How to count number of arguments of a function, in javascript, INCLUDING optional arguments? [duplicate]

Question

For example, if I have the following function in javascript:

var f1 = function(a, b, c) {}
console.log(f1.length); // 3

But with this function, the output is different:

var f2 = function(a, b, c = 6) {}
console.log(f2.length); // 2

How do I count number of arguments of f2 INCLUDING optional arguments?

Answer 1

Well, it's a bit of a mess but I believe this should cover most edge cases.

It works by converting the function to a string and counting the commas, but ignoring commas that are in strings, in function calls, or in objects/arrays. I can't think of any scenarios where this won't return the proper amount, but I'm sure there is one, so this is in no way foolproof, but should work in most cases.

function getNumArgs(func) {
  var funcStr = func.toString();
  var commaCount = 0;
  var bracketCount = 0;
  var lastParen = 0;
  var inStrSingle = false;
  var inStrDouble = false;
  for (var i = 0; i < funcStr.length; i++) {
    if (['(', '[', '{'].includes(funcStr[i]) && !inStrSingle && !inStrDouble) {
      bracketCount++;
      lastParen = i;
    } else if ([')', ']', '}'].includes(funcStr[i]) && !inStrSingle && !inStrDouble) {
      bracketCount--;
      if (bracketCount < 1) {
        break;
      }
    } else if (funcStr[i] === "'" && !inStrDouble && funcStr[i - 1] !== '\\') {
      inStrSingle = !inStrSingle;
    } else if (funcStr[i] === '"' && !inStrSingle && funcStr[i - 1] !== '\\') {
      inStrDouble = !inStrDouble;
    } else if (funcStr[i] === ',' && bracketCount === 1 && !inStrSingle && !inStrDouble) {
      commaCount++;
    }
  }

  // Handle no arguments (last opening parenthesis to the last closing one is empty)
  if (commaCount === 0 && funcStr.substring(lastParen + 1, i).trim().length === 0) {
    return 0;
  }

  return commaCount + 1;
}

Here are a few tests I tried it on: https://jsfiddle.net/ekzuvL0c/