React State at Five Levels

1. To a Child:

Let’s imagine you’re playing a video game. In this game, you can collect points, change tools, and move around. All of this information - how many points you have, what tool you’re using, where you are in the game - is stored somewhere so the game knows what to show you next. This is like the “state” in a React app. It’s a way to remember things like what’s in your shopping cart, what page you’re on, or whether you’re logged in or not.

2. To a Teenager:

You know how on social media you can like a post and the number of likes instantly goes up, or send a message and it appears straight away? This happens thanks to something called “state”. It’s a way for a website to keep track of things - like the number of likes a post has, or what messages are in a conversation. React.js uses state to remember these kinds of things and to update the website quickly when they change.

3. To an Undergraduate Computer Science Major:

State in React.js is a way to store and manage data that can change over time and affect what’s rendered in the user interface. When the state changes, the component re-renders, showing the new state in the UI. State is often used for things like user input, form data, and anything else that can change and needs to be tracked by the component.

4. To a Graduate Computer Science Student:

State in React.js can be local or lifted up to a common ancestor component when it needs to be shared among multiple components. When a component’s state changes, it triggers a re-render. React’s virtual DOM helps to do this efficiently. For complex state logic, React provides hooks like useState and useReducer that encapsulate state update logic. For app-level state, there are also options like context API and third-party libraries like Redux, Mobx etc.

5. To a Colleague (Professional Developer):

State in React.js is an essential tool for building dynamic user interfaces. It is encapsulated within components or shared among components using lifted state or more advanced state management solutions like Redux, MobX or even the Context API. React’s built-in useState and useReducer hooks offer a more function-based approach to state management, which can lead to more predictable code compared to class-based lifecycle methods. One of the main considerations when managing state in large applications is maintaining a balance between prop drilling and overly centralized state to ensure components are self-contained but still easy to manage and understand.


What is the purpose of using useState in React, why does React not use the variables that is available in JavaScript for maintaining state?

The useState function in React is used to create a piece of state - a value that can change over time, and for which changes can trigger a re-render of the component. While we could theoretically just use JavaScript variables to hold these values, it wouldn’t give us the behavior we want for a few reasons:

  1. Reactivity: JavaScript variables themselves do not trigger a re-render of a component when they change. In a functional component, when a variable changes, it does not automatically cause the component to re-render, meaning the UI would not update to reflect this change. useState provides a mechanism to store values that, when updated, will cause the component to re-render and display the new state.

  2. Persistence across renders: When a React component re-renders, all variables within the component function get recreated. So if you’re storing some value in a variable and it changes triggering a re-render, the new render wouldn’t “remember” the previous value of the variable. On the other hand, React manages state variables and ensures they persist across re-renders.

  3. Updates during event handlers and effects: Updates to JavaScript variables are synchronous and immediate, whereas state updates can be batched and/or deferred for performance reasons. State updates also provide the ability to specify a dependency list, so that an update only occurs when specific pieces of data change.

So, in short, while JavaScript variables can hold values, they do not provide the reactivity or persistence across renders that useState offers, which are crucial for building interactive UIs with React.

Why not use metaprogramming to figure out when the variable is changed without using a useState hook?

Metaprogramming could technically be used to create some sort of system to track changes in variables and cause re-renders. However, there are several reasons why this approach isn’t typically used in practice, particularly in a library like React:

  1. Performance: Metaprogramming, especially operations like proxies that intercept all operations on an object, can introduce significant overhead. React’s model, where changes are batched and the virtual DOM diffing algorithm is used to make efficient updates to the actual DOM, is designed to be highly performant.

  2. Predictability: One of the benefits of React’s model, and the useState hook in particular, is that it’s explicit. When you call the state update function returned by useState, you know a re-render is going to happen (unless the same state value is set). This makes the behavior of the application easier to reason about.

  3. Compatibility and interference: Metaprogramming can interfere with other JavaScript code in non-obvious ways and can cause compatibility issues with third-party libraries. For instance, if React used metaprogramming to make every object reactive, it could potentially break third-party code that expects normal JavaScript semantics.

Overall, while it’s theoretically possible to use metaprogramming to track changes to variables, the explicitness, performance, and compatibility of React’s current model makes it a better choice for a library intended to be widely used across a variety of applications and use cases.

Is metaprogramming not widely used in JavaScript library or frameworks?

Metaprogramming is indeed used in various libraries and frameworks, but its usage is usually judicious and carefully considered because of the complexity and potential performance implications it can introduce.

In the context of JavaScript, metaprogramming often involves using features such as:

  1. Reflection: This includes the ability to inspect and dynamically call classes or functions. It’s used in many different libraries for things like dependency injection, plugin architectures, or testing/mocking.

  2. Proxies: Proxies can intercept and redefine fundamental operations for objects, such as property lookup, assignment, enumeration, function invocation, etc. Proxies are a powerful tool for creating APIs that appear to behave like ordinary JavaScript objects but have customized behavior behind the scenes.

One example of a popular JavaScript library that uses metaprogramming extensively is Vue.js. Specifically, Vue.js uses the Object.defineProperty() method to make the data object properties reactive. In its new version (Vue 3), it uses JavaScript Proxies for this purpose, which allow it to intercept a wider range of operations and offer better performance.

However, while metaprogramming can be very powerful, it can also make code harder to reason about and debug, since it can change the standard behavior of JavaScript constructs. It can also have performance implications. Because of these reasons, it is usually used sparingly and only when necessary.