rollup-extensions.mjs

// Source: https://github.com/rollup/rollup/issues/1052
import { basename, dirname, isAbsolute, resolve } from "path";
import * as fs from "fs";

function isFile(file) {
	try {
		return fs.statSync(file).isFile();
	} catch (err) {
		return false;
	}
}

function addExtensionIfNecessary(file, extensions) {
	try {
		const name = basename(file);
		const files = fs.readdirSync(dirname(file));

		if (~files.indexOf(name) && isFile(file)) return file;
		for (const ext of extensions) {
			if (~files.indexOf(`${name}${ext}`) && isFile(`${file}${ext}`)) {
				return `${file}${ext}`;
			}
		}
	} catch (err) {
		// noop
	}

	return null;
}

export default function extensions({ extensions }) {
	if (!extensions || !extensions.length) {
		throw new Error(`Must specify { extensions: [..] } as non-empty array!`);
	}

	return {
		name: "extensions",

		resolveId(importee, importer) {
			// absolute paths are left untouched
			if (isAbsolute(importee)) {
				return addExtensionIfNecessary(resolve(importee), extensions);
			}

			// if this is the entry point, resolve against cwd
			if (importer === undefined) {
				return addExtensionIfNecessary(resolve(process.cwd(), importee), extensions);
			}

			// external modules are skipped at this stage
			if (importee[0] !== ".") return null;

			return addExtensionIfNecessary(resolve(dirname(importer), importee), extensions);
		},
	};
}