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@@ -0,0 +1,167 @@
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+import * as ts from 'typescript';
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+
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+export interface PathTransformerOptions {
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+ baseUrl: string;
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+ paths: Record<string, string[]>;
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+}
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+
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+interface PathMatcher {
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+ pattern: string;
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+ regex: RegExp;
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+ targets: string[];
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+ hasWildcard: boolean;
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+}
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+
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+/**
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+ * Creates a TypeScript custom transformer that rewrites import/export paths
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+ * from tsconfig path aliases to their resolved relative paths.
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+ *
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+ * This is necessary for ESM mode where tsconfig-paths.register() doesn't work
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+ * because it only hooks into CommonJS require(), not ESM import().
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+ *
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+ * The transformer assumes that both the importing file and imported file compile
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+ * to the same flat output directory. For complex monorepo setups with nested output
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+ * structures, the `pathAdapter.transformTsConfigPathMappings` callback should be
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+ * used to adjust paths appropriately.
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+ *
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+ * Known limitations:
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+ * - Only the first path target is used when multiple fallbacks are configured
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+ * - `require()` calls via `createRequire` are not transformed
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+ */
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+export function createPathTransformer(options: PathTransformerOptions): ts.TransformerFactory<ts.SourceFile> {
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+ const { paths } = options;
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+
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+ // Compile the path patterns into matchers
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+ const pathMatchers = Object.entries(paths).map(([pattern, targets]) => {
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+ const hasWildcard = pattern.includes('*');
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+
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+ // Escape special regex chars, then replace * with capture group
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+ const regexStr: string = pattern
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+ .replace(/[.+?^${}()|[\]\\]/g, String.raw`\$&`)
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+ .split('*')
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+ .join('(.*)');
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+
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+ const regex = new RegExp('^' + regexStr + '$');
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+
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+ return { pattern, regex, targets, hasWildcard };
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+ });
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+
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+ return context => {
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+ const visitor: ts.Visitor = node => {
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+ // Handle import declarations: import { X } from 'module';
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+ if (
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+ ts.isImportDeclaration(node) &&
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+ node.moduleSpecifier &&
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+ ts.isStringLiteral(node.moduleSpecifier)
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+ ) {
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+ const resolvedPath = resolvePathAlias(node.moduleSpecifier.text, pathMatchers);
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+ if (resolvedPath) {
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+ return context.factory.updateImportDeclaration(
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+ node,
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+ node.modifiers,
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+ node.importClause,
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+ context.factory.createStringLiteral(resolvedPath),
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+ node.attributes,
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+ );
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+ }
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+ }
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+
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+ // Handle export declarations: export { X } from 'module';
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+ if (
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+ ts.isExportDeclaration(node) &&
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+ node.moduleSpecifier &&
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+ ts.isStringLiteral(node.moduleSpecifier)
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+ ) {
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+ const resolvedPath = resolvePathAlias(node.moduleSpecifier.text, pathMatchers);
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+ if (resolvedPath) {
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+ return context.factory.updateExportDeclaration(
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+ node,
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+ node.modifiers,
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+ node.isTypeOnly,
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+ node.exportClause,
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+ context.factory.createStringLiteral(resolvedPath),
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+ node.attributes,
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+ );
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+ }
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+ }
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+
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+ // Handle dynamic imports: import('module')
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+ if (
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+ ts.isCallExpression(node) &&
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+ node.expression.kind === ts.SyntaxKind.ImportKeyword &&
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+ node.arguments.length > 0 &&
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+ ts.isStringLiteral(node.arguments[0])
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+ ) {
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+ const resolvedPath = resolvePathAlias(node.arguments[0].text, pathMatchers);
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+ if (resolvedPath) {
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+ return context.factory.updateCallExpression(node, node.expression, node.typeArguments, [
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+ context.factory.createStringLiteral(resolvedPath),
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+ ...node.arguments.slice(1),
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+ ]);
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+ }
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+ }
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+
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+ return ts.visitEachChild(node, visitor, context);
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+ };
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+
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+ return sourceFile => ts.visitNode(sourceFile, visitor) as ts.SourceFile;
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+ };
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+}
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+
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+/**
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+ * Resolves a path alias to its actual path.
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+ * Returns undefined if the module specifier doesn't match any path alias.
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+ */
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+function resolvePathAlias(moduleSpecifier: string, pathMatchers: PathMatcher[]): string | undefined {
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+ if (moduleSpecifier.startsWith('.') || moduleSpecifier.startsWith('/')) {
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+ return undefined;
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+ }
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+
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+ for (const { regex, targets, hasWildcard } of pathMatchers) {
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+ const match = regex.exec(moduleSpecifier);
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+ if (match) {
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+ const target = targets[0];
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+ const resolved = hasWildcard && match[1] ? target.split('*').join(match[1]) : target;
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+
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+ return normalizeResolvedPath(resolved);
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+ }
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+ }
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+
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+ return undefined;
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+}
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+
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+/**
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+ * Normalizes a resolved path to a relative path with ./ prefix
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+ * and converts TypeScript extensions to JavaScript equivalents.
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+ */
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+function normalizeResolvedPath(resolved: string): string {
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+ // Normalize to relative path with ./ prefix
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+ let result = resolved.startsWith('./') ? resolved.substring(2) : resolved;
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+ result = `./${result}`;
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+ result = result.split('\\').join('/');
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+
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+ // Convert TypeScript extensions to JavaScript equivalents for ESM
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+ return convertExtension(result);
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+}
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+
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+/**
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+ * Converts TypeScript extensions to JavaScript equivalents for ESM.
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+ * .ts -> .js, .tsx -> .js, .mts -> .mjs, .cts -> .cjs
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+ */
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+function convertExtension(filePath: string): string {
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+ if (filePath.endsWith('.ts') || filePath.endsWith('.tsx')) {
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+ return filePath.replace(/\.tsx?$/, '.js');
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+ }
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+ if (filePath.endsWith('.mts')) {
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+ return filePath.replace(/\.mts$/, '.mjs');
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+ }
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+ if (filePath.endsWith('.cts')) {
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+ return filePath.replace(/\.cts$/, '.cjs');
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+ }
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+ // No extension - assume directory import, add /index.js
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+ if (!/\.\w+$/.test(filePath)) {
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+ return `${filePath}/index.js`;
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+ }
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+ // Files with other extensions (.json, .js, etc.) are left as-is
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+ return filePath;
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+}
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Michael Bromley