test.js

function abc(x,y) {
  //we used logical OR(||)  operator. It returns the first truthy value. 
  //if first value is undefined, second value is returned
  x = x || 100;    
 // value of x is x which is given at the function call. If not given value is 100
  y = y || 50; 
 // value of y is y which is given at the function call. If not given value is 50
  console.log(x,y);
}

abc();            //   100    50
abc(5)            //   5      50
abc(5,10)         //   5      10
abc(0,0)          //   100    50 
abc (null,null)   //   100    50