8000 docs: Document current string concatenation behavior in sum methods (#5818) by flavioespinoza · Pull Request #6097 · lodash/lodash · GitHub
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docs: Document current string concatenation behavior in sum methods (#5818) #6097

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@flavioespinoza flavioespinoza commented Jan 27, 2026 •
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Context

This PR is a direct response to @ljharb's feedback on #6095:

"In the meantime, it would be better to have a PR that adds tests for the current behavior, and updates the docs to match."

Done. ✅

Summary

Documents the current behavior where sum, sumBy, mean, and meanBy perform string concatenation when given non-numeric values, as reported in #5818.

Changes

JSDoc Updates

  • Added Note sections explaining that non-numeric values cause string concatenation
  • Updated @returns from {number} to {number|string} to accurately reflect possible return types
  • Added examples showing the non-numeric behavior
  • Referenced Method sumBy returns string  #5818 for context

New Tests

QUnit.test('with non-numeric values performs string concatenation (current behavior, see #5818)', function(assert) {
  assert.strictEqual(func(['a', 'b']), 'ab', 'strings are concatenated');
  assert.strictEqual(func(['1', '2', '3']), '123', 'numeric strings are concatenated');
  assert.strictEqual(func([1, '2']), '12', 'number + string results in string');
  assert.strictEqual(func([1, 'f', null]), '1fnull', 'mixed types produce unexpected results');
});

Example Documentation Added

// Non-numeric values result in string concatenation (not recommended)
_.sum(['1', '2']);
// => '12'

_.sumBy([{ 'a': '1' }, { 'a': '2' }], 'a');
// => '12'

Test Plan

  • npm run build passes
  • npm test passes (6808 + 327 = 7135 tests)
  • No breaking changes - behavior is unchanged, only documentation added

Related

@flavioespinoza @claude
docs: Document current string concatenation behavior in sum methods (l…
58fe2bb 
…odash#5818)

Update JSDoc and add tests to document the current behavior where non-numeric
values in sum/sumBy/mean/meanBy result in string concatenation rather than
numeric addition.

This documents the existing behavior as reported in lodash#5818. The behavior may
change in a future major version (see lodash#6095 for the proposed fix).

Changes:
- Update JSDoc for sum, sumBy, mean, meanBy to note string concatenation behavior
- Update @returns to reflect that string may be returned
- Add examples showing the non-numeric behavior
- Add tests documenting string concatenation with various non-numeric inputs
- Reference lodash#5818 in documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
@flavioespinoza flavioespinoza mentioned this pull request Jan 27, 2026
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@flavioespinoza
docs: Update dist files with JSDoc changes
e9ebf88 
Include updated documentation for sum, sumBy, mean, and meanBy
methods describing current string concatenation behavior.
ljharb
ljharb approved these changes Jan 27, 2026
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I'm not sure if the dist file should be updated in individual PRs; typically that sort of thing is only updated for releases. Otherwise, LGTM.

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@ljharb ljharb ljharb approved these changes

@UlisesGascon UlisesGascon UlisesGascon approved these changes

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