How to use npm module in DENO?

Deno provides a Node Compatibility Library, that will allow to use some NPM packages that do not use non-polyfilled Node.js APIs. You'll be able to require the package by using https://deno.land/std/node/module.ts

The following works on deno 1.0.0

import { createRequire } from "https://deno.land/std/node/module.ts";

const require = createRequire(import.meta.url);
const esprima = require("esprima");

const program = 'const answer = 42';
console.log(esprima.tokenize(program))

The above code will use esprima from node_modules/.

To run it, you'll need --allow-read flag

deno run --allow-read esprima.js

You can restrict it only to node_modules

deno run --allow-read=node_modules esprima.js

Which outputs:

[
 { type: "Keyword", value: "const" },
 { type: "Identifier", value: "answer" },
 { type: "Punctuator", value: "=" },
 { type: "Numeric", value: "42" }
]

Note: many APIs used by std/ are still unstable, so you may need to run it with --unstable flag.

---

Although since that whole project is written in TypeScript already, and it's not using any dependencies, it will be very easy for them to adapt it to Deno. All they need to do is use .ts extension on their imports. You can also fork the project and do the changes.

// import { CommentHandler } from './comment-handler';
import { CommentHandler } from './comment-handler.ts';
// ...

Once they do, you'll be able to just do:

// Ideally they would issue a tagged release and you'll use that instead of master
import esprima from 'https://raw.githubusercontent.com/jquery/esprima/master/src/esprima.ts';

const program = 'const answer = 42';
console.log(esprima.tokenize(program))

Alternative

You can also use https://jspm.io/ which will convert NPM modules to ES Modules

All modules on npm are converted into ES modules handling full CommonJS compatibility including strict mode conversions.

import esprima from "https://dev.jspm.io/esprima";

const program = 'const answer = 42';
console.log(esprima.tokenize(program))

For packages that use Node.js modules not supported by jspm it will throw an error:

Uncaught Error: Node.js fs module is not supported by jspm core. 
Deno support here is tracking in 
https://github.com/jspm/jspm-core/issues/4, +1's are appreciated!

For now, you can use packages that only use Buffer, for that you'll have to include std/node.

// import so polyfilled Buffer is exposed                                                                                                  
import "https://deno.land/std/node/module.ts";
import BJSON from 'https://dev.jspm.io/buffer-json';

const str = BJSON.stringify({ buf: Buffer.from('hello') })

console.log(str);

As of version Deno 1.2.5 (released today) deno is now included with experimental npm support.

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

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

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

You can now run deno run --unstable --A main.ts and express will be downloaded.

Issue

In general, there are two issues with npm packages in Deno:

1. ES Module (ESM) conformity is not given.

• Bare imports like import _ from "lodash" don't work - no "magic" node_modules resolution
• All import specifiers need to include the file extension - .ts,.js etc.
• CommonJS module system is not usable in Deno

2. The npm package uses native Node.js builtins like fs or path.

---

Solutions to issue 1

1.1: Third party modules

The Third Party Modules section is the quickest way to discover compatible packages.

1.2: ESM CDN providers

Also take a look at CDN providers, that can auto-convert npm packages to ES Modules (ESM):

• Skypack CDN
• jspm.io
• unpkg.com with ?module query parameter

Skypack CDN can deliver auto-converted packages, that e.g. have set a "module" entrypoint in package.json. For TypeScript users: It fetches .d.ts type definitions along with .js files (via X-TypeScript-Types HTTP headers used by Deno).

unpkg.com describes its ?module flag as follows: "Expands all 'bare' import specifiers in JavaScript modules to unpkg URLs. This feature is very experimental".

Esprima does not depend on Node.js builtins, so we can simplify its import by a CDN URL:

import esprima from "https://cdn.skypack.dev/esprima@^4.0.1"; // Option 1: Skypack
import esprima from "https://dev.jspm.io/esprima"; // Option 2: jspm 
// your program
const tokens = esprima.tokenize("const foo = 'bar'"); // works

jspm would be a good choice here - Skypack TS types didn't work for me in this particular case.

1.3: Other approaches

You might also try to import an ESM compatible version directly from repository sources (e.g. an ESM branch). Though for Esprima it won't work because of missing file extensions in code.

Snowpack and jspm stand in for a more manual approach to convert CommonJS → ESM. The rollup plugin @rollup/plugin-commonjs (internally used by Snowpack) is even a more low-level tool.

---

Solution to issue 2

Deno provides a Node compatibility layer, see Marcos Casagrande's answer. However, not all native Node.js built-ins are fully supported.

As Esprima doesn't rely on Node builtins, you can go with the simpler CDN option.