Javascript Interview Question: Given a string, what character appears the most?

max-char.js

/*
  Given a string, find the character that has the highest frequency.
*/

function maxChar(str) {
  //Step 1: Create a data store.
  let ds = {};
  
  //Step 2: Populate it.
  for (let char of str) {
    ds[char] = ds[char] + 1 || 1;
  }
  
  //Step 3: Use a comparator to find the character with highest frequency.
  const keyArr = Object.keys(ds);
  const comparator = (a, b) => (ds[a] > ds[b]) ? a : b;
  
  return keyArr.reduce(comparator);
}

//Jest test
test('Finds the character most frequently used', () => {
  expect(maxChar('x')).toEqual('x');
  expect(maxChar('abbccc')).toEqual('c');
});

//Try it
console.log(maxChar('abbccc'));