markerikson

Replay Case Study: How Replay Helped Find a React Bug Faster than Dan Abramov Did

Authors: Mark Erikson Cover photo: No Date: April 17, 2026 hidden: Yes tags: Case studies, Story

We’ve always said that Replay time-travel recordings enable developers to solve really hard bugs, by giving them the ability to inspect the app’s behavior at any point in time. This is especially true for timing bugs and race conditions, where it may be impossible to actually debug them with standard developer tools that would block the race condition from even happening. Now, with Replay MCP, we’re giving agents those same time-travel investigation superpowers.

markerikson

// See https://blog.isquaredsoftware.com/presentations/react-redux-ts-intro-2020-12/#/36 for slides
// My basic render function structure:
function RenderLogicExample({
  someBoolean, // 1) Destructure values from `props` object
  someList,
}) {
  // 2) Declare state values
  const [a, setA] = useState(0);
  const [b, setB] = useState(0);

markerikson

RTK ESM/TS Discussion

Attendees:

• Nathan Bierema
• Mateusz Burzynski
• Mark Erikson

Notes

• Mateusz: what do you want besides "ship widely compat code?" What preferences?

markerikson

Questions

Company

• company location / remote?
• what project management method?
• good and bad company culture?
• performance reviews?
• what's the path to profitability?

markerikson

// Example of using multiple / nested `createEntityAdapter` calls within a single Redux Toolkit slice

interface Message {
  id: string;
  roomId: string;
  text: string;
  timestamp: string;
  username: string;
}

markerikson

Cheng Lou - On the Spectrum of Abstraction

Cheng Lou, a former member of the React team, gave an incredible talk at React Europe 2016 entitled "On the Spectrum of Abstraction". That talk is available for viewing here: https://www.youtube.com/watch?v=mVVNJKv9esE

It's only a half-hour, but it is mind-blowing. It's worth re-watching two or three times, to let the ideas sink in.

I just rewatched the talk for some research, and wrote down a summary that's semi-transcript-ish. I didn't see any other transcripts for this talk, other than the auto-generated closed captions, so I wanted to share for reference.

Summary

markerikson

import React from "react";
import ReactDOM from "react-dom";

import configureStore from "./store/configureStore";


const store = configureStore();


const rootEl = document.getElementById("root");

markerikson

// The classic AJAX call - dispatch before the request, and after it comes back
function myThunkActionCreator(someValue) {
    return (dispatch, getState) => {
        dispatch({type : "REQUEST_STARTED"});
        
        myAjaxLib.post("/someEndpoint", {data : someValue})
            .then(
                response => dispatch({type : "REQUEST_SUCCEEDED", payload : response}),
                error => dispatch({type : "REQUEST_FAILED", error : error})
            );    

markerikson

// approach 1: define action object in the component
this.props.dispatch({
    type : "EDIT_ITEM_ATTRIBUTES", 
    payload : {
        item : {itemID, itemType},
        newAttributes : newValue,
    }
});

// approach 2: use an action creator function

markerikson

class ParentComponent extends Component {
    constructor() {
        super();
        
        this.state = {
            data : [
                {id : 1, date : "2014-04-18", total : 121.0, status : "Shipped", name : "A", points: 5, percent : 50},
                {id : 2, date : "2014-04-21", total : 121.0, status : "Not Shipped", name : "B", points: 10, percent: 60},
                {id : 3, date : "2014-08-09", total : 121.0, status : "Not Shipped", name : "C", points: 15, percent: 70},
                {id : 4, date : "2014-04-24", total : 121.0, status : "Shipped", name : "D", points: 20, percent : 80},