How to find the number of parameters expected by a function?

The function's object has a length property which tells you how many formal parameters expected by a function. This is a static value defined by the function, not the number of arguments the function is called with(arguments.length). The basic usage of length propery is,

function multiply(x, y) {
  return x * y;
}

function sum(a, b, c) {
  return a + b + c;
}

console.log(multiply.length); //2
console.log(sum.length); //3

But there are few important rules which needs to be noted while using length property.

1. Default values: Only the parameters which exists before a default value are considered.

   function sum(a, b = 2, c = 3) {
     return a + b + c;
   }
   console.log(sum.length); // 1
   

1. Rest params: The rest parameters are excluded with in length property.

   function sum(a, b, ...moreArgs) {
     let total = a + b;
     for (const arg of moreArgs) {
       total += arg;
     }
     return total;
   }
   console.log(sum.length); // 2
   

1. Destructuring patterns: Each destructuring pattern counted as a single parameter.

   function func([a, b], { x, y }) {
     console.log(a + b, x, y);
   }

   console.log(func.length); // 2
   

Note: The Function constructor is itself a function object and it has a length property of 1.