Implementation of proper FormURLEncoded class in JS

FormURLEncoded.js

class FormURLEncoded extends Map {
    /**
     * @param {Nullable<String>} formString - Form URL encoded string (ex: `'hello=world&foo=bar&tar=gz'`)
     */
    constructor(formString = null) {
        super(formString ?
            formString
                .trim()
                .split('&')
                .map(
                    element => element.split('=').map(decodeURIComponent)
                )
            : undefined
        );
    }

    /**
     * Translate the Map to a form URL encoded string
     * @returns {String}
     */
    toString() {
        return Array.from(this)
            .map(
                element => element.map(encodeURIComponent).join('=')
            )
            .join('&')
        ;
    }
}