Add --generate-types option (#589)

Author: developit

src/index.js

@@ -364,6 +364,7 @@ function createConfig(options, entry, format, writeMeta) {
 			: () => resolve(options.cwd, 'mangle.json');
 
 	const useTypescript = extname(entry) === '.ts' || extname(entry) === '.tsx';
+	const emitDeclaration = !!(options.generateTypes || pkg.types || pkg.typings);
 
 	const escapeStringExternals = ext =>
 		ext instanceof RegExp ? ext.source : escapeStringRegexp(ext);
@@ -476,7 +477,7 @@ function createConfig(options, entry, format, writeMeta) {
 							map: null,
 						}),
 					},
-					useTypescript &&
+					(useTypescript || emitDeclaration) &&
 						typescript({
 							typescript: require(resolveFrom.silent(
 								options.cwd,
@@ -488,6 +489,8 @@ function createConfig(options, entry, format, writeMeta) {
 								compilerOptions: {
 									sourceMap: options.sourcemap,
 									declaration: true,
+									allowJs: true,
+									emitDeclarationOnly: options.generateTypes && !useTypescript,
 									declarationDir: getDeclarationDir({ options, pkg }),
 									jsx: 'preserve',
 									jsxFactory: