JS parse query string/serialize object - Supports arrays - Working example - https://codepen.io/Samuel87/pen/QWWZopj

parse.js

/**
 * Parses a serialized query string to an object
 *
 * @param {string} query - query string to parse
 * @returns {Object}
 * @example
 *      parse('a=b&c=d');
 *      // => { a: 'b', c: 'd' }
 * @example
 *      parse('a[]=b&a[]=c');
 *      // => { a: ['b', 'c'] }
 */
const parse = (query) => {
    if (!query || Object.prototype.toString.call(query) !== '[object String]') return {};
    
    const hashes = query.slice(query).split('&');

    return hashes.reduce((params, hash) => {
        const split = hash.indexOf('=');
        
        // If no `=` or nothing after it
        if (split < 0 || split >= hash.length - 1) {
            const key = hash.replace(/\=/g, '');
            
            return Object.assign(
                params, 
                {
                    [key]: null,
                }
            );
        }

        const key = decodeURIComponent(hash.slice(0, split));
        let val = decodeURIComponent(hash.slice(split + 1).replace(/\+/g, ' '));
        
        // Coercions for int and bool
        if (!isNaN(+val)) val = +val;
        if (val === 'true' || val === 'false') val = val === 'true';
        
        // Array query should have `[]` in key
        const isArr = key.indexOf('[]') !== -1;
        const keyName = key.replace('[]', '');
        
        if (params.hasOwnProperty(keyName)) {
            params[keyName].push(val);
            return params;
        }

        return Object.assign(
            params, 
            { 
                [keyName]: isArr ? [val] : val 
            }
        );
    }, {});
};

serialize.js

/**
 * Serializes an object to a query string
 *
 * @param {Object} obj - object to serialize
 * @returns {string}
 * @example
 *      serialize({ a: 'b', c: 'd' });
 *      // => 'a=b&c=d'
 * @example
 *      serialize({ a: ['b', 'c'] });
 *      // => 'a[]=b&a[]=c'
 */
const serialize = (obj) => {
    if (Object.prototype.toString.call(obj) !== '[object Object]') return '';

    return Object.keys(obj).map((key) => {
        const val = obj[key];

        if (val === '' || val === null || val === undefined) return '';

        if (Array.isArray(val)) {
            return val.reduce((results, val2) => {
                if (val2) {
                    // Faster than string interpolation
                    results.push(encodeURIComponent(key) + '[]=' + encodeURIComponent(val2));
                }

                return results;
            }, []).join('&');
        }

        return encodeURIComponent(key) + '=' + encodeURIComponent(val);
    }).filter((x) => x.length).join('&');
};