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Durable Objects
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Build a counter

Build a counter using Durable Objects.

The complete example code is included for both the Worker and the Durable Object for a basic counter below. Refer to Durable Objects template for the full code template.

// Worker
export default {
async fetch(request, env) {
return await handleRequest(request, env);
async function handleRequest(request, env) {
let url = new URL(request.url);
let name = url.searchParams.get("name");
if (!name) {
return new Response(
"Select a Durable Object to contact by using" +
" the `name` URL query string parameter. e.g. ?name=A"
// Every unique ID refers to an individual instance of the Counter class that
// has its own state. `idFromName()` always returns the same ID when given the
// same string as input (and called on the same class), but never the same
// ID for two different strings (or for different classes).
let id = env.COUNTER.idFromName(name);
// Construct the stub for the Durable Object using the ID.
//A stub is a client Object used to send messages to the Durable Object.
let obj = env.COUNTER.get(id);
// Send a request to the Durable Object, then await its response.
let resp = await obj.fetch(request.url);
let count = await resp.text();
return new Response(`Durable Object '${name}' count: ${count}`);
// Durable Object
export class Counter {
constructor(state, env) {
this.state = state;
// Handle HTTP requests from clients.
async fetch(request) {
// Apply requested action.
let url = new URL(request.url);
// Durable Object storage is automatically cached in-memory, so reading the
// same key every request is fast.
// You could also store the value in a class member if you prefer.
let value = (await"value")) || 0;
switch (url.pathname) {
case "/increment":
case "/decrement":
case "/":
// Serves the current value.
return new Response("Not found", { status: 404 });
// You do not have to worry about a concurrent request having modified the value in storage.
// "input gates" will automatically protect against unwanted concurrency.
// Read-modify-write is safe.
await"value", value);
return new Response(value);