JS: Add helper for doing path resolution with JS rules

Author: asgerf

javascript/ql/lib/semmle/javascript/internal/paths/JSPaths.qll

@@ -0,0 +1,65 @@
+/**
+ * Provides predicates for use in `Folder::ResolveSig` in order to resolve
+ * paths using JavaScript semantics.
+ */
+
+private import javascript
+private import semmle.javascript.TSConfig
+
+/**
+ * Gets a folder name that is a common source folder name.
+ */
+string getASrcFolderName() { result = ["ts", "js", "src", "lib"] }
+
+/**
+ * Gets a folder name that is a common build output folder name.
+ */
+string getABuildOutputFolderName() { result = ["dist", "build", "out", "lib"] }
+
+/**
+ * Provides predicates for use in a `Folder::ResolveSig` in order to resolve
+ * paths using JavaScript semantics.
+ *
+ * This accounts for two things:
+ * - automatic file extensions (e.g `./foo` may resolve to `./foo.js`)
+ * - mapping compiled-generated files back to their original source files
+ */
+module JSPaths {
+  /**
+   * Gets an additional child of `base` to include when resolving JS paths.
+   */
+  pragma[nomagic]
+  Container getAnAdditionalChild(Container base, string name) {
+    // Automatically fill in file extensions
+    result = base.(Folder).getJavaScriptFileOrTypings(name)
+    or
+    // When importing a .js file, map to the original file that compiles to the .js file.
+    exists(string stem |
+      result = base.(Folder).getJavaScriptFileOrTypings(stem) and
+      name = stem + ".js"
+    )
+    or
+    // Redirect './bar' to 'foo' given a tsconfig like:
+    //
+    //   { include: ["foo"], compilerOptions: { outDir: "./bar" }}
+    //
+    exists(TSConfig tsconfig |
+      name =
+        tsconfig.getCompilerOptions().getPropStringValue("outDir").regexpReplaceAll("^\\./", "") and
+      base = tsconfig.getFolder() and
+      result = tsconfig.getEffectiveRootDir()
+    )
+    or
+    // Heuristic version of the above based on commonly used source and build folder names
+    exists(Folder folder | base = folder |
+      folder = any(PackageJson pkg).getFolder() and
+      name = getABuildOutputFolderName() and
+      not exists(folder.getChildContainer(name)) and
+      (
+        result = folder.getChildContainer(getASrcFolderName())
+        or
+        result = folder
+      )
+    )
+  }
+}