How to share changes between instances with prototype(aka static properties)

static-properties-proto.js

function Robot(){
  this.parts = [];
}

Robot.prototype.legs = 2;
Robot.prototype.changeLegs = function(num){
  this.__proto__.legs = num;
}

robby = new Robot();
robby.parts.push('head');
console.log(robby, robby.legs);
robby.changeLegs(1);
console.log('legs:', robby.legs);
cranky = new Robot();
console.log(cranky, cranky.legs);