Add support for nested main field selectors (#218)

* Add support for nested main field selectors

* Update description of `mainFields`

README.md

@@ -146,7 +146,7 @@ The public API consists of these functions:
 export interface ExplicitParams {
   baseUrl: string;
   paths: { [key: string]: Array<string> };
-  mainFields?: Array<string>;
+  mainFields?: (string | string[])[];
   addMatchAll?: boolean;
   cwd?: string;
 }
@@ -202,14 +202,14 @@ export interface MatchPath {
  * Creates a function that can resolve paths according to tsconfig paths property.
  * @param absoluteBaseUrl Absolute version of baseUrl as specified in tsconfig.
  * @param paths The paths as specified in tsconfig.
- * @param mainFields A list of package.json field names to try when resolving module files.
+ * @param mainFields A list of package.json field names to try when resolving module files. Select a nested field using an array of field names.
  * @param addMatchAll Add a match-all "*" rule if none is present
  * @returns a function that can resolve paths.
  */
 export function createMatchPath(
   absoluteBaseUrl: string,
   paths: { [key: string]: Array<string> },
-  mainFields: string[] = ["main"],
+  mainFields: (string | string[])[] = ["main"],
   addMatchAll: boolean = true
 ): MatchPath {
 ```
@@ -226,7 +226,7 @@ The `createMatchPath` function will create a function that can match paths. It a
  * @param readJson Function that can read json from a path (useful for testing).
  * @param fileExists Function that checks for existence of a file at a path (useful for testing).
  * @param extensions File extensions to probe for (useful for testing).
- * @param mainFields A list of package.json field names to try when resolving module files.
+ * @param mainFields A list of package.json field names to try when resolving module files. Select a nested field using an array of field names.
  * @returns the found path, or undefined if no path was found.
  */
 export function matchFromAbsolutePaths(
@@ -235,7 +235,7 @@ export function matchFromAbsolutePaths(
   readJson: Filesystem.ReadJsonSync = Filesystem.readJsonFromDiskSync,
   fileExists: Filesystem.FileExistsSync = Filesystem.fileExistsSync,
   extensions: Array<string> = Object.keys(require.extensions),
-  mainFields: string[] = ["main"]
+  mainFields: (string | string[])[] = ["main"]
 ): string | undefined {
 ```
 

src/__tests__/data/match-path-data.ts

@@ -7,7 +7,7 @@ export interface OneTest {
   readonly skip?: boolean;
   readonly absoluteBaseUrl: string;
   readonly paths: { [key: string]: Array<string> };
-  readonly mainFields?: string[];
+  readonly mainFields?: (string | string[])[];
   readonly addMatchAll?: boolean;
   readonly existingFiles: ReadonlyArray<string>;
   readonly requestedModule: string;
@@ -149,6 +149,17 @@ export const tests: ReadonlyArray<OneTest> = [
     extensions: [".ts", ".js"],
     expectedPath: join("/root", "location", "mylibjs", "kalle.js"),
   },
+  {
+    name: "should resolve nested main fields",
+    absoluteBaseUrl: "/root/",
+    paths: { "lib/*": ["location/*"] },
+    mainFields: [["esnext", "main"]],
+    packageJson: { esnext: { main: "./main.js" } },
+    existingFiles: [join("/root", "location", "mylibjs", "main.js")],
+    extensions: [".ts", ".js"],
+    requestedModule: "lib/mylibjs",
+    expectedPath: join("/root", "location", "mylibjs", "main.js"),
+  },
   {
     name: "should ignore advanced field mappings in package.json",
     absoluteBaseUrl: "/root/",

src/config-loader.ts

@@ -4,7 +4,7 @@ import * as path from "path";
 export interface ExplicitParams {
   baseUrl: string;
   paths: { [key: string]: Array<string> };
-  mainFields?: Array<string>;
+  mainFields?: (string | string[])[];
   addMatchAll?: boolean;
 }
 
@@ -24,7 +24,7 @@ export interface ConfigLoaderSuccessResult {
   baseUrl?: string;
   absoluteBaseUrl: string;
   paths: { [key: string]: Array<string> };
-  mainFields?: Array<string>;
+  mainFields?: (string | string[])[];
   addMatchAll?: boolean;
 }
 

src/filesystem.ts

@@ -4,7 +4,7 @@ import * as fs from "fs";
  * Typing for the fields of package.json we care about
  */
 export interface PackageJson {
-  [key: string]: string;
+  [key: string]: string | PackageJson;
 }
 
 /**

src/match-path-async.ts

@@ -27,7 +27,7 @@ export interface MatchPathAsyncCallback {
 export function createMatchPathAsync(
   absoluteBaseUrl: string,
   paths: { [key: string]: Array<string> },
-  mainFields: string[] = ["main"],
+  mainFields: (string | string[])[] = ["main"],
   addMatchAll: boolean = true
 ): MatchPathAsync {
   const absolutePaths = MappingEntry.getAbsoluteMappingEntries(
@@ -64,7 +64,7 @@ export function matchFromAbsolutePathsAsync(
   fileExists: Filesystem.FileExistsAsync = Filesystem.fileExistsAsync,
   extensions: ReadonlyArray<string> = Object.keys(require.extensions),
   callback: MatchPathAsyncCallback,
-  mainFields: string[] = ["main"]
+  mainFields: (string | string[])[] = ["main"]
 ): void {
   const tryPaths = TryPath.getPathsToTry(
     extensions,
@@ -88,7 +88,7 @@ export function matchFromAbsolutePathsAsync(
 
 function findFirstExistingMainFieldMappedFile(
   packageJson: Filesystem.PackageJson,
-  mainFields: string[],
+  mainFields: (string | string[])[],
   packageJsonPath: string,
   fileExistsAsync: Filesystem.FileExistsAsync,
   doneCallback: (err?: Error, filepath?: string) => void,
@@ -108,7 +108,11 @@ function findFirstExistingMainFieldMappedFile(
       index + 1
     );
 
-  const mainFieldMapping = packageJson[mainFields[index]];
+  const mainFieldSelector = mainFields[index];
+  const mainFieldMapping =
+    typeof mainFieldSelector === "string"
+      ? packageJson[mainFieldSelector]
+      : mainFieldSelector.reduce((obj, key) => obj[key], packageJson);
   if (typeof mainFieldMapping !== "string") {
     // Skip mappings that are not pointers to replacement files
     return tryNext();
@@ -136,7 +140,7 @@ function findFirstExistingPath(
   fileExists: Filesystem.FileExistsAsync,
   doneCallback: MatchPathAsyncCallback,
   index: number = 0,
-  mainFields: string[] = ["main"]
+  mainFields: (string | string[])[] = ["main"]
 ): void {
   const tryPath = tryPaths[index];
   if (

src/match-path-sync.ts

@@ -20,14 +20,14 @@ export interface MatchPath {
  *
  * @param absoluteBaseUrl Absolute version of baseUrl as specified in tsconfig.
  * @param paths The paths as specified in tsconfig.
- * @param mainFields A list of package.json field names to try when resolving module files.
+ * @param mainFields A list of package.json field names to try when resolving module files. Select a nested field using an array of field names.
  * @param addMatchAll Add a match-all "*" rule if none is present
  * @returns a function that can resolve paths.
  */
 export function createMatchPath(
   absoluteBaseUrl: string,
   paths: { [key: string]: Array<string> },
-  mainFields: string[] = ["main"],
+  mainFields: (string | string[])[] = ["main"],
   addMatchAll: boolean = true
 ): MatchPath {
   const absolutePaths = MappingEntry.getAbsoluteMappingEntries(
@@ -60,7 +60,7 @@ export function createMatchPath(
  * @param readJson Function that can read json from a path (useful for testing).
  * @param fileExists Function that checks for existence of a file at a path (useful for testing).
  * @param extensions File extensions to probe for (useful for testing).
- * @param mainFields A list of package.json field names to try when resolving module files.
+ * @param mainFields A list of package.json field names to try when resolving module files. Select a nested field using an array of field names.
  * @returns the found path, or undefined if no path was found.
  */
 export function matchFromAbsolutePaths(
@@ -69,7 +69,7 @@ export function matchFromAbsolutePaths(
   readJson: Filesystem.ReadJsonSync = Filesystem.readJsonFromDiskSync,
   fileExists: Filesystem.FileExistsSync = Filesystem.fileExistsSync,
   extensions: Array<string> = Object.keys(require.extensions),
-  mainFields: string[] = ["main"]
+  mainFields: (string | string[])[] = ["main"]
 ): string | undefined {
   const tryPaths = TryPath.getPathsToTry(
     extensions,
@@ -86,13 +86,16 @@ export function matchFromAbsolutePaths(
 
 function findFirstExistingMainFieldMappedFile(
   packageJson: Filesystem.PackageJson,
-  mainFields: string[],
+  mainFields: (string | string[])[],
   packageJsonPath: string,
   fileExists: Filesystem.FileExistsSync
 ): string | undefined {
   for (let index = 0; index < mainFields.length; index++) {
-    const mainFieldName = mainFields[index];
-    const candidateMapping = packageJson[mainFieldName];
+    const mainFieldSelector = mainFields[index];
+    const candidateMapping =
+      typeof mainFieldSelector === "string"
+        ? packageJson[mainFieldSelector]
+        : mainFieldSelector.reduce((obj, key) => obj[key], packageJson);
     if (candidateMapping && typeof candidateMapping === "string") {
       const candidateFilePath = path.join(
         path.dirname(packageJsonPath),
@@ -111,7 +114,7 @@ function findFirstExistingPath(
   tryPaths: ReadonlyArray<TryPath.TryPath>,
   readJson: Filesystem.ReadJsonSync = Filesystem.readJsonFromDiskSync,
   fileExists: Filesystem.FileExistsSync,
-  mainFields: string[] = ["main"]
+  mainFields: (string | string[])[] = ["main"]
 ): string | undefined {
   for (const tryPath of tryPaths) {
     if (