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_subtract(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_subtract(a,b));