Add module to specify for older systems

PR-URL: #79
Credit: @mpodwysocki
Close: #79
Reviewed-by: @isaacs

EDIT(@isaacs): Add to docs

README.md

@@ -561,7 +561,7 @@ If the `selfLink` config is not explicitly set, and creating the
 symlink fails (common on Windows systems where `fs.symlink()` may
 require elevated permissions), then the error will be ignored.
 
-### Old Style Exports
+### Old Style Exports (top-level `main`, `module`, `types`)
 
 Versions of node prior to 12.10.0, published in early to mid
 2016, did not have support for `exports` as a means for defining
@@ -574,6 +574,12 @@ If there is a `commonjs` export of the `"."` subpath, and the
 tshy will use that to set the `main` and `types` fields, for
 compatibility with these tools.
 
+Similarly, some tools rely on a top-level `module` field, which
+is the ESM equivalent to `"main"`. If there is an `esm` export of
+the `"."` subpath, and the `tshy.module` field in package.json is
+not set to `false`, then tshy will use that to set the `module`
+field, for compatibility with these tools.
+
 **Warning: relying on top-level main/types will likely cause
 incorrect types to be loaded in some scenarios.**
 
@@ -588,6 +594,11 @@ not created, or if the `"."` export does not support the
 `commonjs` dialect, and `main` is explicitly set to `true`, then
 the build will fail.
 
+If the `esm` dialect is not built, or if a `"."` export is not
+created, or if the `"."` export does not support the `esm`
+dialect, and `module` is explicitly set to `true`, then the build
+will fail.
+
 For example, this config:
 
 ```json
@@ -605,6 +616,7 @@ will produce:
 ```json
 {
   "main": "./dist/commonjs/index.js",
+  "module": "./dist/esm/index.js",
   "types": "./dist/commonjs/index.d.ts",
   "type": "module",
   "exports": {

src/exports.ts

@@ -207,10 +207,24 @@ export const setMain = (
     delete pkg.types
   }
   pkg.type = pkg.type === 'commonjs' ? 'commonjs' : 'module'
+
+  // Set the package module to exports["."]
+  const importMod = resolveExport(pkg.exports['.'], ['import'])
+  const module = c?.module ?? !!importMod
+  if (module) {
+    if (!importMod) {
+      fail(`could not resolve exports['.'] for tshy.module 'import'`)
+      return process.exit(1)
+    }
+
+    pkg.module = importMod
+  } else {
+    if (c && c.module !== false) delete c.module
+    delete pkg.module
+  }
 }
 
-// These are all defined by exports, so it's just confusing otherwise
-delete pkg.module
 pkg.exports = getExports(config)
+
 setMain(config, pkg as Package & { exports: ExportsSubpaths })
 export default pkg.exports

src/types.ts

@@ -9,6 +9,7 @@ export type TshyConfigMaybeGlobExports = {
   dialects?: Dialect[]
   selfLink?: boolean
   main?: boolean
+  module?: boolean
   commonjsDialects?: string[]
   esmDialects?: string[]
   project?: string

tap-snapshots/test/exports.ts.test.cjs

@@ -417,6 +417,14 @@ Array [
 ]
 `
 
+exports[`test/exports.ts > TAP > setting top level main > invalid module esm, no exports > must match snapshot 1`] = `
+Array [
+  Array [
+    "could not resolve exports['.'] for tshy.module 'import'",
+  ],
+]
+`
+
 exports[`test/exports.ts > TAP > setting top level main > main explicit true, no commonjs module > must match snapshot 1`] = `
 Array [
   Array [

test/exports.ts

@@ -90,11 +90,13 @@ t.test('setting top level main', async t => {
       tshy?: TshyConfig
       exports: Record<string, ConditionalValue>
       main?: string
+      module?: string
       types?: string
       type?: string
     },
     {
       main?: string
+      module?: string
       types?: string
     },
     boolean,
@@ -205,6 +207,58 @@ t.test('setting top level main', async t => {
       {},
       false,
     ],
+    [
+      'module defaults true',
+      {
+        exports: {
+          '.': {
+            require: { types: './r.d.ts', default: './r.js' },
+            import: { types: './i.d.ts', default: './i.js' },
+          },
+        },
+      },
+      { main: './r.js', types: './r.d.ts', module: './i.js' },
+      true,
+    ],
+    [
+      'module explicit true',
+      {
+        tshy: { module: true },
+        exports: {
+          '.': {
+            require: { types: './r.d.ts', default: './r.js' },
+            import: { types: './i.d.ts', default: './i.js' },
+          },
+        },
+      },
+      { main: './r.js', types: './r.d.ts', module: './i.js' },
+      true,
+    ],
+    [
+      'module explicit false, removes',
+      {
+        tshy: { module: false },
+        main: './r.js',
+        types: './r.d.ts',
+        exports: {
+          '.': {
+            require: { types: './r.d.ts', default: './r.js' },
+            import: { types: './i.d.ts', default: './i.js' },
+          },
+        },
+      },
+      { main: './r.js', types: './r.d.ts' },
+      true,
+    ],
+    [
+      'invalid module esm, no exports',
+      {
+        tshy: { module: true },
+        exports: {},
+      },
+      {},
+      false,
+    ],
     [
       'type defaults module',
       {
@@ -232,7 +286,7 @@ t.test('setting top level main', async t => {
       true,
     ],
     [
-      'invalide type',
+      'invalid type',
       {
         type: 'invalid type',
         exports: {},