Truncate a string at full words

JavaScript version
const truncate = (str, max, suffix) => (str.length < max ? str : `${str.substr(0, str.substr(0, max - suffix.length).lastIndexOf(' '))}${suffix}`);
TypeScript version
const truncate = (str: string, max: number, suffix: string = '...'): string => (str.length < max ? str : `${str.substr(0, str.substr(0, max - suffix.length).lastIndexOf(' '))}${suffix}`);
Examples
truncate('This is a long message', 20, '...'); // 'This is a long...'