getMostFrequentWords.js

/**
* @desc Find the k most frequently occurring words in a body of text.
*
* @param {String} paragraph
* @param {Integer} k
* @returns {Array}
*/
function getMostFrequentWords(paragraph = '', k = 0) {
  if (typeof paragraph !== 'string' || !Number.isInteger(k) || k < 0) {
    throw new TypeError('Invalid paragraph text or frequency count.');
  }
  if (k === 0) {
    // Go home; you're drunk.
    return [];
  }
  // Make the search case-insensitive
  const words = paragraph.split(' ').map((word) => word.toLowerCase());

  // Map each word to the frequency of its appearance
  // E.g. {"dog": 2, "fish": 3}
  const frequencyMap = words.reduce((map, string) => {
    map[string] = Number.isInteger(map[string]) ? ++map[string] : 1;
    return map;
  }, {});

  // Frequency of the word that appears most often
  const maxFrequency = Math.max(...Object.values(frequencyMap));

  /* Construct an array where each index corresponds to the frequency
  of a word. Continuing the example above, "dog" would go at index 2;
  "fish" would go at index 3, etc. */
  let frequencyArray = new Array(maxFrequency);
  for (const string in frequencyMap) {
    const frequency = frequencyMap[string];
    
    // Use an array to account for words that occur in the same frequency
    let wordsWithFreq = (frequencyArray[frequency] || []);
    frequencyArray[frequency] = wordsWithFreq.concat(string);
  }
  
  // Iterate through the frequency array in reverse
  let index = maxFrequency;
  let result = [];
  
  while (index > 1 && k > 0) {
    // We check each value to see if it's an array because some values
    // may be undefined. 
    const words = frequencyArray[index];
    
    if (Array.isArray(words)) {
      const maxWordsAddable = Math.min(k, words.length);
      result = result.concat( words.slice(0, maxWordsAddable) );
      // Subtract these words from the total number of words requested
      k-= maxWordsAddable;
    }
    --index;
  }
  
  return result;
}