This is just a browser friendly version of https://github.com/StephanGeorg/trigram-similarity

trigram-similarity.js

const nGram = function(n) {
  if (
    typeof n !== 'number' ||
    Number.isNaN(n) ||
    n < 1 ||
    n === Number.POSITIVE_INFINITY
  ) {
    throw new Error('`' + n + '` is not a valid argument for `n-gram`')
  }

  return grams

  /**
   * Create n-grams from a given value.
   *
   * @template {string|string[]} T
   * @param {T} [value]
   * @returns {T[]}
   */
  function grams(value) {
    /** @type {T[]} */
    var nGrams = []
    /** @type {number} */
    var index
    /** @type {string|string[]} */
    var source

    if (value === null || value === undefined) {
      return nGrams
    }

    source = value.slice ? value : String(value)
    index = source.length - n + 1

    if (index < 1) {
      return nGrams
    }

    while (index--) {
      // @ts-ignore
      nGrams[index] = source.slice(index, index + n)
    }

    return nGrams
  }
};

const bigram = nGram(2);
const trigram = nGram(3);

/**
 * Two blank spaces are added at the beginning, and one at the end,
 * and single spaces are replaced by double ones.
 * @param {string} input
 * @returns {string}
 */
const convertString = (input = '') => {
  if (!input.trim()) return '';
  return `  ${input
    .trim()
    .replace(/\s+/g, ' ') // replace multiple spaces w/ single spaces
    .replace(/\s/g, '  ')} ` // replace single spaces w/ double spaces
    .toLowerCase();
};

/**
 * Sorting them, and taking out repetitions (via Set)
 * @param {string} input
 * @returns {string}
 */
const generateTrigram = (input = '') => [...new Set(trigram(convertString(input))
  .filter((trigramItem) => !/^[\p{Letter}\p{Mark}0-9]\s\s$/giu.test(trigramItem)))];

/**
 * Calculate trigram similarity between 2 strings
 * @param {string} input1
 * @param {string} input2
 * @returns {number}
 */
const trigramSimilarity = (input1 = '', input2 = '') => {
  const trigrams1 = generateTrigram(input1);
  const trigrams2 = generateTrigram(input2);

  // Total trigrams
  const total = [...new Set([...trigrams1, ...trigrams2])];
  // Trigrams both have in common
  const common = [];
  trigrams1.forEach((trigramItem) => {
    if (trigrams2.includes(trigramItem)) common.push(trigramItem);
  });

  return (total.length === 0)
    ? 0
    : common.length / total.length;
};

console.log(trigramSimilarity('Chateau blanc', 'chateau cheval blanc')); // 0.7368421052631579