countOnly.js

const assertEqual = require('./assertEqual');

const countOnly = function(allItems, itemsToCount) {
  const result = {};
  for (const item of allItems) {
    if (itemsToCount[item]) {
      if (result[item]) {
        result[item] += 1;
      } else {
        result[item] = 1;
      }
    }
  }
  return result;
};


const firstNames = [
  "Karl",
  "Salima",
  "Agouhanna",
  "Fang",
  "Kavith",
  "Jason",
  "Salima",
  "Fang",
  "Joe"
];

const result1 = countOnly(firstNames, {
  "Jason": true,
  "Karima": true,
  "Fang": true,
  "Agouhanna": false
});

assertEqual(result1["Jason"], 1);
assertEqual(result1["Karima"], undefined);
assertEqual(result1["Fang"], 2);
assertEqual(result1["Feda"], undefined);