Truncate the middle of a string

truncate.js

const truncate = function(fullStr, strLen, separator) {
  if (fullStr.length <= strLen) return fullStr;
    
  separator = separator || '…';

  const sepLen = separator.length;
  const charsToShow = strLen - sepLen;
  const frontChars = Math.ceil(charsToShow/2);
  const backChars = Math.floor(charsToShow/2);

  return [
    fullStr.substr(0, frontChars),
    separator, 
    fullStr.substr(fullStr.length - backChars)
  ].join('');
}