index.js

import {Octokit} from '@octokit/rest';

async function searchCommits(octokit, email) {
	const {data} = await octokit.search.commits({
		q: `author-email:${email}`,
		sort: 'author-date',
		// eslint-disable-next-line camelcase
		per_page: 1,
	});

	if (data.total_count === 0) {
		throw new Error(`Couldn't find username for \`${email}\``);
	}

	return data.items[0].author.login;
}

export default async function githubUsername(email, {token} = {}) {
	if (!(typeof email === 'string' && email.includes('@'))) {
		throw new Error('Email required');
	}

	const octokit = new Octokit({
		auth: token,
		userAgent: 'https://github.com/sindresorhus/github-username',
	});

	const {data} = await octokit.search.users({
		q: `${email} in:email`,
	});

	if (data.total_count === 0) {
		return searchCommits(octokit, email);
	}

	return data.items[0].login;
}