Custom date format prototype.

dateFormat.js

/**
 * Custom date format prototype.
 *
 * @param {string} format Format character inspired by PHP. @link https://www.php.net/manual/en/datetime.format.php
 *
 * @returns {string|Date}
 */
Date.prototype.format = function (format) {
    const to2digits = n => (n < 10 ? '0' + n : n);

    if (format === 'w') {
        return `${this.getFullYear()}-${to2digits(
            this.getMonth() + 1
        )}-${to2digits(this.getDate())}`;
    }

    return this;
};