Calculate the Jaccard similarity index between two strings. Strings are treated as sets of words, and duplicate words are removed. (https://en.wikipedia.org/wiki/Jaccard_index)

getJaccardSimilarity.js

function getJaccardSimilarity(item, otherItem) {
  function makeUnique(prev, current, index) { 
    return (index === 1 
            ? [prev].concat(prev !== current ? current : []) // handle when first two items are identical
            : (prev.indexOf(current) > -1 
               ? prev
               : (prev.push(current), prev)))
  }
  const union = [].concat(item.split(/\s+/)).concat(otherItem.split(/\s+/)).reduce(makeUnique);
  const otherUnique = otherItem.split(/\s+/).reduce(makeUnique);
  const intersection = item.split(/\s+/).reduce(makeUnique).filter((word) => otherUnique.indexOf(word) > -1);
  return intersection.length / union.length;
}