blah.js

const findWordsInMatrix = (matrix, dictionary) => {
  // Convert dictionary to map for O(1) lookup
  // We could also use a trie here (more efficient, but verbose implementation)
  const dictionary = dictionary.reduce((set, word) => {
    set.add(word);
    return set;
  }, new Set());

  const wordsFound = new Set();
  const visited = new Map();

  // Start search in upper-lefthand corner
  const search = [{
    x: 0,
    y: 0,
    word: matrix[0][0]
  }];

  while (search.length) {
    const { x, y, word } = search.pop();
    
    if (isOutOfBounds(matrix, x, y)) {
      continue;
    }
    // Don't terminate when a word is found because there 
    // may be compound words
    if (dictionary.has(word)) {
      wordsFound.add(word);
    }
    const nearby = getAdjacentSpots(matrix, x, y);
    const letterInThisSpot = matrix[x][y];
    
    nearby
      .filter(spot => !visited[spot])
      .forEach(spot => {
        visited[spot] = true;
        search.push({
          x: spot.x,
          y: spot.y,
          word: word + letterInThisSpot
        })
      });
  }

  return wordsFound;
};