Bartek Iwańczuk bartlomieju

What is a Dev Server?

A dev server is a local HTTP server optimized for the development loop. Its job is to serve your application locally with features that make iteration fast.

Core Features

Hot Module Replacement (HMR) — when you save a file, only the changed module is swapped in the browser without a full page reload. State (like form inputs, scroll position) is preserved.
On-demand transformation — instead of bundling everything upfront, modules are transformed (TypeScript → JS, JSX → JS, etc.) only when the browser requests them. This makes startup near-instant even for large projects.
File watching — monitors the filesystem and triggers rebuilds/HMR updates.
Error overlay — compilation errors are shown directly in the browser.

Issue #26142: Worker blob URL race condition — Analysis & Fix Plan

The Bug

URL.revokeObjectURL() after new Worker(blobURL) causes intermittent "Module not found" errors because blob content is fetched asynchronously on the worker thread, not during construction.

Works in Chrome/Firefox. Fails in Deno.

Race Condition

Fix: node:worker_threads Worker Idle Termination (denoland/deno#23169)

Problem

Workers created with node:worker_threads are terminated when idle, even if they have ref'd transferable objects like MessagePort or SharedArrayBuffer that should keep them alive.

Root Cause: Two Competing Keepalive Systems

There are two independent keepalive decision systems, and that's the root problem:

	/******/ (() => { // webpackBootstrap
	/******/ "use strict";
	/******/ var __webpack_modules__ = ({

	/***/ "./src/base.ts":
	/!*******************!\
	!* ./src/base.ts *!
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	/***/ (function(__unused_webpack_module, exports, __webpack_require__) {

	❯ cargo run -- task
	Compiling deno v1.19.2 (/Users/ib/dev/deno/cli)
	Finished dev [unoptimized + debuginfo] target(s) in 20.58s
	Running `target/debug/deno task`
	Available tasks:
	- async
	sleep 1 && echo 2 &
	- boolean_list
	echo 1 && false \|\| echo 2
	- env_var_substitution

	jupyter notebook
	[I 2021-12-09 17:20:32.804 LabApp] JupyterLab extension loaded from /usr/local/Cellar/jupyterlab/3.2.4/libexec/lib/python3.9/site-packages/jupyterlab
	[I 2021-12-09 17:20:32.804 LabApp] JupyterLab application directory is /usr/local/Cellar/jupyterlab/3.2.4/libexec/share/jupyter/lab
	[I 17:20:32.813 NotebookApp] Serving notebooks from local directory: /Users/biwanczuk/dev/ideno
	[I 17:20:32.813 NotebookApp] Jupyter Notebook 6.4.6 is running at:
	[I 17:20:32.813 NotebookApp] http://localhost:8888/?token=6a1b4c6d4743e5dff339982ec80ac39e758c0dd3b7e7cded
	[I 17:20:32.813 NotebookApp] or http://127.0.0.1:8888/?token=6a1b4c6d4743e5dff339982ec80ac39e758c0dd3b7e7cded
	[I 17:20:32.813 NotebookApp] Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).
	[C 17:20:32.823 NotebookApp]

	class Emitter {
	#on = Object.create(null);
	#once = Object.create(null);

	on(n, fn) { (this.#on[n] ?? (this.#on[n] = [])).push(fn); }
	once(n, fn) { (this.#once[n] ?? (this.#once[n] = [])).push(fn); }
	emit(n, ...args) { this.#on[n]?.forEach(_ => _(...args)); if (this.#once[n]) this.#once[n] = (this.#once[n].forEach(_ => _(...args)), undefined); }
	off(n, fn) { if (!fn) (delete this.#on[n], delete this.#once[n]); else { let o = this.#on[n]?.indexOf(fn) ?? -1; if (o !== -1) this.#on[n].splice(o, 1); o = this.#once[n]?.indexOf(fn) ?? -1; if (o !== -1) this.#once[n].splice(o, 1); } }
	}

	addEventListener("fetch", (event) => {
	new ArrayBuffer(1 << 30);
	});

	addEventListener("fetch", (event) => {
	throw new Error("boom!");
	});