Yusinto Ngadiman
March 28, 2018·1 min read

Javascript Lessons: toJSON and valueOf

For most javascript objects, the prototype chain ends at the Object prototype. This prototype provides a few commonly used methods like toString, isPrototypeOf and hasOwnProperty. Today I'll be looking at two less commonly used methods (at least by me!): toJSON and valueOf.


If you need a custom output when json stringifying your object, you can define a function called toJSON() which will be invoked automatically by JSON.stringify():

class Person {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;

  // JSON.stringify will invoke this method if it is defined
  toJSON() {
    return {
      fullName: `${this.firstName} ${this.lastName}`,

const yus = new Person('Yusinto', 'Ngadiman');
console.log(JSON.stringify(yus)); // {"fullName":"Yusinto Ngadiman"}

Without the custom toJSON() implementation, the code above will use the default implementation:



Object.prototype has a built-in valueOf method which returns the primitive value of the object. You can override this method to return a custom primitive value for your object:

class Car {  
  constructor(make, model, price) {
    this.make = make;
    this.model = model;
    this.price = price;
  // override Object.prototype.valueOf to return a 
  // custom primitive value
  valueOf() {
    return this.price;

const myCar = new Car('Tesla', 'Model 3', 55000);
const tax = myCar * 0.33;
console.log(`Total car price: ${tax + myCar}`); // 73150

The override allows us to perform arithmetic with our object. Without the override, the valueOf myCar will be NaN (Not-a-Number).