migrate-to-deno

Migrating to Deno

Requires Deno 2.9 or later. For general Deno usage once migrated, see the deno skill.

Most Node projects already run under Deno

Deno reads an existing package.json, resolves the same npm packages, writes a real node_modules, runs the same scripts, and supports node: built-ins. TypeScript runs with no build step.

There is usually no code to change — only which binary you invoke. Don't start by rewriting imports to jsr:, swapping dependencies for Deno-specific ones, or restructuring directories. Proposing that is the most common way this goes wrong.

Migrate in rungs

Each rung is independently useful and reversible. Stop wherever suits the project; plenty of teams stop at rung 1.

Rung 1 — Deno as the package manager only

deno install

Reads package.json, resolves the same dependencies, writes node_modules, and creates deno.lock — seeded from any existing package-lock.json, yarn.lock, bun.lock, or pnpm lockfile, so pins and integrity hashes carry over instead of drifting.

The app still runs under node; teammates are unaffected. Commit deno.lock once verified. To back out: delete deno.lock and node_modules, then npm install.

Rung 2 — Run it with Deno

deno run -A main.js      # or: deno -A main.js
deno task build          # runs scripts.build from package.json

Use -A here. The goal is confirming the program works, not designing a permission policy — changing both at once makes failures ambiguous.

Rung 3 — Tighten permissions

Replace -A with the narrowest set that works: run it, read what it asks for, grant exactly that.

deno run --allow-net=api.example.com --allow-read=./config --allow-env=PORT main.js

This buys something Node cannot offer, and is worth doing before deploying.

Rung 4 — Optionally, adopt the built-in toolchain

deno fmt for prettier, deno lint for eslint, deno test for jest or vitest, deno check for tsc, deno watch for nodemon, deno compile for pkg.

Optional, and usually not worth it for an existing project. These are not drop-in replacements; parity is incomplete, so this is a real migration, not a config change. A project happy with prettier, eslint, and vitest should keep them and use Deno as runtime and package manager only. Prefer the built-in tools for new projects. If you do move an existing one, go a tool at a time.

Command equivalents

† Yarn Berry (v2+) spelling. Yarn Classic (v1) uses yarn install --frozen-lockfile and yarn audit.

‡ Yarn Classic only — Berry removed yarn outdated.

dx is a separate binary installed alongside Deno, and an alias for deno x. It does not appear in the top-level deno --help output. Like npx, it runs the package with the sandbox disabled.

The four things that actually break

Requires net access to "..." (or read, env, run)

Deno grants nothing by default. Add that specific permission, or -A while still establishing the program works at all.

ReferenceError: require is not defined

A file containing CommonJS is being parsed as ESM. .cjs is always CommonJS, .mjs always ESM; .js and .ts follow "type" in the nearest package.json. For a CommonJS project, set "type": "commonjs".

A dependency is broken, or postinstall never ran

Lifecycle scripts don't run by default. Native addons notice immediately. Approvals are recorded in the config file, so this is one-time:

deno approve-scripts                              # interactive picker
deno install --allow-scripts=npm:better-sqlite3   # or name them directly

A tool cannot find files inside node_modules

Deno's layout is pnpm-style: real files in node_modules/.deno/, exposed via symlinks. Tools assuming npm's flat hoisted tree need:

{ "nodeModulesLinker": "hoisted" }

What has no Deno equivalent

Say so rather than improvising a workaround that won't hold:

• Yarn Plug'n'Play. Deno creates a real node_modules; .pnp.cjs is unused

and .yarnrc.yml resolver settings don't transfer.

• `yarn patch` / pnpm patched dependencies. Vendor or fork.
• `overrides` / `resolutions`. Pin via an import map entry instead.
• Registry and resolver tuning in .npmrc / .yarnrc.yml.
• Bun build features — macros, HTMLRewriter, HTML entrypoints.

Per-tool details

• references/FROM_NPM.md — lockfile seeding, node_modules layout, overrides
• references/FROM_YARN.md — Plug'n'Play, Berry vs Classic, workspaces
• references/FROM_PNPM.md — pnpm-workspace.yaml, catalog:, patches
• references/FROM_BUN.md — Bun API translation, bunfig.toml
• references/NODE_APIS.md — node: built-ins, DENO_COMPAT, CJS/ESM

Further reading

• <https://docs.deno.com/runtime/migrate/> — official migration guides
• <https://docs.deno.com/runtime/fundamentals/node/> — Node and npm

compatibility

• <https://docs.deno.com/runtime/reference/node_apis/> — per-module Node API

status