Durable Object in-memory state

This example shows you how Durable Objects are stateful, meaning in-memory state can be retained between requests. After a brief period of inactivity, the Durable Object will be evicted, and all in-memory state will be lost. The next request will reconstruct the object, but instead of showing the city of the previous request, it will display a message indicating that the object has been reinitialized. If you need your applications state to survive eviction, write the state to storage by using the Storage API, or by storing your data elsewhere.

import { DurableObject } from 'cloudflare:workers';

// Worker
export default {
  async fetch(request, env) {
    return await handleRequest(request, env);
  }
}

async function handleRequest(request, env) {
  let id = env.LOCATION.idFromName("A");
  let obj = env.LOCATION.get(id);
  // Forward the request to the remote Durable Object.
  let resp = await obj.fetch(request);
  // Return the response to the client.
  return new Response(await resp.text());
}

// Durable Object
export class Location extends DurableObject {
  constructor(state, env) {
    this.state = state;
    // Upon construction, you do not have a location to provide.
    // This value will be updated as people access the Durable Object.
    // When the Durable Object is evicted from memory, this will be reset.
    this.location = null
  }

  // Handle HTTP requests from clients.
  async fetch(request) {
    let response = null

    if (this.location == null) {
      response = new String(`
This is the first request, you called the constructor, so this.location was null.
You will set this.location to be your city: (${request.cf.city}). Try reloading the page.`);
    } else {
      response = new String(`
The Durable Object was already loaded and running because it recently handled a request.

Previous Location: ${this.location}
New Location: ${request.cf.city}`);
    }

    // You set the new location to be the new city.
    this.location = request.cf.city;
    console.log(response);
    return new Response(response);
  }
}

Finally, configure your wrangler.toml file to include a Durable Object binding and migration based on the namespace and class name chosen previously.

wrangler.toml

name = "durable-object-in-memory-state"

[[durable_objects.bindings]]
name = "LOCATION"
class_name = "Location"

[[migrations]]
tag = "v1"
new_classes = ["Location"]