multiplication of complex numbers javascript

function Complex(real, imaginary) {
  this.real = 0;
  this.imaginary = 0;
  this.real = (typeof real === 'undefined') ? this.real : parseFloat(real);
  this.imaginary = (typeof imaginary === 'undefined') ? this.imaginary : parseFloat(imaginary);
}
Complex.transform = function(num) {
  var complex;
  complex = (num instanceof Complex) ? num : complex;
  complex = (typeof num === 'number') ? new Complex(num, 0) : num;
  return complex;
};
function display_complex(re, im) {
  if(im === '0') return '' + re;
  if(re === 0) return '' + im + 'i';
  if(im < 0) return '' + re + im + 'i';
  return '' + re + '+' + im + 'i';
}
function complex_num_add(first, second) {
  var num1, num2;
  num1 = Complex.transform(first);
  num2 = Complex.transform(second);
  var real = num1.real + num2.real;
  var imaginary = num1.imaginary + num2.imaginary;
  return display_complex(real, imaginary);
}
 var a = new Complex(2, -7);
 var b = new Complex(4,  3);
console.log(complex_num_add(a,b));