compatibility with explicit extensions imports

resources/build-npm.js

@@ -52,7 +52,7 @@ if (require.main === module) {
   fs.copyFileSync('./README.md', './npmDist/README.md');
 
   // Should be done as the last step so only valid packages can be published
-  const packageJSON = buildPackageJSON();
+  const packageJSON = buildPackageJSON(srcFiles);
   writeGeneratedFile('./npmDist/package.json', JSON.stringify(packageJSON));
 
   showDirStats('./npmDist');
@@ -72,7 +72,7 @@ function babelBuild(srcPath, options) {
   return code + '\n';
 }
 
-function buildPackageJSON() {
+function buildPackageJSON(srcFiles) {
   const packageJSON = JSON.parse(
     fs.readFileSync(require.resolve('../package.json'), 'utf-8'),
   );
@@ -103,5 +103,19 @@ function buildPackageJSON() {
     packageJSON.publishConfig = { tag: publishTag };
   }
 
+  /**
+   * This prevents breaking previous versions of imports with explicit extensions
+   * Like `require("graphql/language/parser.js")`
+   */
+  for (const srcFile of srcFiles) {
+    if (srcFile.endsWith('.ts')) {
+      const srcFilePath = srcFile.slice(0, srcFile.length - 3);
+      packageJSON.exports[`./${srcFilePath}.js`] = {
+        require: `./${srcFilePath}.js`,
+        import: `./${srcFilePath}.mjs`,
+      };
+    }
+  }
+
   return packageJSON;
 }