IconVault's JS Module Resolution lab visualizes the algorithm that turns an import specifier into a file. Type any specifier and importing file, pick CommonJS or ESM, then step through the resolution one lookup at a time while the fake project tree highlights every path being probed. Watch bare specifiers walk node_modules upward, relative imports probe extensions, and package.json exports maps win over main. It is free and runs fully in your browser, with a copyable step-by-step trace.

Preset scenarios cover the classic gotchas: ./lib resolving in CJS but failing in ESM without an extension, lodash/fp subpath resolution, @scope/pkg exports maps, and nested node_modules winning for deep dependencies but losing from the project root. Comparing the two modes side by side is the fastest way to understand why ERR_MODULE_NOT_FOUND happens and how to fix it.