Earlier function parameters are available to default parameters

Here's Alex MacArthur with a nice JavaScript fun fact: when using JavaScript default parameters, earlier parameters are available to later default parameters.

Let's look at some code from his post because the previous sentence is a mouthful.

// reuse `arg1` and set it as `arg2`
function myFunc(arg1, arg2 = arg1) {
  console.log(arg1, arg2);
}

myFunc("arg1!"); // "arg1!" "arg1!"

That's pretty cool, but when would I use this?

Alex describes using this behavior to make a JavaScript class more testable. Fair, but I rarely use classes paired with dependency injection, so this example isn't convincing for me.

He also came up with the least readable reduce function at all times.

const numbers = [1, 2, 3];

// don't do this 😅
const total = numbers.reduce(
  (acc, value, _index, _array, result = acc + value) => result
);

But the post's comments include some gems.

// Make an image a square if not defined otherwise 
// Kudos to Colum
function Image(width, height = width) {}

// Find a string with defined boundaries
// if no boundaries are defined, search the entire string
// Kudos to Axel Rauschmayer
function findIn(str, start = 0, end = str.length) {}

Great examples! I'm convinced and will put reusable function parameters into my JavaScript toolbelt.