Tree-shaking also happens in GC heap space (both in the server and in the browser): V8 and SpiderMonkey and JavaScriptCore will all eventually treeshake unused

Tree-shaking also happens in GC heap space (both in the server and in the browser): V8 and SpiderMonkey and JavaScriptCore will all eventually treeshake unused code out of memory entirely. The ESM format was designed to allow that. Modules actually only reference each other through weak references and things like import * build proxy objects of weak references designed for tree-shaking. Depending on how your application is structured, an ESM version of a server-only Node application may still benefit from the performance optimizations of the server being able to do dead code elimination at runtime.