Truncate a string at full words

const truncate = (str, max, suffix) =>
    str.length < max ? str : `${str.substr(0, str.substr(0, max - suffix.length).lastIndexOf(' '))}${suffix}`;

const truncate = (str: string, max: number, suffix: string = '...'): string =>
    str.length < max ? str : `${str.substr(0, str.substr(0, max - suffix.length).lastIndexOf(' '))}${suffix}`;

truncate('This is a long message', 20, '...'); // 'This is a long...'