Truncates a string up to a specified length.
Determine if the string's length is greater than num.
Return the string truncated to the desired length, with '...' appended to the end or the original string.
const truncateString = (str, num) =>
str.length > num ? str.slice(0, num > 3 ? num - 3 : num) + '...' : str;truncateString('boomerang', 7); // 'boom...'