feat: support importing json modules

src/helpers.ts

@@ -1,3 +1,15 @@
+export const TS_EXTENSIONS = /\.([cm]ts|[tj]sx?)$/;
+
+export const DTS_EXTENSIONS = /\.d\.(c|m)?tsx?$/;
+
+export const JSON_EXTENSIONS = /\.json$/;
+
+const SUPPORTED_EXTENSIONS = /((\.d)?\.(c|m)?(t|j)sx?|\.json)$/;
+
 export function trimExtension(path: string) {
-  return path.replace(/((\.d)?\.(c|m)?(t|j)sx?)$/, "")
+  return path.replace(SUPPORTED_EXTENSIONS, "")
+}
+
+export function getDeclarationId(path: string) {
+  return path.replace(SUPPORTED_EXTENSIONS, ".d.ts")
 }

src/index.ts

@@ -2,9 +2,9 @@ import * as path from "node:path";
 import type { PluginImpl, Plugin } from "rollup";
 import ts from "typescript";
 import { type Options, resolveDefaultOptions, type ResolvedOptions } from "./options.js";
-import { createProgram, createPrograms, dts, DTS_EXTENSIONS, formatHost, getCompilerOptions } from "./program.js";
+import { createProgram, createPrograms, formatHost, getCompilerOptions } from "./program.js";
 import { transform } from "./transform/index.js";
-import { trimExtension } from "./helpers.js";
+import { trimExtension, DTS_EXTENSIONS, JSON_EXTENSIONS, getDeclarationId } from "./helpers.js";
 
 export type { Options };
 
@@ -119,7 +119,7 @@ const plugin: PluginImpl<Options> = (options = {}) => {
     },
 
     transform(code, id) {
-      if (!TS_EXTENSIONS.test(id)) {
+      if (!TS_EXTENSIONS.test(id) && !JSON_EXTENSIONS.test(id)) {
         return null;
       }
 
@@ -144,7 +144,7 @@ const plugin: PluginImpl<Options> = (options = {}) => {
       };
 
       const treatTsAsDts = () => {
-        const declarationId = id.replace(TS_EXTENSIONS, dts);
+        const declarationId = getDeclarationId(id);
         const module = getModule(ctx, declarationId, code);
         if (module) {
           watchFiles(module);
@@ -153,12 +153,12 @@ const plugin: PluginImpl<Options> = (options = {}) => {
         return null;
       };
 
-      const generateDtsFromTs = () => {
+      const generateDts = () => {
         const module = getModule(ctx, id, code);
         if (!module || !module.source || !module.program) return null;
         watchFiles(module);
 
-        const declarationId = id.replace(TS_EXTENSIONS, dts);
+        const declarationId = getDeclarationId(id);
 
         let generated!: ReturnType<typeof transformPlugin.transform>;
         const { emitSkipped, diagnostics } = module.program.emit(
@@ -185,8 +185,13 @@ const plugin: PluginImpl<Options> = (options = {}) => {
       // if it's a .d.ts file, handle it as-is
       if (DTS_EXTENSIONS.test(id)) return handleDtsFile();
 
+      // if it's a json file, use the typescript compiler to generate the declarations,
+      // requires `compilerOptions.resolveJsonModule: true`.
+      // This is also commonly used with `@rollup/plugin-json` to import JSON files.
+      if (JSON_EXTENSIONS.test(id)) return generateDts();
+
       // first attempt to treat .ts files as .d.ts files, and otherwise use the typescript compiler to generate the declarations
-      return treatTsAsDts() ?? generateDtsFromTs();
+      return treatTsAsDts() ?? generateDts();
     },
 
     resolveId(source, importer) {

src/program.ts

@@ -1,8 +1,6 @@
 import * as path from "node:path";
 import ts from "typescript";
-
-export const DTS_EXTENSIONS = /\.d\.(c|m)?tsx?$/;
-export const dts = ".d.ts";
+import { DTS_EXTENSIONS } from "./helpers.js";
 
 export const formatHost: ts.FormatDiagnosticsHost = {
   getCurrentDirectory: () => ts.sys.getCurrentDirectory(),
@@ -26,6 +24,8 @@ const DEFAULT_OPTIONS: ts.CompilerOptions = {
   preserveSymlinks: true,
   // Ensure we can parse the latest code
   target: ts.ScriptTarget.ESNext,
+  // Allows importing `*.json`
+  resolveJsonModule: true,
 };
 
 const configByPath = new Map<string, ts.ParsedCommandLine>();

src/transform/index.ts

@@ -6,7 +6,7 @@ import { preProcess } from "./preprocess.js";
 import { convert } from "./Transformer.js";
 import { ExportsFixer } from "./ExportsFixer.js";
 import { TypeOnlyFixer } from "./TypeOnlyFixer.js";
-import { trimExtension } from "../helpers.js";
+import { JSON_EXTENSIONS, trimExtension } from "../helpers.js";
 
 function parse(fileName: string, code: string): ts.SourceFile {
   return ts.createSourceFile(fileName, code, ts.ScriptTarget.Latest, true);
@@ -81,9 +81,10 @@ export const transform = () => {
       const moduleIds = this.getModuleIds()
       const moduleId = Array.from(moduleIds).find((id) => trimExtension(id) === name)
       const isEntry = Boolean(moduleId && this.getModuleInfo(moduleId)?.isEntry)
+      const isJSON = Boolean(moduleId && JSON_EXTENSIONS.test(moduleId))
 
       let sourceFile = parse(fileName, code);
-      const preprocessed = preProcess({ sourceFile, isEntry });
+      const preprocessed = preProcess({ sourceFile, isEntry, isJSON });
       // `sourceFile.fileName` here uses forward slashes
       allTypeReferences.set(sourceFile.fileName, preprocessed.typeReferences);
       allFileReferences.set(sourceFile.fileName, preprocessed.fileReferences);

src/transform/preprocess.ts

@@ -16,6 +16,7 @@ type Range = [start: number, end: number];
 interface PreProcessInput {
   sourceFile: ts.SourceFile;
   isEntry: boolean;
+  isJSON?: boolean;
 }
 
 interface PreProcessOutput {
@@ -39,7 +40,7 @@ interface PreProcessOutput {
  * - [ ] Duplicate the identifiers of a namespace `export`, so that renaming does
  *   not break it
  */
-export function preProcess({ sourceFile, isEntry }: PreProcessInput): PreProcessOutput {
+export function preProcess({ sourceFile, isEntry, isJSON }: PreProcessInput): PreProcessOutput {
   const code = new MagicString(sourceFile.getFullText());
 
   // Only treat as global module if it's not an entry point,
@@ -235,6 +236,23 @@ export function preProcess({ sourceFile, isEntry }: PreProcessInput): PreProcess
   if (exportedNames.size) {
     code.append(`\nexport { ${[...exportedNames].join(", ")} };\n`);
   }
+  if(isJSON && exportedNames.size) {
+    /**
+     * Add default export for JSON modules.
+     * 
+     * The typescript compiler only generate named exports for each top-level key,
+     * but we also need a default export for JSON modules in most cases.
+     * This also aligns with the behavior of `@rollup/plugin-json`.
+     */
+    defaultExport = uniqName("export_default");
+    code.append([
+      `\ndeclare const ${defaultExport}: {`,
+      [...exportedNames].map(name => `  ${name}: typeof ${name};`).join("\n"),
+      `};`,
+      `export default ${defaultExport};\n` 
+      ].join('\n')
+    );
+  }
   for (const [fileId, importName] of inlineImports.entries()) {
     code.prepend(`import * as ${importName} from "${fileId}";\n`);
   }

tests/testcases/import-json/bar.json

@@ -0,0 +1 @@
+["bar"]

tests/testcases/import-json/expected.d.ts

@@ -0,0 +1,8 @@
+declare let name: string;
+declare let age: number;
+declare const export_default: {
+  name: typeof name;
+  age: typeof age;
+};
+declare const _exports: string[];
+export { age, _exports as bar, export_default as foo, name };

tests/testcases/import-json/foo.json

@@ -0,0 +1,4 @@
+{
+  "name": "foo",
+  "age": 42
+}

tests/testcases/import-json/index.ts

@@ -0,0 +1,6 @@
+import foo from './foo.json'
+import bar from './bar.json'
+
+export { name, age } from './foo.json'
+
+export { foo, bar }

tests/testcases/import-json/meta.js

@@ -0,0 +1,18 @@
+// @ts-check
+import url from "url";
+import path from "path";
+
+/** @type {import('../../testcases').Meta} */
+export default {
+  /**
+   * TypeScript <5.1 will generate `declare const name: ...` for JSON modules,
+   * instead of current `decalre let name: ...`.
+   * But it doesn't matter, both can work.
+   * I'll just omit the test results for TypeScript <5.1.
+   */
+  tsVersion: "5.1",
+  options: {
+    tsconfig: path.resolve(url.fileURLToPath(new URL(".", import.meta.url)), "tsconfig.json"),
+  },
+  rollupOptions: {},
+};

tests/testcases/import-json/tsconfig.json

@@ -0,0 +1,21 @@
+{
+  "compilerOptions": {
+    "resolveJsonModule": true,
+
+    "lib": ["es2018", "esnext", "dom"],
+    "skipDefaultLibCheck": true,
+
+    "strict": true,
+
+    "allowSyntheticDefaultImports": true,
+    "moduleResolution": "node",
+    "module": "esnext",
+    "stripInternal": true,
+
+    "newLine": "lf",
+
+    "noEmit": true,
+    "declaration": true,
+    "emitDeclarationOnly": true
+  }
+}