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Writing on React, web performance, and the craft of building software.

8 min read•Breaking apart the intimidating one-sentence definition of JavaScript, high-level, prototype-based, multi-paradigm, dynamically typed, single-threaded, into three plain questions worth actually understanding.

The Javascript Engine And Runtime

6 min read•What a JavaScript engine actually is, why modern engines are neither purely compiled nor purely interpreted, and what a JavaScript runtime bundles around that engine to make browser code and Node.js code both possible.

Execution Contexts And The Call Stack

6 min read•What an execution context actually contains, why arrow functions quietly skip two of its pieces, and how the call stack keeps track of which function gets to resume when another one finishes.

What Is Asynchronous Javascript

8 min read•The first time this really clicked for me was at a fast food counter, watching a cashier freeze the entire line waiting on one order. JavaScript has that exact problem, and it solves it with a buzzer.

Your First Ajax Call With Xmlhttprequest

6 min read•Last post, calling a supplier for data was still just a metaphor. This one actually places the call, the old-school way, with XMLHttpRequest.

Dns Ip Addresses And Tcp Connections

Optional4 min read•Post 2 fired a request at a URL and got country data back. This is what actually happens in the gap between those two moments, before a single byte of HTTP is sent.

Anatomy Of Http Requests And Responses

Optional4 min read•The last post got a real, open connection to the actual server. This one is about what you actually write once that connection exists, and what comes back.

Callback Hell And Chained Ajax Requests

5 min read•A few posts back, chaining two AJAX calls together already gave callback hell away by name. Time to actually build one and see exactly why it's a problem.

What A Promise Actually Is

3 min read•The last post ended in callback hell, deeply nested and hard to read. Promises are the actual way out, and seeing one appear is the first step.

Consuming A Promise With Then

3 min read•The last post left a Promise sitting there, pending. This one actually does something with it once it settles.

Chaining Multiple Promises

4 min read•The last post chained two .then() calls to parse JSON. This one chains a whole second network request onto the same flat pattern.

Error Handling With Catch And Finally

4 min read•The last post's guard clause quietly breaks the chain when there's no neighbor. Time to actually catch that instead of pretending it can't happen.

Throwing Errors And A Getjson Helper

4 min read•The last post ended with a real gap: a 404 is still a fulfilled Promise, so .catch() never fires for it. Here's the actual fix.

Coding Challenge Where Am I

Exercise4 min read•A real coding challenge: turn GPS coordinates into a rendered country card, chaining two completely different APIs with everything from the last several posts.

The Event Loop And Microtasks

4 min read•Way back in post 1, a buzzer stood in for how JavaScript handles waiting. This is the actual kitchen behind that buzzer.

A Slow Microtask Delays A Timer Too

2 min read•The last post proved microtasks jump the line with a step-through visualizer. This one proves the sharper, more surprising claim: a slow microtask delays a timer exactly the way slow synchronous code does.

Building A Promise From Scratch

3 min read•Post 6 mentioned building a Promise is a separate skill from consuming one. This is that skill: the Promise constructor, resolve and reject, and wrapping old callback-based code in one.

Promisifying Geolocation

3 min read•The last post promisified setTimeout. This one promisifies a real browser API from all the way back in post 1, geolocation, and uses it to rebuild whereAmI without hardcoded coordinates.

Coding Challenge Promisifying Image Loading

Exercise3 min read•A second coding challenge, on purpose harder than the first: promisify DOM image loading, then chain a real two-image slideshow out of it.

Async Await

3 min read•Consuming and building promises are both second nature by now. Since ES2017 there's an even easier way to consume them: async/await.

Try Catch With Async Await

4 min read•The last post left async/await with no error handling at all. .catch() doesn't attach to it the same way, but try/catch, plain JavaScript from long before Promises existed, does.

Returning Values From Async Functions

7 min read•An async function always returns a Promise, never the value written after return. Getting whereAmI's result out means consuming that Promise, and rethrowing an error inside it is what makes .catch() actually fire.

Running Promises In Parallel With Promise All

5 min read•Three independent API calls, awaited one after another, waste real time waiting on requests that never depended on each other. Promise.all runs them together instead, and still hands back results in the order they were requested.

Promise Race Allsettled And Any

5 min read•Promise.all isn't the only combinator. Promise.race settles on whichever promise finishes first, win or lose, allSettled never short circuits, and any ignores rejections entirely.

Coding Challenge Image Loading With Promise All

Exercise6 min read•The last challenge of the section, in two parts: rebuild the image slideshow with async/await, then hit the classic gotcha of .map() with an async callback returning promises instead of values, and fix it with Promise.all.

Blog

How Js Works Behind The Scenes

Asynchronous Javascript

How Js Works Behind The Scenes

Asynchronous Javascript