Clone a object in JavaScript

deepCopy.js

function deepCopy(subject) {
  let copySubject;

  const subjectIsObject = isObject(subject);
  const subjectIsArray = isArray(subject);

  if (subjectIsArray) {
    copySubject = [];
  } else if (subjectIsObject) {
    copySubject = {};
  } else {
    return subject;
  }

  for (key in subject) {
    const keyIsObject = isObject(subject[key]);

    if (keyIsObject) {
      copySubject[key] = deepCopy(subject[key]);
    } else {
      if (subjectIsArray) {
        copySubject.push(subject[key]);
      } else {
        copySubject[key] = subject[key];
      }
    }
  }

  return copySubject;
}