feat: added some coding exercise challenges

Author: xingarr

coding-exercise/README.md

@@ -227,3 +227,332 @@ If you try to use **{ref.current}** in the render method, the number won’t be
 ---
 
 **[⬆ Back to Top](#table-of-contents)**
+
+#### 6. What is the output in the console after mounting and unmounting the component?
+
+```javascript
+import { useEffect, useState } from 'react';
+
+export default function Timer() {
+  const [count, setCount] = useState(0);
+
+  useEffect(() => {
+    console.log('Effect ran');
+    const timer = setInterval(() => {
+      setCount(c => c + 1);
+    }, 1000);
+
+    console.log('Cleanup registered');
+    return () => {
+      console.log('Cleanup executed');
+      clearInterval(timer);
+    };
+  }, []);
+
+  return <div>Count: {count}</div>;
+}
+```
+
+- 1: "Effect ran", "Cleanup registered", "Cleanup executed" (on mount), "Cleanup executed" (on unmount)
+- 2: "Effect ran", "Cleanup registered" (on mount), "Cleanup executed" (on unmount)
+- 3: "Effect ran" (on mount), "Cleanup executed" (on unmount)
+- 4: "Effect ran", "Cleanup registered", "Cleanup executed" (on mount and unmount)
+
+<details><summary><b>Answer</b></summary>
+<p>
+
+##### Answer: 2
+
+When a component mounts, React runs the effect function completely, which logs both "Effect ran" and "Cleanup registered". The cleanup function (returned from useEffect) is **not** executed during mount - it's only registered at this point.
+
+The cleanup function executes in two scenarios:
+1. **Before re-running the effect** (if dependencies change)
+2. **When the component unmounts**
+
+Since the dependency array is empty `[]`, the effect only runs once on mount and never re-runs. Therefore, the cleanup function will only execute when the component unmounts, logging "Cleanup executed" and clearing the interval timer.
+
+This pattern is crucial for preventing memory leaks when using timers, subscriptions, or event listeners in React components.
+
+</p>
+</details>
+
+---
+
+**[⬆ Back to Top](#table-of-contents)**
+
+#### 7. What will be the output after clicking the button?
+
+```javascript
+import { useState } from 'react';
+
+export default function App() {
+  const [items, setItems] = useState([1, 2, 3]);
+
+  function handleClick() {
+    items.push(4);
+    setItems(items);
+  }
+
+  return (
+    <>
+      <div>Items: {items.join(', ')}</div>
+      <button onClick={handleClick}>Add Item</button>
+    </>
+  );
+}
+```
+
+- 1: Items: 1, 2, 3, 4
+- 2: Items: 1, 2, 3
+- 3: Error: Cannot mutate state directly
+- 4: Items: 1, 2, 3, 4, 4 (duplicates on each click)
+
+<details><summary><b>Answer</b></summary>
+<p>
+
+##### Answer: 2
+
+React uses `Object.is()` comparison to detect state changes. When you mutate the array directly using `items.push(4)` and then pass the same array reference to `setItems(items)`, React sees it as the same object (same reference) and doesn't trigger a re-render.
+
+To properly update an array in state, you should create a new array:
+```javascript
+setItems([...items, 4]); // or
+setItems(items.concat(4));
+```
+
+This is a common mistake that leads to bugs where the UI doesn't update even though the underlying data has changed. Always treat state as immutable in React.
+
+</p>
+</details>
+
+---
+
+**[⬆ Back to Top](#table-of-contents)**
+
+#### 8. What is the output after the component mounts?
+
+```javascript
+import { useState, useEffect } from 'react';
+
+export default function Counter() {
+  const [count, setCount] = useState(0);
+
+  useEffect(() => {
+    setCount(1);
+  });
+
+  useEffect(() => {
+    setCount(2);
+  }, []);
+
+  console.log('Rendered with count:', count);
+
+  return <div>Count: {count}</div>;
+}
+```
+
+- 1: "Rendered with count: 0", "Rendered with count: 2"
+- 2: "Rendered with count: 0", "Rendered with count: 1", "Rendered with count: 2"
+- 3: Infinite loop / Maximum update depth exceeded error
+- 4: "Rendered with count: 0", "Rendered with count: 2", "Rendered with count: 1"
+
+<details><summary><b>Answer</b></summary>
+<p>
+
+##### Answer: 3
+
+This code creates an **infinite loop** because the first `useEffect` has no dependency array, which means it runs after **every render**. Here's what happens:
+
+1. Initial render with `count: 0`
+2. After render, both effects run (first sets count to 1, second sets count to 2)
+3. State update triggers re-render with `count: 2`
+4. After render, the first effect runs again (no dependencies), setting count to 1
+5. This triggers another re-render, and the cycle continues infinitely
+
+React will detect this and throw an error: **"Maximum update depth exceeded"**.
+
+The fix is to add a dependency array to the first useEffect:
+```javascript
+useEffect(() => {
+  setCount(1);
+}, []); // Add empty dependency array
+```
+
+</p>
+</details>
+
+---
+
+**[⬆ Back to Top](#table-of-contents)**
+
+#### 9. What will be displayed on the screen?
+
+```javascript
+import { useMemo } from 'react';
+
+export default function App() {
+  const expensiveCalculation = useMemo(() => {
+    console.log('Calculating...');
+    return 100 + 200;
+  });
+
+  return <div>Result: {expensiveCalculation}</div>;
+}
+```
+
+- 1: Result: 300
+- 2: Result: function() { ... }
+- 3: Result: undefined
+- 4: Error: useMemo requires a dependency array
+
+<details><summary><b>Answer</b></summary>
+<p>
+
+##### Answer: 4
+
+The `useMemo` hook **requires** a dependency array as the second argument. Without it, React will throw an error during development:
+
+```
+Error: useMemo requires a dependency array as the second argument
+```
+
+The correct usage is:
+```javascript
+const expensiveCalculation = useMemo(() => {
+  console.log('Calculating...');
+  return 100 + 200;
+}, []); // Dependency array is required
+```
+
+With an empty dependency array `[]`, the calculation runs once during the initial render and the memoized value (300) is reused on subsequent renders. If you include dependencies, the calculation re-runs whenever those dependencies change.
+
+</p>
+</details>
+
+---
+
+**[⬆ Back to Top](#table-of-contents)**
+
+#### 10. What is the output after clicking the button twice?
+
+```javascript
+import { useState } from 'react';
+
+export default function Form() {
+  const [person, setPerson] = useState({
+    name: 'John',
+    age: 30
+  });
+
+  function handleClick() {
+    person.age = person.age + 1;
+    setPerson(person);
+  }
+
+  return (
+    <>
+      <div>Age: {person.age}</div>
+      <button onClick={handleClick}>Increment Age</button>
+    </>
+  );
+}
+```
+
+- 1: Age: 32
+- 2: Age: 31
+- 3: Age: 30
+- 4: Error: Cannot mutate state
+
+<details><summary><b>Answer</b></summary>
+<p>
+
+##### Answer: 3
+
+Similar to the array mutation issue, directly mutating an object's property and then passing the same object reference to the state setter doesn't trigger a re-render. React uses `Object.is()` to compare the old and new state values. Since `person` is still the same object reference, React doesn't detect any change.
+
+Even though `person.age` is being incremented internally, the UI shows the initial value (30) because React never re-renders the component.
+
+The correct approach is to create a new object:
+```javascript
+function handleClick() {
+  setPerson({
+    ...person,
+    age: person.age + 1
+  });
+}
+```
+
+This creates a new object with a new reference, which React recognizes as a state change and triggers a re-render. This is a fundamental principle in React: **always treat state as immutable**.
+
+</p>
+</details>
+
+---
+
+**[⬆ Back to Top](#table-of-contents)**
+
+#### 11. What will be logged to the console?
+
+```javascript
+import { useEffect, useState } from 'react';
+
+export default function App() {
+  const [count, setCount] = useState(0);
+
+  useEffect(() => {
+    console.log('Effect A');
+    return () => console.log('Cleanup A');
+  }, [count]);
+
+  useEffect(() => {
+    console.log('Effect B');
+    return () => console.log('Cleanup B');
+  }, []);
+
+  console.log('Render');
+
+  return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
+}
+```
+
+**On initial mount:**
+- 1: "Render", "Effect A", "Effect B"
+- 2: "Effect A", "Effect B", "Render"
+- 3: "Render", "Effect B", "Effect A"
+- 4: "Effect A", "Cleanup A", "Effect B", "Render"
+
+**After first button click:**
+- 1: "Render", "Cleanup A", "Effect A"
+- 2: "Cleanup A", "Render", "Effect A"
+- 3: "Render", "Effect A", "Cleanup A"
+- 4: "Render", "Cleanup B", "Effect B", "Cleanup A", "Effect A"
+
+<details><summary><b>Answer</b></summary>
+<p>
+
+##### Answer for initial mount: 1, Answer for first click: 1
+
+**On initial mount:**
+1. Component renders first: "Render"
+2. After render, effects run in the order they're defined: "Effect A", "Effect B"
+3. Cleanup functions are registered but not executed
+
+**After first button click:**
+1. State changes, component re-renders: "Render"
+2. Before running Effect A again (because `count` dependency changed), its cleanup runs: "Cleanup A"
+3. Effect A runs again: "Effect A"
+4. Effect B doesn't run because its dependency array `[]` is empty (no changes)
+5. Cleanup B is not called because Effect B doesn't re-run
+
+**Key takeaways:**
+- Render happens first, then effects run
+- Effects run in the order they're defined
+- Cleanup runs before re-running an effect (when dependencies change)
+- Effects with empty dependency arrays only run once on mount
+
+</p>
+</details>
+
+---
+
+**[⬆ Back to Top](#table-of-contents)**