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Fluent Interfaces in JavaScript


A couple of days ago, I had a technical interview and one of the exercises was OOP and Fluent Interface, I had seen its use a couple of times, but I had never had to develop anything similar.

Fluent Interface

The Fluent Interface is often referred to as Method Chaining, Fluent API or jQuery Style. A fluent interface is an object-oriented API whose design relies extensively on method chaining. Its goal is to increase code legibility by creating a domain-specific language (DSL). The term was coined in 2005 by Eric Evans and Martin Fowler [1].

If you ever worked with jQuery, you had to see code like:

  .attr("src", "")
  .attr("alt", "pic of velocity")
  .attr("width", "99")

this keyword

Basically this refers to the instance of our object that is created. Is the key to creating your own Fluent Interface, is to use the instance object of your JavaScript class.

Calculator API

class Calculator {
  constructor(number) {
    this.number = number;

  sum(n) {
    this.number += n;
    return this;

  subtraction(n) {
    this.number -= n;
    return this;

  multiplication(n) {
    this.number *= n;
    return this;

  division(n) {
    this.number /= n;
    return this;

  equal() {

This is a very simple example of use:

const operation = new Calculator(3)

// => 8
const operation = new Calculator(2)

// => -1.5

I don't know for sure how popular this pattern could be, but at least I found it quite interesting.

[1] FluenInterface

this in JavaScript