Browse a folder and try to change all esm import `from`, by auto prefixing `.js` or `/index.js` only if this file exists.

fix-esm-import-paths.js

import * as fs from 'fs';
import * as path from 'path';

// https://gist.github.com/lovasoa/8691344
async function* walk(dir) {
	for await (const d of await fs.promises.opendir(dir)) {
		const entry = path.join(dir, d.name);
		if (d.isDirectory()) {
			yield* walk(entry);
		} else if (d.isFile()) {
			yield entry;
		}
	}
}

function resolveImportPath(sourceFile, importPath, options) {
	const sourceFileAbs                        = path.resolve(process.cwd(), sourceFile);
	const root                                 = path.dirname(sourceFileAbs);
	const {moduleFilter = defaultModuleFilter} = options;

	if (moduleFilter(importPath)) {
		const importPathAbs = path.resolve(root, importPath);
		let possiblePath    = [
			path.resolve(importPathAbs, './index.ts'),
			path.resolve(importPathAbs, './index.js'),
			importPathAbs + '.ts',
			importPathAbs + '.js',
		];

		if (possiblePath.length) {
			for (let i = 0; i < possiblePath.length; i++) {
				let entry = possiblePath[i];
				if (fs.existsSync(entry)) {
					const resolved = path.relative(root, entry.replace(/\.ts$/, '.js'));

					if (!resolved.startsWith('.')) {
						return './' + resolved;
					}

					return resolved;
				}
			}
		}
	}

	return null;
}

function replace(filePath, outFilePath, options) {
	const code    = fs.readFileSync(filePath).toString();
	const newCode = code.replace(
		/(import|export) (.+?) from ('[^\n']+'|"[^\n"]+");/gs,
		function (found, action, imported, from) {
			const importPath   = from.slice(1, -1);
			const resolvedPath = resolveImportPath(filePath, importPath, options);

			if (resolvedPath) {
				console.log('\t', importPath, resolvedPath);
				return `${action} ${imported} from '${resolvedPath}';`;
			}

			return found;
		});

	if (code !== newCode) {
		fs.writeFileSync(outFilePath, newCode);
	}
}

// Then, use it with a simple async for loop
async function run(srcDir, options = defaultOptions) {
	const {
			  sourceFileFilter = defaultSourceFileFilter,
		  } = options;

	for await (const entry of walk(srcDir)) {
		if (sourceFileFilter(entry)) {
			console.log(entry);
			replace(entry, entry, options);
		}
	}
}

const defaultSourceFileFilter = function (sourceFilePath) {
	return /\.(js|ts)$/.test(sourceFilePath) && !/node_modules/.test(sourceFilePath);
};

const defaultModuleFilter = function (importedModule) {
	return !path.isAbsolute(importedModule) && !importedModule.startsWith('@') && !importedModule.endsWith('.js');
};

const defaultOptions      = {
	sourceFileFilter: defaultSourceFileFilter,
	moduleFilter    : defaultModuleFilter,
};

// Switch this to test on one file or directly run on a directory.
const DEBUG = true;

if (DEBUG) {
	replace('./path/to/an/esm/module/index.ts', './out.ts', defaultOptions);
} else {
	await run('./path/to/an/esm/project/src/directory/', defaultOptions);
}

Browse a folder and try to change all esm import from, by auto prefixing .js or /index.js only if one of these files exists.
This come in help when you're concerned with one of these issues:

• microsoft/TypeScript#40878
• microsoft/TypeScript#13422
• microsoft/TypeScript#16577