Deno: error: Uncaught (in promise) ReferenceError: require is not defined

ReferenceError: require is not defined in Deno means the file uses CommonJS require() but Deno loaded it as an ES module, where require does not exist. Deno 2 treats .js and .ts files as ES modules unless told otherwise. Replace require("fs") with import fs from "node:fs", or keep CommonJS by renaming the file to .cjs.

Reproduce it

echo 'const fs = require("fs"); console.log(typeof fs.readFileSync);' > main.js
npx --yes deno@2 run main.js 2>&1 | head -1
exit "${PIPESTATUS[0]}"

error: Uncaught (in promise) ReferenceError: require is not defined

Below that line Deno prints an info: note and hints listing the same options as the fixes here.

Fix

Convert to ESM:

echo 'import fs from "node:fs"; console.log(typeof fs.readFileSync);' > main.js
npx --yes deno@2 run main.js

function

Or keep CommonJS with the .cjs extension:

echo 'const fs = require("fs"); console.log(typeof fs.readFileSync);' > main.cjs
npx --yes deno@2 run main.cjs

function

Node.js shows a similar message, require is not defined in ES module scope, when a .js file sits under a package.json with "type": "module".