[JS] Singleton

Singleton is a design pattern that restricts the instantiation of a class to one object. A singleton should be immutable by the consuming code. Usually, the goal is to manage global application state.

• It provides a global point of access to it.
• Source of config settings for a web app (initiated with an API key)
• It saves memory because only single obect is reused not created at each request.

ES6 Module instance

let instance = null;

class _API {
  constructor() {
    if (!instance) {
      instance = this;
    }

    this.url = Constants.API_URL;
    return instance;
  }

  getCities() {
    return fetch(this.url, { method: 'get' })
      .then(response => response.json());
  }
}

export default _API;

// index.js
import _API from './_API';
const instance1 = new _API();

Singleton Class

class _API {
  constructor() {
    this.url = Constants.API_URL;
  }

  getCities() {
    return fetch(this.url, { method: 'get' })
      .then(response => response.json());
  }
}

const API = new _API();
Object.freeze(API)
export default API;