index.js

import https from 'node:https';

const regexp = /class="(.*)branch-name(.*)>(.*)</g; // The default branch is always the first on the page

export default function defaultBranch(path) {
	// Normalize path to be URL relative to https://github.com/ by default
	const url = new URL(path, 'https://github.com/');
	// Force https connection
	url.protocol = 'https:';
	const repositoryPath = url.toString();

	return new Promise((resolve, reject) => {
		https.get(`${repositoryPath}/branches`, response => {
			if (response.statusCode < 200 || response.statusCode > 299) {
				reject(new Error(`Failed to load url: ${response.statusCode}`));
				return;
			}

			response.setEncoding('utf8');
			let data = '';

			response.on('data', body => {
				data += body;
			});

			response.on('end', () => {
				try {
					// The first item (0) will be at the top and will be the default branch
					const regexMatch = data.match(regexp)[0].split('>')[1].split('<')[0];
					return resolve(regexMatch);
				} catch (error) {
					return reject(new Error(`Failed to get default branch: ${error}`));
				}
			});
		}).on('error', error => reject(error));
	});
}