From Nodejs docs https://nodejs.org/api/modules.html#all-together
require(X) from module at path Y
1. If X is a core module,
a. return the core module
b. STOP
2. If X begins with '/'
a. set Y to be the filesystem root
3. If X begins with './' or '/' or '../'
a. LOAD_AS_FILE(Y + X)
b. LOAD_AS_DIRECTORY(Y + X)
c. THROW "not found"
4. If X begins with '#'
a. LOAD_PACKAGE_IMPORTS(X, dirname(Y))
5. LOAD_PACKAGE_SELF(X, dirname(Y))
6. LOAD_NODE_MODULES(X, dirname(Y))
7. THROW "not found"
What does no.4 refer to?
What does it mean for a module to start with #?
LOAD_PACKAGE_IMPORTS(X, DIR)
1. Find the closest package scope SCOPE to DIR.
2. If no scope was found, return.
3. If the SCOPE/package.json "imports" is null or undefined, return.
4. let MATCH = PACKAGE_IMPORTS_RESOLVE(X, pathToFileURL(SCOPE),
["node", "require"]) defined in the ESM resolver.
5. RESOLVE_ESM_MATCH(MATCH).
This is the LOAD_PACKAGE_IMPORTS but it looks enigma to me.
The character "#" at the beginning of an import specifier refers to an imports field defined in package.json.
Imports fields are useful to alias file names or other dependencies that are only accessed inside a package.
Considering the example in the documentation:
"imports": {
"#dep": {
"node": "dep-node-native",
"default": "./dep-polyfill.js"
}
}
When a module in this package contains an import statement like
import dep from "#def";
This tells Node.js to import the package "dep-node-native"; and it instructs other build tools (for the browser) to use the local file "./dep-polyfill.js".
This is explained in https://nodejs.org/api/packages.html#subpath-imports:
Subpath imports
Added in: v14.6.0, v12.19.0
In addition to the
"exports"field, it is possible to define internal package import maps that only apply to import specifiers from within the package itself.Entries in the imports field must always start with
#to ensure they are disambiguated from package specifiers.
(emphasis mine)