add: react/finite-state-machine.md

react/finite-state-machine.md

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+# Simplify Your Components with Finite State Machines and Derived States
+
+Consider a common situation of a React component with multiple UI states such as `loading`, `error`, and `success`. The common paatern is to use multiple `useState` hooks to manage these states. This results in code that is hard to read and error-prone -
+
+```js
+const MyComponent = () => {
+  const [loading, setLoading] = useState(false)
+  const [error, setError] = useState(false)
+  const [success, setSuccess] = useState(false)
+
+  return (
+    <div>
+      {loading && !error && !success && <p>Loading...</p>}
+      {error && !loading && !success && <p>Error occurred</p>}
+      {success && !loading && !error && <p>Operation completed successfully</p>}
+    </div>
+  )
+}
+```
+
+These states are **distinct from each other**. When `loading` is true, the `error` and `success` states should be false. Using multiple useState hooks can cause unexpected behaviors, like accidentally setting two states to true simultaneously.
+
+Instead, consider using the ["finite state machine" (FSM) pattern](https://en.wikipedia.org/wiki/Finite-state_machine). A FSM allows only a finite number of states. In the UI example above, a single useState can manage the current state more robustly and with less risk of error, as shown here:
+
+```js
+import { useState } from 'react'
+
+type State = 'loading' | 'error' | 'success'
+
+const MyComponent = () => {
+  const [state, setState] = useState<State>('loading')
+
+  const handleClick = () => {
+    setState('loading')
+    // Simulate an async operation
+    setTimeout(() => {
+      setState('success')
+    }, 2000)
+  }
+
+  return (
+    <div>
+      {state === 'loading' && <p>Loading...</p>}
+      {state === 'error' && <p>Error occurred</p>}
+      {state === 'success' && <p>Operation completed successfully</p>}
+      <button onClick={handleClick}>Click me</button>
+    </div>
+  )
+}
+```
+
+An even better way would be to use something like [Tanstack Query](https://tanstack.com/query/latest/docs/framework/react/overview) to fetch data. `useQuery` eliminates the need for separate `useState` hooks for `loading`, `error`, and `success` states.
+
+[source](https://www.nico.fyi/blog/you-dont-need-usestate-in-react)