Prefer optional objects to optional arguments

02 Aug, 2024

Every time I make a function with this type of signature, I end up regretting it:

const formatDate = (
  date: Date,
  format = "ddd MMM D, h:mm A",
): string => {
  // some logic to format a date
};

This is because invariably at some point in time I'm going to want to have more customizations on how to format a date. In this situation I wanted to, in some cases only, change the format of the date to exclude the year if the year was the same year as today.

I could do this check on the calling side but then I have date logic spread across the codebase which is not great. Adding a third parameter called hide_year_if_current: boolean (the second one that's optional) makes the caller having to specify the second parameter of format even when they really just want control over whether the year is specified or not.

Taking some guidance from Uncle Bob's work from way back:

The ideal number of arguments for a function is zero (niladic). Next comes one (monadic), followed closely by two (dyadic). Three arguments (triadic) should be avoided where possible.

When a function seems to need more than two or three arguments, it is likely that some of those arguments ought to be wrapped into a class of their own. Consider, for example, the difference between the two following declarations:

Circle makeCircle(double x, double y, double radius);
Circle makeCircle(Point center, double radius);

Reducing the number of arguments by creating objects out of them may seem like cheating, but it’s not. When groups of variables are passed together, the way x and y are in the example above, they are likely part of a concept that deserves a name of its own.

Since I've been living in the Elixir world lately and practically every function's last argument is an optional keyword list opts \\ [], I'm starting to take a page from that book and write the above instead as:

const formatDate = (
  date: Date,
  options?: { format?: string; hide_year_if_current?: boolean },
): string => {
  // some logic to format a date based on options
}

This adheres to the Clean Code worldview, makes testing easier, and allows for options to be added or removed without changing the function signature.

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Prefer optional objects to optional arguments

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