Understanding React in 2 minutes: components, hooks and best practices!

1// Simple functional component
2const Welcome = ({ name }) => {
3  return <h1>Hello, {name}! 👋</h1>;
4};
5
6// Component with props and children
7const Card = ({ title, children }) => {
8  return (
9    <div className="card">
10      <h2>{title}</h2>
11      <div className="card-content">{children}</div>
12    </div>
13  );
14};
15
16// Usage
17const App = () => {
18  return (
19    <Card title="My first card">
20      <Welcome name="Developer" />
21      <p>Welcome to the world of React!</p>
22    </Card>
23  );
24};

1import { useState } from 'react';
2
3const Counter = () => {
4  const [count, setCount] = useState(0);
5  const [isActive, setIsActive] = useState(false);
6
7  const handleIncrement = () => {
8    setCount((prevCount) => prevCount + 1);
9    setIsActive(true);
10  };
11
12  return (
13    <div className="counter">
14      <p>Counter: {count}</p>
15      <p>Status: {isActive ? 'Active' : 'Inactive'}</p>
16      <button onClick={handleIncrement} className={isActive ? 'active' : ''}>
17        Increment
18      </button>
19    </div>
20  );
21};

1import { useState, useEffect } from 'react';
2
3const WindowResizeTracker = () => {
4  const [windowSize, setWindowSize] = useState({
5    width: window.innerWidth,
6    height: window.innerHeight,
7  });
8
9  useEffect(() => {
10    // Event handler to update dimensions
11    const handleResize = () => {
12      setWindowSize({
13        width: window.innerWidth,
14        height: window.innerHeight,
15      });
16    };
17
18    // Add event listener
19    window.addEventListener('resize', handleResize);
20
21    // Cleanup function: VERY IMPORTANT
22    // Remove event listener when component unmounts
23    return () => {
24      window.removeEventListener('resize', handleResize);
25    };
26  }, []); // Empty dependency array because we want the effect only on mount
27
28  return (
29    <div className="window-size">
30      <h2>Window Dimensions:</h2>
31      <p>Width: {windowSize.width}px</p>
32      <p>Height: {windowSize.height}px</p>
33    </div>
34  );
35};
36
37// Another example: Timer with cleanup
38const Timer = () => {
39  const [seconds, setSeconds] = useState(0);
40  const [isRunning, setIsRunning] = useState(false);
41
42  useEffect(() => {
43    let intervalId;
44
45    if (isRunning) {
46      // Start timer
47      intervalId = setInterval(() => {
48        setSeconds((prevSeconds) => prevSeconds + 1);
49      }, 1000);
50    }
51
52    // Cleanup function
53    return () => {
54      if (intervalId) {
55        clearInterval(intervalId);
56      }
57    };
58  }, [isRunning]); // Triggers only when isRunning changes
59
60  const handleToggle = () => {
61    setIsRunning((prev) => !prev);
62  };
63
64  const handleReset = () => {
65    setIsRunning(false);
66    setSeconds(0);
67  };
68
69  return (
70    <div className="timer">
71      <h2>Timer</h2>
72      <p>Time elapsed: {seconds} seconds</p>
73      <button onClick={handleToggle}>{isRunning ? 'Pause' : 'Start'}</button>
74      <button onClick={handleReset}>Reset</button>
75    </div>
76  );
77};

1import { useCallback, useMemo } from 'react';
2
3const ExpensiveComponent = ({ items, onItemSelect }) => {
4  // Function memoization
5  const handleSelect = useCallback(
6    (id) => {
7      onItemSelect(id);
8    },
9    [onItemSelect],
10  );
11
12  // Expensive calculation memoization
13  const sortedItems = useMemo(() => {
14    return [...items].sort((a, b) => b.value - a.value);
15  }, [items]);
16
17  return (
18    <ul>
19      {sortedItems.map((item) => (
20        <li key={item.id} onClick={() => handleSelect(item.id)}>
21          {item.name}
22        </li>
23      ))}
24    </ul>
25  );
26};

1// Custom hook for form management
2const useForm = (initialValues = {}) => {
3  const [values, setValues] = useState(initialValues);
4  const [errors, setErrors] = useState({});
5
6  const handleChange = useCallback((e) => {
7    const { name, value } = e.target;
8    setValues((prev) => ({ ...prev, [name]: value }));
9  }, []);
10
11  const resetForm = useCallback(() => {
12    setValues(initialValues);
13    setErrors({});
14  }, [initialValues]);
15
16  return {
17    values,
18    errors,
19    handleChange,
20    resetForm,
21  };
22};
23
24// Usage
25const LoginForm = () => {
26  const { values, handleChange, resetForm } = useForm({
27    email: '',
28    password: '',
29  });
30
31  const handleSubmit = (e) => {
32    e.preventDefault();
33    // Submission logic
34    console.log(values);
35    resetForm();
36  };
37
38  return (
39    <form onSubmit={handleSubmit}>
40      <input
41        type="email"
42        name="email"
43        value={values.email}
44        onChange={handleChange}
45      />
46      <input
47        type="password"
48        name="password"
49        value={values.password}
50        onChange={handleChange}
51      />
52      <button type="submit">Login</button>
53    </form>
54  );
55};