. assert.deepEqual([0], 0) throws even though [0] == 0).

return true;
}
} else {
memos.actual.map.set(actual, memos.actual.position++);
}
if (!memos.expected.map.has(expected)) {
memos.expected.map.set(expected, memos.expected.position++);
}
memos.actual.push(actual);
memos.expected.push(expected);

return objEquiv(actual, expected, strict, memos);
}
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