Feature: ESM exports support (#55)

- Support ESM `package.json:exports` in Modern Node.js (CJS and ESM style). Fixes #51

CHANGELOG.md

@@ -1,6 +1,11 @@
 Changes
 =======
 
+## UNRELEASED
+
+* Feature: Add full support for modern Node.js ESM and `exports`.
+  [#49](https://github.com/FormidableLabs/trace-deps/issues/51)
+
 ## 0.3.9
 
 * Bug/Feature: Support relative paths from package name root in `allowMissing`.

README.md

@@ -109,6 +109,12 @@ Examples:
 
 * **Only handles single string dependencies**: `require`, `require.resolve`, and dynamic `import()` support calls with variables or other expressions like `require(aVar)`, `import(process.env.VAL + "more-stuff")`. This library presently only supports calls with a **single string** and nothing else. We have a [tracking ticket](https://github.com/FormidableLabs/trace-deps/issues/2) to consider expanding support for things like partial evaluation.
 
+* **Modern Node.js ESM / `package.json:exports` Support**: Node.js v12 and newer now support modern ESM, and `trace-deps` will correctly package your application in any Node.js runtime. Unfortunately, the implementation of how to [resolve an ESM import](https://nodejs.org/api/packages.html) in modern Node.js is quite complex.
+    * **It's complicated**: For example, for the same import of `my-pkg`, a `require("my-pkg")` call in Node.js v10 might match a file specified in `package.json:main`, while `require("my-pkg")` in Node.js v12 might match a second file specified in `package.json:exports:".":require`, and `import "my-pkg"` in Node,js v12 might match a _third_ file specified in `package.json:exports:".":import`. Then, throw in [conditions](https://nodejs.org/api/packages.html#packages_conditional_exports), [subpaths](https://nodejs.org/api/packages.html#packages_subpath_exports), and even subpath conditions, and it becomes exceedingly difficult to statically analyze what is actually going to be imported at runtime by Node.js ahead of time, which is what `trace-deps` needs to do. 🤯
+    * **Our solution**: Our approach is to basically give up on trying to figure out the exact runtime conditions that will be used in module resolution, and instead package all reasonable conditions for a given module import. This means that maintain correctness at the cost of slightly larger zip sizes for libraries that ship multiple versions of exports.
+    * **Our implementation**: When `trace-deps` encounters a dependency, it resolves the file according to old CommonJS (reading `package.json:main`) and then in modern Node.js `package.json:exports` mode with each of the following built-in / suggested official conditions: `import`, `require`, `node`, `default`, `development`, and `production`. (We ignore `browser`).
+    * **Missing Features**: `trace-deps` does not support the deprecated [subpath folder mappings](https://nodejs.org/api/packages.html#packages_subpath_folder_mappings) feature. Some advanced ESM features are still under development.
+
 * **Includes `package.json` files used in resolution**: As this is a Node.js-focused library, to follow the Node.js [module resolution algorithm](https://nodejs.org/api/modules.html#modules_all_together) which notably uses intermediate encountered `package.json` files to determine how to resolve modules. This means that we include a lot of `package.json` files that seemingly aren't directly imported (such as a `const pkg = require("mod/package.json")`) because they are needed for the list of all traced files to together resolve correctly if all on disk together.
 
 * **Using the `allowMissing` option**: The `allowMissing` function field helps in situations where you want to allow certain dependencies to have known missing sub-dependencies, often seen in patterns like: `try { require("optional-dep"); } catch (e) {}`. If the sub-dependency is found, then it will be returned just like any normal one. If not, the module not found error is just swallowed and normal processing resumes.

lib/package.js

@@ -51,8 +51,71 @@ const getLastPackageSegment = (filePath) => {
   return relPath || null;
 };
 
+// Get path up to and including the package name (but no further).
+const getLastPackageRoot = (filePath) => {
+  if (!filePath) { return null; }
+
+  // Iterate all normalized parts of the file path and extract packages.
+  const parts = path.normalize(filePath).split(path.sep);
+  const lastModsIdx = parts.lastIndexOf("node_modules");
+
+  // Not within node_modules or potential path possibility.
+  if (lastModsIdx === -1 || lastModsIdx + 1 === parts.length) { return null; }
+
+  // Find package root index. Start with unscoped.
+  let pkgRootIdx = lastModsIdx + 1;
+  if (parts[pkgRootIdx][0] === "@") {
+    // Check possible scoped path possibility.
+    if (pkgRootIdx + 1 === parts.length) { return null; }
+    // Scoped.
+    pkgRootIdx++;
+  }
+
+  return parts.slice(0, pkgRootIdx + 1).join(path.sep);
+};
+
+// Return module name and relative path (as array of path parts).
+const getDependencyParts = (dep) => {
+  if (
+    !dep
+    || path.isAbsolute(dep)
+    || dep.startsWith(".")
+  ) {
+    return null;
+  }
+
+  // Note that `package.json:exports` don't normalize/resolve `..` or file
+  // paths, so leave them intact (which means manually replace `\\` instead
+  // of using `toPosixPath`).
+  let parts = dep
+    .replace(/\\/g, "/")
+    .split("/")
+    .filter(Boolean);
+
+  let name;
+  if (parts.length > 0) {
+    if (parts[0][0] === "@") {
+      if (parts[1]) {
+        name = `${parts[0]}/${parts[1]}`;
+        parts = parts.slice(2); // eslint-disable-line no-magic-numbers
+      }
+    } else if (parts[0]) {
+      name = parts[0];
+      parts = parts.slice(1);
+    }
+  }
+
+  if (!name) {
+    return null;
+  }
+
+  return { name, parts };
+};
+
 module.exports = {
   getPackages,
   getLastPackage,
-  getLastPackageSegment
+  getLastPackageSegment,
+  getLastPackageRoot,
+  getDependencyParts
 };

lib/path.js

@@ -0,0 +1,9 @@
+"use strict";
+
+const path = require("path");
+
+const toPosixPath = (file) => !file ? file : path.normalize(file).replace(/\\/g, "/");
+
+module.exports = {
+  toPosixPath
+};