calc.js

'use stirct';

function calculator(){
  this.ans = 0;
  var self = this;


  this.add = add;
  this.miniz = miniz;
  this.div = div;
  this.multi = multi;
  this.result = result;

  function add(num){
    self.ans = self.ans + num;
    return self;
  }

  function miniz(num){
    self.ans = self.ans - num;
    return self;
  }

  function div(num){
    self.ans = self.ans / num;
    return self;
  }

  function multi(num){
    self.ans = self.ans * num;
    return self;
  }

  function result(){
    return self.ans;
  }
}

var cal = new calculator;
cal.add(10).add(20).miniz(2).div(2);
console.log(cal.result()); // result 14