Task: Write some code that, giving an array of strings, returns the number of occurrences where at least a char is repeated.

occurrences.js

/**
 * production code
 */

const _countOccurrences = charMap => {
  let occ = 0;
  for (let char in charMap) {
    if (charMap[char] >= 2) {
      occ++;
    }
  }

  return occ;
}

const _occurrencesInAString = str => {
  const charMap = {};
  for (let char of str){
    if (!charMap[char]) {
      charMap[char] = 1;
    } else {
      charMap[char]++;
    }
  }

  return _countOccurrences(charMap);
};

const occurrences = words => {
  if (!words || !words.length) {
    return 0;
  }
  return Math.max(...words.map(word => _occurrencesInAString(word)));
}

/**
 * unit tests
 */

mocha.setup('bdd');
const assert = chai.assert;

describe('given an array, counting its occurrences', () => {

  describe('when no character is repeated', () => {
    it('should return 0', function () {
      assert.equal(occurrences(["a","b","c"]), 0);
    });
  });

  describe('when just 1 character is repeated in just one string', () => {
    it('should return 1', function () {
      assert.equal(occurrences(["a","b","cc"]), 1);
    });
  });

  describe('when just 1 character is repeated in different strings', () => {
    it('should return 1', function () {
      assert.equal(occurrences(["a","bb","cc"]), 1);
    });
  });

  describe('when 2 characters are repeated in just one string', () => {
    it('should return 2', function () {
      assert.equal(occurrences(["ABC","deef","hi", "mnmn"]), 2);
    });
  });


  describe('when 2 characters are repeated in different strings', () => {
    it('should return 2', function () {
      assert.equal(occurrences(["ABC","ddeef","hii", "mnmn"]), 2);
    });
  });

  describe('when it is empty', () => {
    it('should return 0', function () {
      assert.equal(occurrences([]), 0);
    });
  });

  describe('when it is null or undefined', () => {
    it('should return 0', function () {
      assert.equal(occurrences(null), 0);
      assert.equal(occurrences(undefined), 0);
    });
  });

});

mocha.run();