esbuildBabel.ts

import babel, { TransformOptions } from '@babel/core';
import { Loader, Plugin, OnLoadArgs, OnLoadResult } from 'esbuild';
import fs from 'fs';
import path from 'path';

/**
 * Original: https://github.com/nativew/esbuild-plugin-babel
 * Copied, because there was a problem with `type: "module"` in `package.json`
 */
export interface ESBuildPluginBabelOptions {
	config?: TransformOptions;
	filter?: RegExp;
	namespace?: string;
	loader?: Loader | ((path: string) => Loader);
}

export const esbuildPluginBabel = (options: ESBuildPluginBabelOptions = {}): Plugin => ({
	name: 'babel',

	setup(build) {
		const { filter = /.*/, namespace = '', config = {}, loader } = options;

		const resolveLoader = (args: OnLoadArgs): Loader | undefined => {
			if (typeof loader === 'function') {
				return loader(args.path);
			}
			return loader;
		};

		const transformContents = async (args: OnLoadArgs, contents: string): Promise<OnLoadResult> => {
			const babelOptions = babel.loadOptions({
				filename: args.path,
				...config,
				caller: {
					name: 'esbuild-plugin-babel',
					supportsStaticESM: true,
				},
			}) as TransformOptions;

			if (!babelOptions) {
				return { contents, loader: resolveLoader(args) };
			}

			if (babelOptions.sourceMaps) {
				babelOptions.sourceFileName = path.relative(process.cwd(), args.path);
			}

			return new Promise((resolve, reject) => {
				babel.transform(contents, babelOptions, (error, result) => {
					error
						? reject(error)
						: resolve({
							contents: result?.code ?? '',
							loader: resolveLoader(args),
						});
				});
			});
		};

		build.onLoad({ filter, namespace }, async args => {
			const contents = await fs.promises.readFile(args.path, 'utf8');

			return transformContents(args, contents);
		});
	},
});