ESM and CommonJS Differences and Interoperability

We already mentioned several important differences between ESM and CommonJS, such as having to explicitly specify file extensions in imports with ESM, while file extensions are totally optional with the CommonJS require function.

Let's close this chapter by discussing some other important differences between ESM and CommonJS and how the two module systems can work together when necessary.

ESM runs in strict mode

ES modules run implicitly in strict mode. This means that we don’t have to explicitly add the "use strict" statements at the beginning of every file. Strict mode cannot be disabled; therefore, we cannot use undeclared variables or the with statement or have other features that are only available in non-strict mode, but this is definitely a good thing, as strict mode is a safer execution mode.

Missing references in ESM

In ESM, some important CommonJS references are not defined. These include require, exports, module.exports, __filename, and __dirname. If we try to use any of them within an ES module, we’ll get a ReferenceError because ES also runs in strict mode.