Deno

Deno

A JavaScript and TypeScript runtime with a package manager, formatter, linter, test runner, type checker, and bundler in one binary. Runs TypeScript directly.

Needs Deno 2.9+. Check with deno --version, update with deno upgrade.

Deno works the way npm and bun do

Deno is not a separate ecosystem to port code into:

• deno install reads an existing package.json and writes a real node_modules.
• deno add express installs from npm. Unprefixed names default to npm.
• deno task build runs scripts.build from package.json or tasks.build from deno.json. If both define it, deno.json wins.
• Node built-ins work prefixed or not: node:fs and fs both resolve.
• deno main.js runs a file. deno run is optional.
• Deno reads tsconfig.json.

Don't tell users to rewrite imports, adopt JSR, or restructure as a precondition. The two real differences are permissions and npm lifecycle scripts not running by default.

Single-file scripts need no build step and no tsconfig.json. Applications and framework projects keep their normal setup.

To convert an existing project, see the migrate-to-deno skill.

Dependency management

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deno install                  # install everything declared
deno add express              # from npm (unprefixed = npm)
deno add jsr:@std/path        # from JSR
deno add -D vitest            # dev dependency (package.json only)
deno remove express
deno outdated                 # list outdated deps
deno update                   # alias for `deno outdated --update`
deno update --latest          # ignore existing semver ranges
deno list                     # declared deps + resolved versions (npm ls)
deno why express              # why a package is in the tree
deno audit                    # vulnerability audit
deno ci                       # clean reproducible install for CI
dx cowsay hello               # run a package binary without installing (npx)

deno ci is the CI command, not deno install: it requires deno.lock, deletes node_modules, installs strictly from the lockfile, and fails if the lockfile is stale. --prod skips devDependencies.

dx is npx / bunx / pnpm dlx, and an alias for deno x. It runs with the sandbox disabled, so treat it with the same care as npx.

Deno won't install a version published less than a day ago, limiting the window for a compromised release. Override with --min-dep-age, which takes minutes (120), an ISO-8601 duration (P7D), a cutoff date, or 0 to disable:

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deno add --min-dep-age=0 npm:some-package

Lifecycle scripts (postinstall) don't run by default — a common surprise when a native addon looks broken after install. Approve once per project:

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deno approve-scripts                              # interactive picker
deno install --allow-scripts=npm:better-sqlite3

Where configuration goes

Put dependencies in package.json — every other tool reads it, and Deno resolves it natively. Use deno.json for dependencies only when there is no package.json: a standalone script, or a JSR package. Likewise prefer tsconfig.json over compilerOptions in deno.json, so tsc and editors see the same settings.

Commit deno.lock. Deno seeds it from an existing package-lock.json, yarn.lock, bun.lock, or pnpm lockfile, preserving pins.

node_modules layout

Deno uses pnpm's isolated layout: real files in node_modules/.deno/, exposed by symlinks, so a package can't import what it never declared. For a tool that needs npm's flat hoisted tree:

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{ "nodeModulesLinker": "hoisted" }

nodeModulesDir applies only to projects without a package.json, so it is rarely the right knob.

Permissions

Deno grants no filesystem, network, environment, or subprocess access unless asked.

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deno run --allow-net=api.example.com --allow-read=./data main.ts
deno run -A main.ts          # allow everything

-S is --allow-sys, not --allow-run. Every flag takes an allowlist — --allow-net=example.com:443 beats bare --allow-net. Matching --deny-* flags always win.

On Requires net access to "...", add that specific permission. -A is fine for trusted first-party code and during migration, but a poor default to commit in a task.

Configuration

deno.json (or .jsonc) is auto-discovered from the current directory upward.

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{
  "tasks": {
    "dev": "deno watch -A main.ts",
    "start": "deno run -A main.ts"
  },
  "fmt": { "exclude": ["build/"] },
  "lint": { "rules": { "exclude": ["no-explicit-any"] } },
  "exclude": ["build/", "dist/"]
}

Top-level exclude applies to every subcommand; per-tool exclude narrows it.

deno.json also accepts imports, an import map pointing bare specifiers at real ones. That is how a project without package.json declares dependencies, and how a JSR package declares its own alongside name, version, exports.

Workspaces

npm, Yarn, and Bun workspaces work out of the box — Deno reads package.json "workspaces" directly. pnpm is the exception: pnpm-workspace.yaml is migrated into deno.json on first run, which must then be re-run.

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{ "workspace": ["./packages/core", "./packages/cli"] }

Members are explicit or single-level globs ("packages/*"); ** and negation are unsupported. Run a task across members with deno task --filter '*' build.

Packages: npm and JSR

Prefer npm — it is where the ecosystem is, and deno add express is the normal case. Reach for JSR for the standard library (@std/*), or to publish TypeScript that consumers get types for without a build step. Mixing is fine.

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deno add jsr:@std/path npm:express
deno doc jsr:@std/path        # read a package's API from the terminal

Deno once used full URL imports (https://deno.land/x/...). They still run but aren't recommended; to modernize, deno add the package and import the bare specifier.

Built-in tooling

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deno fmt              # format (--check for CI)
deno lint             # lint (--fix, --rules)
deno test             # tests (--watch, --parallel, --coverage=dir)
deno check main.ts    # type-check without running
deno bench            # benchmarks
deno coverage         # coverage report from --coverage output
deno compile main.ts  # single-file executable (--target cross-compiles)
deno doc mod.ts       # docs (--html for a site)
deno info main.ts     # module graph and cache info

These cover prettier, eslint, jest/vitest, tsc, and pkg/nexe with no config or dependencies — but they are not drop-in replacements. Parity is incomplete, so moving an established project is real work. There is no need to migrate: keep prettier, eslint, and vitest, and use Deno as runtime and package manager. Prefer the built-in tools for new projects.

Suppress with // deno-lint-ignore <rule>, // deno-lint-ignore-file, // deno-fmt-ignore, // deno-fmt-ignore-file. In Markdown, <!-- deno-fmt-ignore --> before a code block protects illustrative snippets that aren't valid standalone code.

Running code

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deno main.ts                 # deno run is optional
deno watch main.ts           # reload on change (replaces nodemon)
deno task dev                # task from package.json or deno.json
deno repl
deno eval "console.log(1)"

deno watch hot-replaces modules, restarting if that fails; it aliases deno run --watch-hmr.

An HTTP server needs no dependencies:

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Deno.serve((_req) => new Response("Hello"));

Starting a new project

Scaffold rather than hand-writing the files:

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deno init my-project          # script + test + deno.json
deno init --empty my-project  # just main.ts and deno.json
deno init --lib my-lib        # library laid out for JSR
deno create vite my-app       # scaffold from a package initializer

deno create is npm create / yarn create and covers that ecosystem (deno create astro, etc). Unprefixed names are npm; --jsr selects JSR.

Publishing

To npm the regular flow still works — npm publish, or deno pack to build the tarball first. deno publish targets JSR only, from a deno.json with name, version, and exports:

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deno publish --dry-run
deno publish

Provenance attestation is automatic on GitHub Actions. deno bump-version patch bumps the version, across every member at a workspace root.

Guide: https://docs.deno.com/runtime/reference/cli/publish/

Reviewing Deno code

• -A committed in a task where a scoped grant would work.
• deno.lock uncommitted, or CI running deno install instead of deno ci.
• Inline jsr:/npm: specifiers in a project with a package.json — use deno add so the version lives in one place. Fine in standalone scripts.
• A specifier with no version constraint.
• Dependencies or compiler options in deno.json when package.json or tsconfig.json exists.
• Missing deno fmt --check, deno lint, deno check in CI.

Further reading

• https://docs.deno.com — runtime documentation
• https://docs.deno.com/api/ — Deno.* API reference
• references/CLI.md — fuller subcommand and flag reference
• deno <subcommand> --help — authoritative and version-accurate; check it before guessing at a flag.