truncate.js

function getIndicesOf(str, delimiter, options) {
  let indices = new Set();
  let fromIdx = 0;
  let idx = null;
  while (idx !== -1 && idx <= options.max) {
    if (idx) {
      indices.add(idx);
    }
    idx = str.indexOf(delimiter, fromIdx);
    fromIdx = idx + 1;
  }
  return indices;
}

function findIdealIdx(indices, options) {
  let maxDiff = options.max - options.ideal;
  let minDiff = options.ideal - options.min;
  for(let i = 0; i <= maxDiff || i <= minDiff; i++) {
    let minIndex = options.ideal - i;
    let maxIndex = options.ideal + i;
    if (i <= maxDiff && indices.has(maxIndex)) {
      return maxIndex;
    } else if (i <= minDiff && indices.has(minIndex)) {
      return minIndex;
    }
  }
  return null;
}


export function truncate (str, options) {
  if (str.split('').length < options.min) {
    return [str, ''];
  }
  let idx =
        findIdealIdx(getIndicesOf(str, '. ', options), options) ||
        findIdealIdx(getIndicesOf(str, ', ', options), options) ||
        findIdealIdx(getIndicesOf(str, ' ', options), options) ||
        findIdealIdx(getIndicesOf(str, '', options), options);
  return [str.substring(0, idx + 1), str.substring(idx + 1)];
}