React cheatsheet

<Student
  firstName="Julie"
  lastName="Johnson"
  age={23}
  pro={true}
/>

import React from "react";

export default function Student(props) {
  return (
    <h1>
      {props.firstName} {props.lastName} is {props.age}.
    </h1>
  );
}

import React, { useState } from "react";

export default function Hello(props) {
  let [name, setName] = useState("Julie");
  function updateName() {
    let newName = prompt("What is your name?");
    setName(newName);
  }

  return (
    <div>
      <h1>{name}</h1>
      <button onClick={updateName}>Update name</button>
    </div>
  );
}

import React from "react";

export default function Hello() {
  function handleClick(event) {
    event.preventDefault();
    alert("Hello World");
  }

  return (
    <a href="/" onClick={handleClick}>
      Say Hi
    </a>
  );
}

Note: The most common event listeners are onClick for links/buttons and onSubmit for forms.

let elements = ["one", "two", "three"];

return (
  <ul>
    {elements.map(function (value, index) {
      return <li key={index}>{value}</li>;
    })}
  </ul>
);

Note: Each list item inside a map loop needs a key attribute with a unique value which is generally the index.

let elements = [
  {
    name: "one",
    value: 1,
  },
  {
    name: "two",
    value: 2,
  },
  {
    name: "three",
    value: 3,
  },
];
return (
  <ul>
    {elements.map(function (element, index) {
      return (
        <li key={index}>
          The value for {element.name} is {element.value}
        </li>
      );
    })}
  </ul>
);

Note: Each list item inside a map loop needs a key attribute with a unique value which is generally the index.

import React, { useState } from "react";

export default function LoginForm() {
  let [username, setUsername] = useState("");
  let [password, setPassword] = useState("");

  function handleSubmit(event) {
    event.preventDefault();
    alert(`Loging in with ${username} and ${password}`);
  }

  function updateUsername(event) {
    setUsername(event.target.value);
  }

  function updatePassword(event) {
    setPassword(event.target.value);
  }

  return (
    <form onSubmit={handleSubmit}>
      <input type="text" placeholder="Username" onChange={updateUsername} />
      <input type="password" placeholder="Password" onChange={updatePassword} />
      <input type="submit" value="Login" />
    </form>
  );
}

import React from "react";
import "./Student.css";

export default function Student() {
  return <div className="Student">Julie Johnson</div>;
}

Note: You'll then have to crate a css file called Student.css

import React from "react";
import axios from "axios";

export default function Weather(props) {
  function handleResponse(response) {
    console.log(response);
  }

  if (notifications) {
    return <p>notifications</p>;
  } else {
    let url = `https://notifications.com`;
    axios.get(url).then(handleResponse);
    return <p>Loading notifications..</p>;
  }
}

Note: Make sure to import Axios first to your project.

import React, { useState } from "react";

function Counter() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

export default Counter;

Note: The useState Hook is a built-in React Hook that allows functional components to manage local state. It provides a way to declare state variables and update them within a functional component. Example code illustrating how to use it

import React, { useState } from "react";

function Counter() {
  const [count, setCount] = useState(0);
  const [name, setName] = useState("");
  const [isCompleted, setIsCompleted] = useState(false);

  const handleIncrement = () => {
    setCount(count + 1);
  };

  const handleNameChange = (event) => {
    setName(event.target.value);
  };

  const toggleCompletion = () => {
    setIsCompleted(!isCompleted);
  };

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={handleIncrement}>Increment</button>

      <input type="text" value={name} onChange={handleNameChange} placeholder="Enter your name" />

      <label>
        <input type="checkbox" checked={isCompleted} onChange={toggleCompletion} />
        Completed
      </label>
    </div>
  );
}

export default Counter;

Note: You can declare multiple state variables using the useState Hook by calling it multiple times in a functional component. Each call to useState manages a separate piece of state.

import { useState } from "react";

function FormExample() {
  const [formData, setFormData] = useState({ name: "", email: "", message: "" });

  const handleChange = (event) => {
    const { name, value } = event.target;
    setFormData((prevFormData) => ({ ...prevFormData, [name]: value }));
  };

  const handleSubmit = (event) => {
    event.preventDefault();
    alert(`Name: ${formData.name}, Email: ${formData.email}, Message: ${formData.message}`);
  };

  return (
    <form onSubmit={handleSubmit}>
      <label htmlFor="name">Name:</label>
      <input type="text" id="name" name="name" value={formData.name} onChange={handleChange} />

      <label htmlFor="email">Email:</label>
      <input type="email" id="email" name="email" value={formData.email} onChange={handleChange} />

      <label htmlFor="message">Message:</label>
      <textarea id="message" name="message" value={formData.message} onChange={handleChange} />

      <button type="submit">Submit</button>
    </form>
  );
}

export default FormExample;

import React, { useState, useEffect } from "react";

function Timer() {
  const [seconds, setSeconds] = useState(0);

  useEffect(() => {
    const interval = setInterval(() => {
      setSeconds((prevSeconds) => prevSeconds + 1);
    }, 1000);

    return () => clearInterval(interval);
  }, []);

  return <div>Seconds: {seconds}</div>;
}

export default Timer;

Note: The useEffect Hook in React is used for performing side effects in functional components. It allows you to execute code based on component lifecycle events like mounting, updating, and unmounting.

import React, { useState, useEffect } from "react";
import axios from "axios";

function UserList() {
  const [users, setUsers] = useState([]);

  useEffect(() => {
    axios
      .get("https://jsonplaceholder.typicode.com/users")
      .then((response) => {
        setUsers(response.data);
      })
      .catch((error) => {
        console.error("Error fetching users:", error);
      });
  }, []);

  return (
    <div>
      <h2>User List</h2>
      <ul>
        {users.map((user) => (
          <li key={user.id}>{user.name}</li>
        ))}
      </ul>
    </div>
  );
}

export default UserList;

Note: Make sure to import Axios first to your project.

import { useState, useEffect } from "react";

function useLocalStorage(key, initialValue) {
  const [value, setValue] = useState(() => {
    const storedValue = localStorage.getItem(key);
    return storedValue !== null ? JSON.parse(storedValue) : initialValue;
  });

  useEffect(() => {
    localStorage.setItem(key, JSON.stringify(value));
  }, [key, value]);

  return [value, setValue];
}

export default useLocalStorage;

Note: Custom Hooks are reusable functions in React that contain logic shared across multiple components. They allow you to extract stateful logic from components into standalone functions.

import React, { Component } from "react";

class MyComponent extends Component {
  constructor(props) {
    super(props);
    this.myRef = React.createRef();
  }

  componentDidMount() {
    console.log(this.myRef.current); // Access the DOM element
  }

  render() {
    return <div ref={this.myRef}>Hello, world!</div>;
  }
}

export default MyComponent;

import React, { useRef, useEffect } from "react";

function MyComponent() {
  const myRef = useRef(null);

  useEffect(() => {
    console.log(myRef.current); // Access the DOM element
  }, []);

  return <div ref={myRef}>Hello, world!</div>;
}

export default MyComponent;

import React, { Component } from "react";

class MyComponent extends Component {
  constructor(props) {
    super(props);
    this.myRef = null;
    this.setRef = element => {
      this.myRef = element;
    };
  }

  componentDidMount() {
    console.log(this.myRef); // Access the DOM element
  }

  render() {
    return <div ref={this.setRef}>Hello, world!</div>;
  }
}

export default MyComponent;

Copy code
import React from "react";

const FancyButton = React.forwardRef((props, ref) => (
  <button ref={ref} className="FancyButton">
    {props.children}
  </button>
));

// Usage
const ref = React.createRef();
<FancyButton ref={ref}>Click me!</FancyButton>;

import React, { useRef, useEffect } from "react";

function FocusInput() {
  const inputRef = useRef(null);

  useEffect(() => {
    // Focus the input element when the component mounts
    inputRef.current.focus();
  }, []);

  return <input ref={inputRef} type="text" />;
}

export default FocusInput;

import React, { useRef } from "react";

function Form() {
  const firstInputRef = useRef(null);
  const secondInputRef = useRef(null);

  const handleKeyDown = (e) => {
    if (e.key === "Enter") {
      secondInputRef.current.focus();
    }
  };

  return (
    <div>
      <input ref={firstInputRef} type="text" onKeyDown={handleKeyDown} />
      <input ref={secondInputRef} type="text" />
    </div>
  );
}

export default Form;