I want to mark the difference between two html texts, I found a solution, everything works as expected (deleted files receive class name "del", inserted text receives class name "ins" and related styles) only One thing is wrong - for some reason the text is marked in the wrong place.

var Match,
  calculate_operations,
  consecutive_where,
  create_index,
  diff,
  find_match,
  find_matching_blocks,
  html_to_tokens,
  is_end_of_tag,
  is_start_of_tag,
  is_tag,
  is_whitespace,
  isnt_tag,
  op_map,
  define,
  recursively_find_matching_blocks,
  render_operations,
  wrap;

is_end_of_tag = function (char) {
  return char === '>';
};

is_start_of_tag = function (char) {
  return char === '<';
};

is_whitespace = function (char) {
  return /^\s+$/.test(char);
};

is_tag = function (token) {
  return /^\s*<[^>]+>\s*$/.test(token);
};

isnt_tag = function (token) {
  return !is_tag(token);
};

Match = class Match {
  constructor(start_in_before1, start_in_after1, length1) {
    console.log('lalala');

    this.start_in_before = start_in_before1;
    this.start_in_after = start_in_after1;
    this.length = length1;
    this.end_in_before = this.start_in_before + this.length - 1;
    this.end_in_after = this.start_in_after + this.length - 1;
  }
};

html_to_tokens = function (html) {
  var char, current_word, i, len, mode, words, letters;
  mode = 'char';
  current_word = '';
  // letters = [];
  words = [];
  for (i = 0, len = html.length; i < len; i++) {
    char = html[i];
    // letters.push(char);
    switch (mode) {
      case 'tag':
        if (is_end_of_tag(char)) {
          current_word += '>';
          words.push(current_word);
          current_word = '';
          if (is_whitespace(char)) {
            mode = 'whitespace';
          } else {
            mode = 'char';
          }
        } else {
          current_word += char;
        }
        break;
      case 'char':
        if (is_start_of_tag(char)) {
          if (current_word) {
            words.push(current_word);
          }
          current_word = '<';
          mode = 'tag';
        } else if (/\s/.test(char)) {
          if (current_word) {
            words.push(current_word);
          }
          current_word = char;
          mode = 'whitespace';
        } else if (/[\w\#@]+/i.test(char)) {
          current_word += char;
        } else {
          if (current_word) {
            words.push(current_word);
          }
          current_word = char;
        }
        break;
      case 'whitespace':
        if (is_start_of_tag(char)) {
          if (current_word) {
            words.push(current_word);
          }
          current_word = '<';
          mode = 'tag';
        } else if (is_whitespace(char)) {
          current_word += char;
        } else {
          if (current_word) {
            words.push(current_word);
          }
          current_word = char;
          mode = 'char';
        }
        break;
      default:
        throw new Error(`Unknown mode ${mode}`);
    }
  }
  if (current_word) {
    words.push(current_word);
  }

  return words;
};

find_match = function (
  before_tokens,
  after_tokens,
  index_of_before_locations_in_after_tokens,
  start_in_before,
  end_in_before,
  start_in_after,
  end_in_after
) {
  var best_match_in_after,
    best_match_in_before,
    best_match_length,
    i,
    index_in_after,
    index_in_before,
    j,
    len,
    locations_in_after,
    looking_for,
    match,
    match_length_at,
    new_match_length,
    new_match_length_at,
    ref,
    ref1;
  best_match_in_before = start_in_before;
  best_match_in_after = start_in_after;
  best_match_length = 0;
  match_length_at = {};
  for (
    index_in_before = i = ref = start_in_before, ref1 = end_in_before;
    ref <= ref1 ? i < ref1 : i > ref1;
    index_in_before = ref <= ref1 ? ++i : --i
  ) {
    new_match_length_at = {};
    looking_for = before_tokens[index_in_before];
    locations_in_after = index_of_before_locations_in_after_tokens[looking_for];

    for (j = 0, len = locations_in_after.length; j < len; j++) {
      index_in_after = locations_in_after[j];
      if (index_in_after < start_in_after) {
        continue;
      }
      if (index_in_after >= end_in_after) {
        break;
      }
      if (match_length_at[index_in_after - 1] == null) {
        match_length_at[index_in_after - 1] = 0;
      }
      new_match_length = match_length_at[index_in_after - 1] + 1;
      new_match_length_at[index_in_after] = new_match_length;
      if (new_match_length > best_match_length) {
        best_match_in_before = index_in_before - new_match_length + 1;
        best_match_in_after = index_in_after - new_match_length + 1;
        best_match_length = new_match_length;
      }
    }
    match_length_at = new_match_length_at;
  }
  if (best_match_length !== 0) {
    match = new Match(
      best_match_in_before,
      best_match_in_after,
      best_match_length
    );
  }
  return match;
};

recursively_find_matching_blocks = function (
  before_tokens,
  after_tokens,
  index_of_before_locations_in_after_tokens,
  start_in_before,
  end_in_before,
  start_in_after,
  end_in_after,
  matching_blocks
) {
  var match;
  match = find_match(
    before_tokens,
    after_tokens,
    index_of_before_locations_in_after_tokens,
    start_in_before,
    end_in_before,
    start_in_after,
    end_in_after
  );
  if (match != null) {
    if (
      start_in_before < match.start_in_before &&
      start_in_after < match.start_in_after
    ) {
      recursively_find_matching_blocks(
        before_tokens,
        after_tokens,
        index_of_before_locations_in_after_tokens,
        start_in_before,
        match.start_in_before,
        start_in_after,
        match.start_in_after,
        matching_blocks
      );
    }
    matching_blocks.push(match);
    if (
      match.end_in_before <= end_in_before &&
      match.end_in_after <= end_in_after
    ) {
      recursively_find_matching_blocks(
        before_tokens,
        after_tokens,
        index_of_before_locations_in_after_tokens,
        match.end_in_before + 1,
        end_in_before,
        match.end_in_after + 1,
        end_in_after,
        matching_blocks
      );
    }
  }
  return matching_blocks;
};

create_index = function (p) {
  var i, idx, index, len, ref, token;
  if (p.find_these == null) {
    throw new Error('params must have find_these key');
  }
  if (p.in_these == null) {
    throw new Error('params must have in_these key');
  }
  index = {};
  const arr = html_to_tokens(p.find_these);
  const arr_number = [];
  for (var i = 0; i < arr.length; i++) {
    arr_number.push(arr[i].length);
  }
  ref = p.find_these;

  for (i = 0, len = ref.length; i < len; i++) {
    token = ref[i];
    index[token] = [];
    idx = p.in_these.indexOf(token);
    while (idx !== -1) {
      index[token].push(idx);
      idx = p.in_these.indexOf(token, idx + 1);
    }
  }
  console.log(token, 'token');
  console.log(arr_number, 'arr_number');
  return index;
};

find_matching_blocks = function (before_tokens, after_tokens) {
  var index_of_before_locations_in_after_tokens, matching_blocks;
  matching_blocks = [];
  index_of_before_locations_in_after_tokens = create_index({
    find_these: before_tokens,
    in_these: after_tokens,
  });
  console.log(
    index_of_before_locations_in_after_tokens,
    'index_of_before_locations_in_after_tokens '
  );
  return recursively_find_matching_blocks(
    before_tokens,
    after_tokens,
    index_of_before_locations_in_after_tokens,
    0,
    before_tokens.length,
    0,
    after_tokens.length,
    matching_blocks
  );
};

calculate_operations = function (before_tokens, after_tokens) {
  var action_map,
    action_up_to_match_positions,
    i,
    index,
    is_single_whitespace,
    j,
    last_op,
    len,
    len1,
    match,
    match_starts_at_current_position_in_after,
    match_starts_at_current_position_in_before,
    matches,
    op,
    operations,
    position_in_after,
    position_in_before,
    post_processed;
  if (before_tokens == null) {
    throw new Error('before_tokens?');
  }
  if (after_tokens == null) {
    throw new Error('after_tokens?');
  }
  position_in_before = position_in_after = 0;
  operations = [];
  action_map = {
    'false,false': 'replace',
    'true,false': 'insert',
    'false,true': 'delete',
    'true,true': 'none',
  };
  matches = find_matching_blocks(before_tokens, after_tokens);
  console.log(matches, 'matches');
  matches.push(new Match(before_tokens.length, after_tokens.length, 0));
  for (index = i = 0, len = matches.length; i < len; index = ++i) {
    match = matches[index];
    match_starts_at_current_position_in_before =
      position_in_before === match.start_in_before;
    match_starts_at_current_position_in_after =
      position_in_after === match.start_in_after;
    action_up_to_match_positions =
      action_map[
        [
          match_starts_at_current_position_in_before,
          match_starts_at_current_position_in_after,
        ].toString()
      ];
    if (action_up_to_match_positions !== 'none') {
      operations.push({
        action: action_up_to_match_positions,
        start_in_before: position_in_before,
        end_in_before:
          action_up_to_match_positions !== 'insert'
            ? match.start_in_before - 1
            : void 0,
        start_in_after: position_in_after,
        end_in_after:
          action_up_to_match_positions !== 'delete'
            ? match.start_in_after - 1
            : void 0,
      });
    }
    if (match.length !== 0) {
      operations.push({
        action: 'equal',
        start_in_before: match.start_in_before,
        end_in_before: match.end_in_before,
        start_in_after: match.start_in_after,
        end_in_after: match.end_in_after,
      });
    }
    position_in_before = match.end_in_before + 1;
    position_in_after = match.end_in_after + 1;
  }
  post_processed = [];
  last_op = {
    action: 'none',
  };
  is_single_whitespace = function (op) {
    if (op.action !== 'equal') {
      return false;
    }
    if (op.end_in_before - op.start_in_before !== 0) {
      return false;
    }
    return /^\s$/.test(
      before_tokens.slice(op.start_in_before, +op.end_in_before + 1 || 9e9)
    );
  };
  for (j = 0, len1 = operations.length; j < len1; j++) {
    op = operations[j];
    if (
      (is_single_whitespace(op) && last_op.action === 'replace') ||
      (op.action === 'replace' && last_op.action === 'replace')
    ) {
      last_op.end_in_before = op.end_in_before;
      last_op.end_in_after = op.end_in_after;
    } else {
      post_processed.push(op);
      last_op = op;
    }
  }
  return post_processed;
};

consecutive_where = function (start, content, predicate) {
  var answer, i, index, last_matching_index, len, token;
  content = content.slice(start, +content.length + 1 || 9e9);
  // console.log(content, 'content');
  // console.log(predicate, 'predicate');
  // console.log(start, 'start');

  last_matching_index = void 0;
  for (index = i = 0, len = content.length; i < len; index = ++i) {
    token = content[index];
    answer = predicate(token);
    if (answer === true) {
      last_matching_index = index;
    }
    if (answer === false) {
      break;
    }
  }
  if (last_matching_index != null) {
    return content.slice(0, +last_matching_index + 1 || 9e9);
  }
  return [];
};

wrap = function (tag, content) {
  var length, non_tags, position, rendering, tags;
  rendering = '';
  position = 0;
  length = content.length;
  while (true) {
    if (position >= length) {
      break;
    }
    non_tags = consecutive_where(position, content, isnt_tag);
    position += non_tags.length;
    if (non_tags.length !== 0) {
      rendering += `<${tag}>${non_tags.join('')}</${tag}>`;
    }
    if (position >= length) {
      break;
    }
    tags = consecutive_where(position, content, is_tag);
    position += tags.length;
    rendering += tags.join('');
  }
  return rendering;
};

op_map = {
  equal: function (op, before_tokens, after_tokens) {
    return before_tokens
      .slice(op.start_in_before, +op.end_in_before + 1 || 9e9)
      .join('');
  },
  insert: function (op, before_tokens, after_tokens) {
    var val;
    const token_length = after_tokens.map(element => element.length);
    val = after_tokens.slice(op.start_in_after, op.end_in_after + 1 || 9e9);

    return wrap('ins', val);
  },
  delete: function (op, before_tokens, after_tokens) {
    var val;
    val = before_tokens.slice(op.start_in_before, +op.end_in_before + 1 || 9e9);

    return wrap('del', val);
  },
};

op_map.replace = function (op, before_tokens, after_tokens) {
  return (
    op_map.delete(op, before_tokens, after_tokens) +
    op_map.insert(op, before_tokens, after_tokens)
  );
};

render_operations = function (before_tokens, after_tokens, operations) {
  var i, len, op, rendering;
  rendering = '';
  console.log(operations.length, 'operations.length');
  console.log(operations, 'operations');
  for (i = 0, len = operations.length; i < len; i++) {
    op = operations[i];

    rendering += op_map[op.action](op, before_tokens, after_tokens);
  }
  return rendering;
};

diff = function (before, after) {
  var ops;
  if (before === after) {
    return before;
  }
  before = html_to_tokens(before);
  after = html_to_tokens(after);
  ops = calculate_operations(before, after);
  return render_operations(before, after, ops);
};

diff.html_to_tokens = html_to_tokens;

diff.find_matching_blocks = find_matching_blocks;

find_matching_blocks.find_match = find_match;

find_matching_blocks.create_index = create_index;

diff.calculate_operations = calculate_operations;

diff.render_operations = render_operations;

if (typeof define === 'function') {
  define([], function () {
    return diff;
  });
} else if (typeof module !== 'undefined' && module !== null) {
  module.exports = diff;
} else {
  this.htmldiff = diff;
}

Call the code I wrote:

let oldTxt = Match.html_to_tokens(a);
    let newTxt = Match.html_to_tokens(b);
    let ops = Match.calculate_operations(a, b);
    const render = Match.render_operations(newTxt, oldTxt, ops);
    document.getElementById('output').innerHTML = render;

a is the first html, b is the changed html (not stringified)

Do I have any ideas on how to proceed? Thank you for your help!

I know the error is somewhere inside the "create_index" function. Because I'm calculating the length of the text and then comparing it to the length of the text, each tag/word counts as one.

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