Make me beautiful

The first step of refactoring is complete. The most important thing is that the function works the same as before the refactoring. It means you can move on. Look, the function duplicates the code for calculating miles. Expressions differ only in the value of percentages:

var calculateMiles = function (distance, isBusinessClass) {
  if (isBusinessClass) {
    return distance * 0.22;
  }
  return distance * 0.18;
};

Let’s take a look at the solution from a different angle: what if we calculated percentage in the conditions, rather than duplicating mile calculation? We can reformulate the task so that the base percentage is 18, and if we fly business class, the percentage is increased by 4. Then at the end of the function, we calculate and return miles once.

var calculateMiles = function (distance, isBusinessClass) {
  // Enter percentage
  if (isBusinessClass) {
    // Change percentage
  }
  // Calculate and return miles
};

The second step of refactoring is as follows: create variable percent, which will vary depending on the condition, and return the result of the expression distance * percent from the function.

This way the code is easier to understand, and there will be only one exit point from the function. The function will become easier to read, because the conditions for changing percentages can now be made not nested, but sequential.

By the way, the exit point from the function is the place where the function completes its work and returns the result.