source: frontend/node_modules/resolve.exports/dist/index.js

/**
 * @param {object} exports
 * @param {Set<string>} keys
 */
function loop(exports, keys) {
        if (typeof exports === 'string') {
                return exports;
        }

        if (exports) {
                let idx, tmp;
                if (Array.isArray(exports)) {
                        for (idx=0; idx < exports.length; idx++) {
                                if (tmp = loop(exports[idx], keys)) return tmp;
                        }
                } else {
                        for (idx in exports) {
                                if (keys.has(idx)) {
                                        return loop(exports[idx], keys);
                                }
                        }
                }
        }
}

/**
 * @param {string} name The package name
 * @param {string} entry The target entry, eg "."
 * @param {number} [condition] Unmatched condition?
 */
function bail(name, entry, condition) {
        throw new Error(
                condition
                ? `No known conditions for "${entry}" entry in "${name}" package`
                : `Missing "${entry}" export in "${name}" package`
        );
}

/**
 * @param {string} name the package name
 * @param {string} entry the target path/import
 */
function toName(name, entry) {
        return entry === name ? '.'
                : entry[0] === '.' ? entry
                : entry.replace(new RegExp('^' + name + '\/'), './');
}

/**
 * @param {object} pkg package.json contents
 * @param {string} [entry] entry name or import path
 * @param {object} [options]
 * @param {boolean} [options.browser]
 * @param {boolean} [options.require]
 * @param {string[]} [options.conditions]
 * @param {boolean} [options.unsafe]
 */
function resolve(pkg, entry='.', options={}) {
        let { name, exports } = pkg;

        if (exports) {
                let { browser, require, unsafe, conditions=[] } = options;

                let target = toName(name, entry);
                if (target !== '.' && !target.startsWith('./')) {
                        target = './' + target; // ".ini" => "./.ini"
                }

                if (typeof exports === 'string') {
                        return target === '.' ? exports : bail(name, target);
                }

                let allows = new Set(['default', ...conditions]);
                unsafe || allows.add(require ? 'require' : 'import');
                unsafe || allows.add(browser ? 'browser' : 'node');

                let key, m, k, kv, tmp, isSingle=false;

                for (key in exports) {
                        isSingle = key[0] !== '.';
                        break;
                }

                if (isSingle) {
                        return target === '.'
                                ? loop(exports, allows) || bail(name, target, 1)
                                : bail(name, target);
                }

                if (tmp = exports[target]) {
                        return loop(tmp, allows) || bail(name, target, 1);
                }

                if (target !== '.') {
                        for (key in exports) {
                                if (k && key.length < k.length) {
                                        // do not allow "./" to match if already matched "./foo*" key
                                } else if (key[key.length - 1] === '/' && target.startsWith(key)) {
                                        kv = target.substring(key.length);
                                        k = key;
                                } else {
                                        tmp = key.indexOf('*', 2);
                                        if (!!~tmp) {
                                                m = RegExp(
                                                        '^\.\/' + key.substring(2, tmp) + '(.*)' + key.substring(1+tmp)
                                                ).exec(target);

                                                if (m && m[1]) {
                                                        kv = m[1];
                                                        k = key;
                                                }
                                        }
                                }
                        }

                        if (k && kv) {
                                // must have value
                                tmp = loop(exports[k], allows);
                                if (!tmp) return bail(name, target);

                                return tmp.includes('*')
                                        ? tmp.replace(/[*]/g, kv)
                                        : tmp + kv;
                        }
                }

                return bail(name, target);
        }
}

/**
 * @param {object} pkg
 * @param {object} [options]
 * @param {string|boolean} [options.browser]
 * @param {string[]} [options.fields]
 */
function legacy(pkg, options={}) {
        let i=0, value,
                browser = options.browser,
                fields = options.fields || ['module', 'main'];

        if (browser && !fields.includes('browser')) {
                fields.unshift('browser');
        }

        for (; i < fields.length; i++) {
                if (value = pkg[fields[i]]) {
                        if (typeof value == 'string') {
                                //
                        } else if (typeof value == 'object' && fields[i] == 'browser') {
                                if (typeof browser == 'string') {
                                        value = value[browser=toName(pkg.name, browser)];
                                        if (value == null) return browser;
                                }
                        } else {
                                continue;
                        }

                        return typeof value == 'string'
                                ? ('./' + value.replace(/^\.?\//, ''))
                                : value;
                }
        }
}

exports.legacy = legacy;
exports.resolve = resolve;