Compat Layer Review

Verify that a compat PR actually fixes a real lodash inconsistency.

Input

$ARGUMENTS — PR number (e.g. 1234) or function name (e.g. chunk)

Workflow

1. Identify target function and PR claims

PR number:

gh pr view {number} --repo toss/es-toolkit --json title,body,files

From the PR description and diff, extract:

• Which compat function is being fixed
• What inconsistency the contributor claims (expected vs actual behavior)
• Any test examples the contributor provides

Function name: Read the function source and its spec to understand current behavior.

2. Build comparison test

Create a temporary vitest spec at src/compat/{category}/_compat-review-{fn}.spec.ts.

import { describe, expect, it } from 'vitest';
import { fn as compatFn } from 'es-toolkit/compat';
import { fn as lodashFn } from 'lodash';

Include two groups of test cases:

A. Contributor's claimed examples — Extract directly from the PR description or test diff. These are the cases the PR claims to fix.

B. ~10 additional edge cases you generate — Based on the function's signature and lodash's known behavior patterns:

• Empty inputs: [], {}, '', 0
• Nullish: null, undefined
• Negative/zero/float numbers: -1, 0, 1.5, NaN, Infinity
• Type coercion: string numbers ('3'), boolean, mixed types
• Boundary: single-element arrays, very long strings, deeply nested objects
• Pick cases that are relevant to the specific function

Each test:

it('description', () => {
  let lodashResult, compatResult;
  let lodashErr: any, compatErr: any;
  try {
    lodashResult = lodashFn(args);
  } catch (e) {
    lodashErr = e;
  }
  try {
    compatResult = compatFn(args);
  } catch (e) {
    compatErr = e;
  }

  if (lodashErr && compatErr) return; // both throw = consistent
  if (lodashErr || compatErr) {
    throw new Error(`Behavior mismatch: ${lodashErr ? 'lodash throws' : 'compat throws'}`);
  }
  expect(compatResult).toEqual(lodashResult);
});

3. Run comparison BEFORE the PR change

yarn vitest run src/compat/{category}/_compat-review-{fn}.spec.ts

Record which tests fail — these are real inconsistencies that exist on main.

4. Apply PR changes and re-run

gh pr diff {number} --repo toss/es-toolkit | git apply

Run the same test again. Confirm that:

• The contributor's claimed examples now pass
• The additional edge cases still pass (no regressions)

Revert after testing:

git checkout -- .

5. Clean up

Delete _compat-review-*.spec.ts.

6. Report

## {fn} — Compat Review

### PR Claim
{What the contributor says they're fixing}

### Verification (BEFORE fix)
| # | Input | lodash | compat | Match? | Source |
|---|-------|--------|--------|--------|--------|
| 1 | ...   | ...    | ...    | MISMATCH | PR example |
| 2 | ...   | ...    | ...    | MATCH    | Edge case |

### Verification (AFTER fix)
| # | Input | lodash | compat | Match? | Source |
|---|-------|--------|--------|--------|--------|

### Verdict
- [ ] PR claim is valid (inconsistency confirmed on main)
- [ ] Fix resolves the claimed inconsistency
- [ ] No regressions in edge cases
- [ ] Existing tests still pass