Named Parameters in JavaScript

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Have you ever read code such as perform(true) or perform(false) and wondered: “what does true or false mean in this context?” If so, the answer was likely found by opening the function definition for perform and reading the parameter’s name.

Or, have you ever read code like perform(null, null, true) and wondered: “why are all of these null values being passed?” If so, the answer was possibly that the 3rd argument needed to be set so the only way to get there was by passing placeholders.

Wouldn’t it be great if there was a way to both avoid opening the function definition to know the significance of a parameter’s value and avoid passing a bunch of null (or false, undefined, {} etc) values only to get to the nth argument?

Ruby supports this functionality with what it calls keyword arguments:

def perform(one: nil, two: nil, skip_something: false)
  # ...
end

With this example, this function can now be invoked as perform(skip_something: true). This solves both problems! The first two parameters aren’t necessary and the boolean value now has a name associated to provide more context on what true is in this context.

But how can this be achieved with JavaScript? Fortunately, something very similar can be done leveraging object destructuring. The above example converted to its equivalent in JavaScript would look something like the following:

function perform({ one = null, two = null, skipSomething = false } = {}) {
  // ...
}

The same invocation from above would then be perform({ skipSomething: true }).

One thing to note is that the entire object has a default value of {}. This allows invoking perform() without any arguments. Without the default value of {} invoking perform() would throw Cannot read property 'one' of undefined since it would be trying to destructure undefined.

However, one thing Ruby supports is required named parameters. This can be achieved by removing the default value. For example, to make two required in the above example it would look like the following:

def perform(one: nil, two:, skip_something: false)
  # ...
end

Calling perform without two will now error with missing keyword: two. What if the same thing was done with the JavaScript example?

function perform({ one = null, two, skipSomething = false } = {}) {
  // ...
}

Rather than throwing an error, two will now default to undefined. Fortunately, there is still a way to emulate Ruby’s behavior with a little extra work:

function required(name) {
  throw new Error(`missing required parameter: ${name}`);
}

function perform({
  one = null,
  two = required("two"),
  skipSomething = false
} = {}) {
  // ...
}

Now, if two is not provided this will now error with missing required parameter: two.

However, if it’s truly required it may make sense to use a positional parameter instead:

function perform(two, { one = null, skipSomething = false } = {}) {
  // ...
}

Conclusion

Named parameters provide a handful of benefits:

  • Providing a name along with the parameter’s value
  • Avoiding unnecessary “empty” values to reach the nth position
  • Adding a new parameter doesn’t change the position of others

Like anything, named parameters are useful in moderation. Two examples of useful scenarios include:

  • Functions that accept configurations or options with a lot of optional parameters
  • Functions that change often because it doesn’t require changing every call site, only the ones that use the specific parameter.

The next time you run into one of the earlier stated problems or one of these scenarios, consider reaching for named parameters.