LWC Event callback to report validity of fields in a form

gistfile1.txt

/**
 * Uses a JS Map structure to easily set & enforce unique keys.
 * Map Iterator is turned into an Array then ultimately reduced to
 * a single boolean for the Save button to disable against
 *
 * @param {Event} event
 */
formValidityCallback(event) {
    const { fieldId, isValid } = event.detail;

    this.validityMap.set(fieldId, isValid);

    this.isSaveDisabled = !Array.from(this.validityMap)
        .map((item) => item[1])
        .reduce((validSoFar, validity) => {
            return validSoFar && validity;
        }, true);
}