What is this length property of a function?

By contrast, arguments.length is local to a function and provides the number of arguments actually passed to the function.

exampleFunction has three parameters (a, b, c), so its length property is 3.

anotherFunction has one regular parameter (x), one parameter with a default value (y = 5), and a rest parameter (...rest). The length property only counts the parameters before the first parameter with a default value, so the value is 1.