notes/node/cjs-loader.md
Node's CommonJS resolution, cache, extension-handler, and ESM-error behavior.
Module._resolveFilename(request, parent, ...) is the CommonJS resolution entry point (v24.15.0 loader.js#L1392). It delegates path searching to Module._findPath, which checks the resolved path before trying extension and directory candidates.
tryExtensions(basePath, exts, isMain) is the later implicit-extension phase. Registered extension handlers can make a request such as ./target.js resolve target.js.ts. Native ESM does not apply this CommonJS-style implicit extension or directory lookup to relative imports.
Node caches CommonJS modules by resolved filename. _load checks Module._cache[filename] before evaluating a module, and modules cached by the ESM loader have dedicated circular-load handling (loader.js#L1297-L1308).
Node initializes Module._extensions at loader.js#L355. It selects the longest registered extension (findLongestRegisteredExtension) and dispatches to that handler (loader.js#L1545-L1553).
When ESM resolution fails with ERR_MODULE_NOT_FOUND or ERR_UNSUPPORTED_DIR_IMPORT, Node's defaultResolve eagerly calls decorateErrorWithCommonJSHints. That helper runs resolveAsCommonJS, creating a temporary CJS module and invoking CJSModule._resolveFilename.
Any loader that probes alternate specifiers by catching expected ESM misses therefore pays both the failed ESM resolution and the CJS hint lookup. This cost is inherent to the current Node mechanism, not to a particular loader.
See cjs-esm-interop.md for Node's CJS export preparse and namespace construction.