twoSum.js

const twoSum = (arr, target) => {
  const hashTable = {};
  populateTable(hashTable, arr);

  for (let i = 0; i < arr.length; i++) {
    const desiredValue = target - arr[i];
    if (validValueExists(desiredValue, hashTable, i)) {
      return [i, hashTable[desiredValue]];
    }
  }

  return [-1, -1];
};

const validValueExists = (desiredValue, hashTable, currentIndex) => {
  return (
    // Make sure there exists a key with `desiredValue`.
    hashTable.hasOwnProperty(desiredValue)
    // Make sure we aren't re-using the same index.
    && hashTable[desiredValue] !== currentIndex
  );
}

const populateTable = (hashTable, arr) => {
  for (let i = 0; i < arr.length; i++) {
    hashTable[arr[i]] = i;
  }
}


console.log(twoSum([2, 1, 4, 3], 6)); // [0, 2]
console.log(twoSum([2, 7, 11, 15], 9)); // [0, 1]