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Using npm packages with npm specifiers

Deno 1.28 stabilizes support for npm specifiers, which allow you to use npm modules directly in Deno with a higher chance of compatibility than importing from CDN's, particularly if the modules depend on artifact files in their package.

It is important to emphasize that even though this feature was stabilized for use with deno run and some other sub commands in Deno 1.28, it is still under development and doesn't work in some scenarios (ex. with deno bundle). You're likely find scenarios where something doesn't work. Please report these problems to the issue tracker. We'll be working hard to improve the compatibility layer and user experience in the near future. You can follow issue 15960 for updates.

The way these work is best described with an example:

// main.ts
import express from "npm:express@^4.18";
const app = express();

app.get("/", function (req, res) {
  res.send("Hello World");
});

app.listen(3000);
console.log("listening on http://localhost:3000/");

These npm specifiers have the following format:

npm:<package-name>[@<version-requirement>][/<sub-path>]

Then doing the following will start a simple express server:

$ deno run --A main.ts
listening on http://localhost:3000/

When doing this, no npm install is necessary and no node_modules folder is created. These packages are also subject to the same permissions as Deno applications. At the moment though, there are some unnecessary permissions that get asked for, but in the future the above program will only require network permissions.

These specifiers currently work with deno run, deno check, deno info, deno lsp, deno test, and deno bench, but do not with deno vendor, deno install, deno repl, and deno bundle at the moment.

npm package binaries can be executed from the command line without an npm install using a specifier in the following format:

npm:<package-name>[@<version-requirement>][/<binary-name>]

For example:

$ deno run --allow-env --allow-read npm:cowsay@1.5.0 Hello there!
 ______________
< Hello there! >
 --------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||
$ deno run --allow-env --allow-read npm:cowsay@1.5.0/cowthink What to eat?
 ______________
( What to eat? )
 --------------
        o   ^__^
         o  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

--node-modules-dir flag

npm specifiers resolve npm packages to a central global npm cache. This works well in most cases and is ideal since it uses less space and doesn't require a node_modules directory. That said, you may find cases where an npm package expects itself to be executing from a node_modules directory. To improve compatibility and support those packages, you can use the --node-modules-dir flag.

For example, given main.ts:

import chalk from "npm:chalk@5";

console.log(chalk.green("Hello"));

Running this script with a --node-modules-dir like so...

deno run --node-modules-dir main.ts

...will create a node_modules folder in the current directory with a similar folder structure to npm.

Note that this is all done automatically when calling deno run and there is no separate install command necessary.

In the case where you want to modify the contents of the node_modules directory before execution, you can run deno cache with --node-modules-dir, modify the contents, then run the script.

For example:

deno cache --node-modules-dir main.ts
deno run --allow-read=. --allow-write=. scripts/your_script_to_modify_node_modules_dir.ts
deno run --node-modules-dir main.ts