func-object.js

const calculator = {
  sum(...args) {
    return args.reduce((acc, n) => typeof n === 'number' ? acc + n : acc, 0);
  },
    
  sumOldSchool: function () {
    return [...arguments].reduce(function (acc, n) {
      if (typeof n === 'number') return acc + n;
      return acc;
    }, 0);
  },  
};

calculator.sum(31, 7, 4);          // return 42
calculator.sumOldSchool(31, 7, 4); // return 42