lodash/lodash

`meanBy` returning `NaN` on empty lists is error prone

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#5901 opened on Jul 10, 2024

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Description

_.meanBy([], 'test') will return NaN

This behavior is not currently documented, meaning that users might not know that it could return NaN.

Possible solutions:

  • Make meanBy return undefined instead of NaN, meaning that users using TypeScript will have to handle that case
  • Improve the documentation to warn users that they mustn't assume that the returned value is a valid number

Contributor guide

Tech stack
javascript
Domain
frontendbackend
Issue type
documentation
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
under 1 hour
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
JavaScriptlodash basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
The issue concerns the behavior of ` .meanBy` when called on an empty list. The current implementation returns `NaN`, which is error prone and undocumented. Two possible solutions are proposed: change the return value to `undefined` (which would require updating TypeScript types) or improve the documentation to warn users. To resolve, examine the source code of `meanBy` in the lodash repository (likely in `src/.internal/` or similar) and consider both technical and documentation changes. The decision should be discussed with maintainers if needed.