JavaScript determine number of arguments a function takes

Question

Say I have some functions stored in an array of objects, each takes a variable amount of arguments:

var ops = [
   {
      id: "plus",
      op: function(a, b) {
         return a + b;
      }
   },
   {
      id: "inverse",
      op: function(a) {
         return -a;
      }
   }
];

How would I determine the number of arguments the function takes from outside the function (Without specifying it as an additional property) e.g.

for (var i = 0; i < ops.length; i++) {
   var op = ops[i].op;
   var args = ? // Determine number of arguments op takes here
}

Is this even possible? If so how?

Answer 1

Yes, it's possible. Use function.length