Lodash Migration & Compat Guide

Guide users through migrating lodash to es-toolkit and understanding the strict vs compat APIs, grounded in actual source code.

Input

$ARGUMENTS — Lodash code to migrate, specific function names, or a question about strict vs compat.

Core Concepts

es-toolkit (strict): Opinionated, simplified API for the 85% use case. Smaller bundle, may differ from lodash in edge cases by design. New functions are added here.

es-toolkit/compat: Aims for full lodash test compatibility within a defined scope. See docs/compatibility.md for out-of-scope behaviors (e.g., implicit type conversions, prototype modifications).

Why source-first matters

The only reliable way to know the difference between strict and compat is to inspect the version installed in the user's project. The skill may be installed as a standalone directory, so never assume the es-toolkit repository or a sibling docs directory is available.

Workflow

If a specific function or lodash code is given

1. Identify lodash functions from input

Extract which lodash functions are used and how they're imported.

2. Verify availability in the installed package

Run from the package that depends on es-toolkit. In a monorepo this is usually a workspace package, not the repository root. If the project uses Yarn Plug'n'Play, use yarn node instead of node.

Replace the example names with the lodash functions found in the input:

node --input-type=module -e "
import { createRequire } from 'node:module';

const require = createRequire(import.meta.url);
const names = ['get', 'chunk', 'map'];
const pkg = require('es-toolkit/package.json');
const entries = Object.keys(pkg.exports)
  .filter(subpath => subpath !== './package.json' && !subpath.includes('*'))
  .map(subpath => subpath === '.' ? 'es-toolkit' : 'es-toolkit/' + subpath.slice(2));
const modules = await Promise.all(entries.map(entry => import(entry).catch(() => null)));
if (modules.every(module => module == null)) {
  console.error('es-toolkit is not resolvable from ' + process.cwd());
  process.exit(1);
}
for (const name of names) {
  const found = entries.filter((_, index) => modules[index] != null && name in modules[index]);
  console.log(name.padEnd(20), found.join(', ') || 'NOT AVAILABLE');
}
"

A function can appear in several entry points, so read the complete result. es-toolkit/types is type-only and cannot be checked through a runtime import.

Resolve the package metadata with the following command, then read declarations under its dist/ directory for exact signatures and JSDoc. Do not assume a conventional node_modules path.

node -e "console.log(require.resolve('es-toolkit/package.json'))"

When the current project is the es-toolkit source repository itself, inspect src/{category}/{fn}.ts, src/compat/{category}/{fn}.ts, and their specs directly instead of requiring a built package.

3. Determine the right migration path

If the function only exists in compat (like get, set, has), explain why — es-toolkit doesn't implement functions replaceable by modern JS (optional chaining ?., Object.hasOwn(), etc.).

4. Generate before/after migration

For each function, provide:

• Availability: es-toolkit and/or es-toolkit/compat
• Doc link: https://es-toolkit.dev/reference/{category}/{fn}.html (strict) or https://es-toolkit.dev/compat/reference/{category}/{fn}.html (compat)
• Before (lodash) and After (es-toolkit) code examples
• Any behavioral differences found in source code
• Feature comparison table: Compare API capabilities side-by-side (e.g., cancel support, flush, maxWait, return values, AbortSignal, callback arguments). Read both implementations to identify all supported options and present them in a table like:

• "When to use which": Based on the feature comparison, provide scenario-based guidance — e.g., "Use es-toolkit if you only need basic debounce; use compat if you rely on cancel/flush; keep lodash if you need X."

For migrations involving many functions, use a summary table instead of repeating the full template for each one.

5. Provide consolidated import rewrite

Show the final import transformation as a single block.

5a. Suggest automation patterns for large-scale migrations

When migrating many files, mention practical automation approaches:

• Bundler alias: Configure resolve.alias in webpack or Vite to redirect lodash imports at build time without changing source files:
  jsCopy

  // vite.config.js or webpack.config.js
  resolve: { alias: { 'lodash': 'es-toolkit/compat' } }
  

• ESLint rule: Use no-restricted-imports to warn or error on remaining lodash imports after migration.
• Codemod: For systematic AST-based transforms, mention tools like jscodeshift if the migration pattern is complex.

6. Note bundle size impact

Explain that strict and compat imports are tree-shakeable, but quote exact size or performance numbers only after measuring the user's project or reading current benchmark data from an es-toolkit source checkout. Otherwise link to the official bundle-size and performance pages.

If no specific function (migration strategy overview)

Provide a strategic overview with three migration options:

• Option A: Direct to es-toolkit — new/small projects
• Option B: Gradual via compat — large codebases (recommended for legacy)
• Option C: Mixed — pragmatic approach

For each option, include a trade-off matrix:

Compat-exclusive functions: Search src/compat/ for functions that don't exist in src/ (strict). List representative examples so users know what can only come from compat (e.g., get, set, has).

For concrete behavioral differences, read a few representative function pairs from source (e.g., chunk, debounce) to give real examples rather than abstract descriptions.

Documentation links

• Migration overview: https://es-toolkit.dev/compat/intro.html
• Bundle size: https://es-toolkit.dev/bundle-size.html
• Performance: https://es-toolkit.dev/performance.html

Function pages use these forms:

• strict: https://es-toolkit.dev/reference/{category}/{fn}.html
• compat: https://es-toolkit.dev/compat/reference/{category}/{fn}.html
• fp: https://es-toolkit.dev/fp/reference/{fn}.html

If you are unsure that a page exists, omit the page link rather than guessing.