longestPrefix.js

const wordArray = ['cranberry', 'crawfish', 'crap'];

const longestPrefix = (wordArray) => {
  // Find the smallest word - our prefix cannot be any longer
  const smallestWord = wordArray.reduce((acc, word) => {
    return word.length <= acc.length ? word : acc;
  });

  let indexNoMatch = [];
  for (let word of wordArray) {
    for (let i = 0; i < smallestWord.length; i++) {
      if (word[i] !== smallestWord[i]) {
        indexNoMatch.push(i);
      }
    }
  }

  // Grab smallest number in indexNoMatch array - that's where the common letters for all words end
  const smallestNumber = indexNoMatch.reduce((acc, num) => {
    return num <= acc ? num : acc;
  });

  const commonPrefix = smallestWord
    .split('')
    .splice(0, smallestNumber)
    .join('');

  console.log(commonPrefix);
};

longestPrefix(wordArray); // "cra"