▶
All 15

The series

01 How the Event Loop Runs Beginner

setTimeout(fn, 0)
02 How Timers Really Run Beginner

setTimeout is a minimum, not a promise
03 How Promises Run Intermediate

Promise vs setTimeout
04 How Promise Chains Run Intermediate

Two chains, interleaved
05 How queueMicrotask Runs Intermediate

queueMicrotask vs Promise.then
06 How await Runs Advanced

async / await
07 How Rejections Run Intermediate

throw · catch · finally
08 How Promise.all Runs Intermediate

fail-fast, but nothing gets cancelled
09 How fetch Runs Intermediate

await over the network
10 How It All Runs Together Advanced

The full interview question
11 How Rendering Fits In Advanced

requestAnimationFrame vs microtasks
12 How Node Runs It Advanced

process.nextTick vs Promise
13 How setImmediate Runs Advanced

setImmediate vs setTimeout(0)
14 How the TDZ Works Beginner

hoisting: var vs let
15 How Closures Capture Variables Intermediate

var vs let in a loop

Privacy Policy

The short version: this site has no accounts and sets no cookies, and we never sell your data or share it for targeted advertising. It does use a cookieless analytics service, described in full below. Last updated 4 August 2026.

What this site collects

No direct identifiers. There is no registration, no login, no comment system and no form that asks for your name, email address or account details. You can use every page of this site without telling us who you are.

Analytics (Umami)

This site uses Umami, a cookieless analytics service. Rather than summarise vaguely, here is what it records:

  • Page and referrer — the URL path including any query string, the page title, the referring page, and UTM or ad-click parameters (such as gclid) if you arrive through a campaign link.
  • Browser and device — browser, operating system, device type, screen size, and browser language.
  • Approximate location — country, region and city, derived from your IP address.
  • Learning events — which case or experiment was viewed, which prediction option was chosen and whether it was correct, whether a visualization was played to the end, and an overall daily quiz score. Your individual quiz answers are not sent; only the score.

Umami sets no cookies and does not store your IP address. The IP is processed transiently to derive the location above and a pseudonymous session identifier — a hash rebuilt from a salt that rotates monthly — which lets us see that "one visitor read three cases in a row" without knowing who that visitor is. We never attach a name, email address or account identifier to a session, and Umami does not follow you across other websites.

To be precise rather than flattering: this is pseudonymous session-level data, not purely aggregate statistics. Umami documents the exact fields in its metric definitions and sessions documentation.

Data stored in your browser (localStorage)

The site keeps three things in your browser's localStorage: which cases you have marked complete, your theme preference, and your answers to each day's quiz.

These records themselves are never uploaded — the site has no account system and no progress-sync backend to receive them. Clearing your browser's site data for howitruns.dev erases them completely. Note the one overlap with the section above: your per-question quiz answers stay local, but the analytics events include your overall daily score.

Hosting

The site is served by Vercel. Like any web host, Vercel processes standard request data (such as IP addresses) to deliver pages and protect against abuse, according to Vercel's privacy policy and this project's retention settings. We do not use that data to build profiles of visitors or for advertising.

Advertising

This site displays no advertising of any kind — no banners, no sponsored content, no third-party ad scripts. It currently carries no Google advertising or conversion-tracking tag either, so no advertising cookies are set on this domain.

We may promote the site through Google Ads in future. If you arrive from such a campaign, the click identifier in the URL is recorded by our analytics as described above — that is the only advertising-related data this site handles today. If we ever install a Google conversion-measurement tag here, this page will be updated before the tag goes live to describe exactly what it collects, as required by Google's advertising technologies policy.

You can review or turn off personalized advertising from Google at any time at My Ad Center.

External links

Case pages link out to standards, official runtime documentation, and supporting references such as MDN. Those sites have their own privacy practices, which we do not control.

Who is responsible, and your rights

This site is run by an individual — Asen, the author of How It Runs — reachable at qianduandaren@gmail.com. Two service providers process data on our behalf: Umami Cloud (analytics) and Vercel (hosting), both named above with links to their own policies. Data may be processed on servers outside your country, including in the United States.

The purpose of the analytics described above is to understand which explanations work — which cases people finish, which predictions people get wrong — so the teaching content can be improved. Analytics records are retained by Umami Cloud under its standard retention; the pseudonymous session identifier is rebuilt from a salt that rotates monthly, so sessions cannot be linked across months.

If you are in a jurisdiction that grants you rights over personal data — such as the EU, UK, or California — you may ask what is held about you, ask for it to be deleted, or object to its processing. Email the address above. Because we hold no account or direct identifier, we may need you to help identify the relevant records, and in some cases the data will not be attributable to you at all.

Changes and contact

If our data practices change — for example, if we add a comment system or expand ad measurement — this page will be updated first, with the date above bumped.

Questions about privacy on this site: qianduandaren@gmail.com.