add_normal.js

function add() {
  // Start from zero
  var total = 0;

  // Floating-points, bah!
  // 1e12 = 1000000000000.
  var factor = 1e12;

  // Undefined, set in the loop
  var value;

  // Something to iterate on
  var i = arguments.length;

  // Loop through all the parameters
  while (i--) {
    // Multiply by 1e12, to account for peculiarities
    // of doing addition with floating-point numbers.
    value = parseFloat(arguments[i]) * factor;

    // Is it not, not a number?
    // Then hey, it's a number!
    if (!isNaN(value)) {
       total += value;
    }
  }

  // Divide back by 1e12, because we multiplied by
  // 1e12 to account for floating-point weirdness.
  return total/factor;
}

exotic_add.js

// can you figure it out?
function add() {
  var args = [].slice.call(arguments), total = 0;
      
  for (;args.length; args.shift()) {
    !args[0] || typeof args[0] === 'boolean' || (total += parseFloat( !(args[0]*1) ? 0 : args[0] ) * 1e12); 
  }
      
  return total/1e12;
}

question.js

// Should equal 15
add(1, 2, 3, 4, 5);

// Should equal 0
add(5, null, -5);

// Should equal 10
add('1.0', false, 1, true, 1, 'A', 1, 'B', 1, 'C', 1, 'D', 1, 'E', 1, 'F', 1, 'G', 1);

// Should equal 0.3, not 0.30000000000000004
add(0.1, 0.2);

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