How to swap keys and values in an Object and how to handle different types in JS

getNumber.js

const swapKeyVal = (obj) =>  Object.assign({}, ...Object.entries(obj).map(([a,b]) => ({ [b]: a })));

const getNumber = (num) => {
  const isStringType = typeof num === 'string';
  const isNumberType = typeof num === 'number';
  let hasNumber = false;
  let keyVal = { one: 1, two: 2, three: 3 };

  if (!isStringType && !isNumberType) {
    throw new Error('Unknown type');
  }

  if (isStringType) {
    hasNumber = Object.keys(keyVal).includes(num);
  } else if (isNumberType) {
    keyVal = swapKeyVal(keyVal);
    hasNumber = Object.keys(keyVal).map(Number).includes(num);
  }

  if (!hasNumber) {
    throw new Error('Unknown number');
  } else {
    return keyVal[num];
  }
};

console.log(getNumber(1)); // one
console.log(getNumber('one')); // 1
console.log(getNumber(4)); // Error: Unknown number
console.log(getNumber('four')); // Error: Unknown number
console.log(getNumber(null)); // Error: Unknown type