fix(compat/meanBy): skip undefined values when averaging to match lodash

[kdelay]

Summary

es-toolkit/compat's meanBy returns NaN whenever the iteratee produces undefined for any element, while lodash skips those values and still divides by the full length.

import { meanBy } from 'es-toolkit/compat';
import lodashMeanBy from 'lodash/meanBy';

meanBy([{ a: 1 }, {}], 'a'); // NaN (before this PR)
lodashMeanBy([{ a: 1 }, {}], 'a'); // 0.5

More cases measured against lodash@4.18.1 on main (81af489):

call	compat (before)	lodash
meanBy([{ a: 1 }, {}], 'a')	NaN	0.5
meanBy([{ a: 1 }, { a: undefined }, { a: 3 }], 'a')	NaN	1.333...
meanBy([1, undefined, 2])	NaN	1
meanBy({ 0: { a: 1 }, 1: {}, length: 2 }, 'a')	NaN	0.5
meanBy([{ a: { b: 1 } }, {}], 'a.b')	NaN	0.5

This is also inconsistent inside compat itself: mean([1, undefined, 2]) already returns 1 like lodash, because compat/mean builds on compat/sum, which skips undefined.

Cause

src/compat/math/meanBy.ts delegated to the main library's meanBy, which sums with the main library's sumBy. That one adds every value as-is, so one undefined turns the whole sum into NaN. lodash's baseMean divides baseSum — which ignores undefined — by array.length.

Changes

• Compute the sum with compat's own sumBy, which already implements lodash's undefined-skipping behavior, and divide by the length of items. When every value is undefined the result stays NaN, matching lodash.
• Drop the intermediate Array.from copy, since compat/sumBy accepts array-likes directly.
• Add regression tests for missing keys, explicit undefined, deep paths, array-likes, and the all-undefined case.
• Document the behavior in the compat reference docs for all four languages.

Only es-toolkit/compat is changed; es-toolkit's own meanBy keeps its stricter getValue: (element: T) => number contract.

Test results

• yarn vitest run src/compat/math src/math — 35 files, 288 tests pass
• yarn vitest run — 659 files pass (tests/check-dist.spec.ts fails to build locally on Node 24.10 with an unrelated tsdown/PnP loader error; it also fails on unmodified main here)
• yarn typecheck, yarn workspace type-tests test — no errors
• yarn eslint on the changed files — no errors
• Differential run of the table above against lodash@4.18.1 in the benchmarks workspace: all cases match after the change

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Project	Deployment	Actions	Updated (UTC)
es-toolkit	Ready	Preview	Aug 8, 2026 1:05pm