Checks how many arguments a function takes in Javascript?

Question

With arguments.length I can see how many arguments were passed into a function.

But is there a way to determine how many arguments a function can take s

Answer 1

Edge cases

Beware, before counting on fn.length, there are some edge cases where the result may not be what you expect:

const fn1 = ( a, b ) => {}; //       length: 2
const fn2 = ( a = 0, b ) => {}; //   length: 0
const fn3 = ( ...params ) => {};//   length: 0
const fn4 = ( a, b = 1, c ) => {};// length: 1

fn.length doesn't seem to recognize default values or rest operator.

You can mess with this codePen