Function Length

Requirements

• Implement functionLength(fn), where fn is a function. Return its expected parameter count without invoking it.
• Count parameters before the first default parameter. Do not count a rest parameter.
• Each destructured parameter counts once. Support regular, arrow, async, generator, native, and bound functions.
• Use built-in function metadata. Parsing source strings is not required; non-function inputs are out of scope.

Examples

Input: functionLength((a, b = 1, ...rest) => {})
Output: 1