We Don’t Know How React State Hooks Work

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We use them all the time, and we think we know state hooks work (useState, useReducer). But under the hood, as expected, things are not what you imagine. This talk is about update queues, batching, eager and lazy updates, and some other cool things learned from looking at Hooks source code. It also contains practical takeaways that will help you better understand and debug your code.

Adam Klein
Adam Klein
7 min
14 May, 2021

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Video Summary and Transcription

This Talk provides an introduction to React Staytools, explaining how to use the state and setState function to update a counter. It also delves into the inner workings of React rendering, discussing the update queue and re-rendering conditions. The Talk concludes by mentioning the different modes of updating and triggering re-renders in React, and encourages further exploration of the source code and discussion in Discord.

Available in Español

1. Introduction to React Staytools

Short description:

Hi! This is We Don't Know How React Staytools Work. My name is Adam Klein and I'm broadcasting from Tel Aviv, Israel. Let's take this code, which uses a state that's initialized to 0, which is a counter. And we have a button that has an onClick event handler. The onClick event handler calls setCounter, which is the setState function, and passes in an update function, which increments the value by 1. Let's see what happens when we call setCounter.

Hi! This is We Don't Know How React Staytools Work. My name is Adam Klein and I'm broadcasting from Tel Aviv, Israel.

Let's take this code, which uses a state that's initialized to 0, which is a counter. And we have a button that has an onClick event handler. The onClick event handler calls setCounter, which is the setState function, and passes in an update function, which increments the value by 1.

Let's see what happens when we call setCounter. Now, you probably know that React manages the state of the hooks for us outside of the component. And what I used to think that happens is that we call setCounter. React takes the current state of the hook, which is in our case 0, calls our update function which returns 1, and updates the state of the hook to the new value, which is 1. And then re-renders our component. And the render function calls useState which gets the new value, which is 1, and the component re-renders. This is pretty straightforward, except that it's wrong.

2. Understanding React Rendering

Short description:

Let's see what React really does. Every hook has its own update queue where React keeps all of the pending actions. When we call setCounter, React adds the update to the queue and schedules a re-render. React decides to re-render based on certain conditions.

So let's see what React really does, and this is a simplified version. So in fact, every hook has its own update queue where React keeps all of the pending actions to this hook. So when we call setCounter, React just adds the update to the queue. It adds an action. What is the action? Whatever we passed inside setState, which could be either a function or a value, or in case of reducers, it's the action that we dispatched. And then it just schedules a re-render, which means it flips some flag that says this component needs to be re-rendered. And there might be more updates to this hook, or other hooks, or hooks in other components. And eventually React decides to re-render. And then it re-renders all of the components according to their order inside the tree. So when does this React decide to re-render? We'll talk in the last part of this talk.

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