JSFiddle - React, Tailwind, and code Playground

JavaScript

function Person(age) {
    this.age = age;
    this.isOld = function (){
        return this.age > 60 ? true : false;
    }
}

// before serialize, ok
var p1 = new Person(77);
alert("Is old: " + p1.isOld());

// after, error
var serialize = JSON.stringify(p1);
var _p1 = JSON.parse(serialize);
alert("Is old: " + _p1.isOld());
// error: Object #<Object> has no method 'isOld'