remeda-utility

Development

Conventions for authoring, modifying, testing, and documenting a Remeda utility function — including the migration mapping pages for Lodash/Ramda/Just users. Use this skill whenever editing a file under `packages/remeda/src/` or `packages/docs/src/content/mapping/{lodash,ramda,just}/`, when reviewing a PR or local changes touching those directories, or whenever an issue, PR, or chat refers to a specific Remeda function or to a migration mapping page.

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A new or modified Remeda function ships across five surfaces: implementation, tests, JSDoc, the public export, and — when there's a Lodash/Ramda equivalent — migration mappings on the docs site. Skipping any one of them leaves the function half-shipped (e.g., a working implementation that isn't re-exported, or a behavior change with no test covering it). Work through the sections in order — later sections assume the earlier ones are in place.

1. Implementation (`src/functionName.ts`)

When any of the edits involves the runtime implementation of a function, one of its overloading function signatures, or any of the type definitions that it uses (either directly on the same file, or indirectly via imports) read reference/implementation.md.

2. Tests

Each function has up to three test files, one per kind:

Runtime tests (functionName.test.ts) — Vitest. Cover happy path, edge cases, empty inputs, and both calling styles (data-first and data-last inside pipe, map or filter). When editing, read reference/testing-runtime.md.
Type tests (functionName.test-d.ts) — expectTypeOf from Vitest. Verify inferred return types, narrowing, and that invalid inputs are compile errors. When editing, read reference/testing-types.md.
Property-based tests (functionName.test-prop.ts) — @fast-check/vitest. Optional but encouraged for functions with well-defined algebraic properties (idempotence, involutions, round-trips). When adding, read reference/testing-properties.md.

Conventions that apply across all three kinds:

Runtime and type assertions are strictly separated — expect() lives in .test.ts, expectTypeOf() lives in .test-d.ts. Never mix them in the same test block, and never put one kind in the other file.
Test names describe what is being tested in the function's own vocabulary — "lazy early exit with hasMany", not "take and flat".
Test names should be terse and concise, and should rely on context from parent describe() blocks and not repeat them.
Test names do not need to read as prose!
Tests for a specific bug must reference the issue number, either in the test name or a comment so that the reporting issue can always be traced back.
Input data needs enough variation to produce distinct outputs — [1, 1, 1] hides bugs that [1, 2, 3] catches.

3. JSDoc

Each overload needs its own JSDoc block. Required tags:

Tag	Notes
Description	Present tense, precise vocabulary. First sentence is a concise one-line summary; further detail goes after a blank line.
`@param`	One per parameter.
`@signature`	The call as it looks at the call site, e.g., `filter(data, predicate)` for data-first, `filter(predicate)(data)` for data-last.
`@example`	Simple, almost trivial — show params and output, not a full usage pattern. Data-last examples use `pipe`, `map`, of `filter`.
`@dataFirst` / `@dataLast`	One per overload, matching the signature style. The docs site uses these to label and group overloads.
`@lazy`	Only if the function supports lazy evaluation in `pipe`.
`@category`	The function's category (`Array`, `Object`, `String`, etc.) — drives docs-site navigation.

If the new function is closely related to others where a user might pick the wrong one (e.g., indexBy vs. fromKeys vs. pullObject vs. fromEntries), list those alternatives with a one-line "use this when…" gloss in the description body of every overload. Update the related functions' JSDoc to cross-reference back, so the disambiguation is symmetric — a user who lands on either function discovers the others.

The function description, @params, @returns, and @deprecated should be copied verbatim between the overloads, unless there is a strong reason not to, and then that reason should be made extremely clear in both blocks.

4. Export

Add export * from "./functionName"; to src/index.ts (alphabetical order).

5. Migration Mappings

Many Remeda functions are genuinely novel and have no Lodash, Ramda, or Just counterpart — those skip this section entirely. But if the function has a matching upstream function (or replaces one), it needs a per-library mapping page on the docs site so migrators landing from a search for the upstream name find a clean answer for their call site.

Read reference/migration.md when:

Authoring a new Remeda function that has a Lodash, Ramda, or Just equivalent.
Editing any existing file under packages/docs/src/content/mapping/{lodash,ramda,just}/.
Reviewing changes that add or modify a mapping page.

reference/migration.md covers the file location and frontmatter, the bullets-then-examples body structure, the comment-header convention inside code blocks (// Lodash / // Remeda / // Native), the linking rules, and the __MISSING.md cleanup. This is interim guidance — eventually migration work will live behind its own skill, but until then this reference is the source of truth.