Resursive convert object to querystring in the format - numfruits=2&fruits[banana]=yellow&fruits[watermelon]=red&arr[]=first&arr[]=second

queryString.js

/**
 * Convert query object to string
 * @param {Object} query - Object
 * @returns {String} Query string
 */
function queryString(query) {
    let encode = encodeURIComponent;
    
    /**
     * Flatten an array
     * @param {Array} arr - Array to flatten
     * @param {Boolean} [deep] - Recursive flatten?
     * @returns {Array} Flattened array
     */
    function flatten(arr, deep) {
        return arr.reduce((result, a) => {
            if (deep && Array.isArray(a)) {
                return [...result, ...flatten(a)];
            } else {
                return [...result, a];
            }
        }, []);
    }
    
    /**
     * Get query parameter for value
     * @param {String} name - Name of value
     * @param {*} value - Value
     * @returns {(String|Array)} Query parameter
     */
    function getQuery(name, value) {
        if (Array.isArray(value)) {
            return value.map((subvalue) => {
                return getQuery(name + '%5B%5D', subvalue);
            });
        } else if (typeof value === 'object') {
            return Object.keys(value).map((subname) => {
                return getQuery(`${name}%5B${encode(subname)}%5D`, value[subname]);
            });
        } else {
            return name + '=' + encode(value);
        }
    }
    
    return flatten(Object.keys(query).map((name) => {
        return getQuery(encode(name), query[name]);
    }), true).join('&');
}