Using an imported module inside Google App Script

Your GAS project is not a Node.js app, so the above will not work. While both Google Apps Script and Node use JavaScript, they provide different runtime environments for executing JS code. In GAS, the environment is a closed ecosystem on Google Servers that end users don't know anything about.

In Node, the runtime consists of V8 (JS engine) and C++ add-ons that expose low-level APIs (access to the file system, etc.). The library you referenced is an NPM package created for Node.js. Installing the package via NPM will make it available for Node projects, but don't think it will magically appear on Google servers as well.

You must either use GAS-specific versions of these dependencies, or, if they don't exist, refactor the source code to make it compatible with GAS (Node and GAS use different ECMAScript versions, so some latest features like arrow functions will break your GAS code).

For example, here's lodash for GAS https://github.com/contributorpw/lodashgs

Using libraries in GAS https://developers.google.com/apps-script/guides/libraries

P.S. In GAS, all ".gs" files share the same namespace, so calling 'require' function is redundant. If you want to mimic this behavior, you'll still need to write your own require function.

I wrote up some guidance on packaging up an npm module for usage in Apps Script in this article.

tl;dr is to create an index.js with the following in a new directory:

import {compareTwoStrings, findBestMatch} from 'string-similarity';
export {compareTwoStrings, findBestMatch};

and then run the following in that directory:

npm init -y
npm install --save-dev string-similarity
npx esbuild index.js --bundle --global-name=stringSimilarity --outfile=StringSimilarity.js

You can then copy the contents of StringSimiliarity.js into a new .gs file in the Apps Script editor. The exported functions will be usable from your own Code.gs as stringSimilarity. compareTwoStrings() and stringSimilarity.findBestMatch().

(In case you'd rather not bundle it yourself, the output of the esbuild process for string-similarity can be found in this gist. But the general steps should apply to most npm modules that don't require a Node or browser-specific runtime environment.)

Google Apps Script files by default have access to Google APIs, such as the Spreadsheet Service in the G Suite Services: https://developers.google.com/apps-script/reference/spreadsheet/

A GAS project (so scripts in that project) doesn't by default have access to Node.js or any other frameworks. However, a GAS project can include another GAS project by reference as a library. In the GAS Script Editor, use menu Resources > Libraries... to add an external GAS project as a library by setting the source project's project key (and some other source project properties).

So if you have a Javascript that depends on external resources (like Node.js) by require then if you can find an external GAS project that provides the same services with the same API you can provide it as a library to the script. You omit the require statements from the original script as they're replaced by the GAS library dependency configuration I mentioned.

If your original script and its dependencies are all open source you could create GAS projects for each level of dependencies in the originals. You can already find some popular JS frameworks scripts available as GAS libraries.

GAS also allows packaging external resources as Web apps and other package formats. They can be used with corresponding techniques, and some of them found already available from other developers.