Frontend | Вопросы собесов

npm init -y   

npm install eslint --save-dev   

npx eslint --init   

npx eslint имя_файла.js   

npx eslint app.js   

npx eslint имя_файла.js --fix   

"scripts": {
    "lint": "eslint ."
}  

npm run lint

{
    "env": {
        "browser": true,
        "es2021": true
    },
    "extends": "eslint:recommended",
    "parserOptions": {
        "ecmaVersion": 12,
        "sourceType": "module"
    },
    "rules": {
        "indent": ["error", 4],
        "linebreak-style": ["error", "unix"],
        "quotes": ["error", "single"],
        "semi": ["error", "always"]
    }
}

import React, { Component } from 'react';

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

  componentDidMount() {
    // Получаем доступ к DOM-элементу через реф
    this.myRef.current.focus();
  }

  render() {
    return <input type="text" ref={this.myRef} />;
  }
}

export default MyComponent;

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

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

  useEffect(() => {
    // Получаем доступ к DOM-элементу через реф
    myRef.current.focus();
  }, []);

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

export default MyComponent;

import React, { useEffect } from 'react';

const MyComponent = () => {
  useEffect(() => {
    const element = document.getElementById('my-element');
    element.style.color = 'red';
  }, []);

  return <div id="my-element">Hello, world!</div>;
};

export default MyComponent;

import React, { useRef, useLayoutEffect } from 'react';

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

  useLayoutEffect(() => {
    // Получаем доступ к DOM-элементу через реф
    myRef.current.style.color = 'blue';
  }, []);

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

export default MyComponent;

console.log('Start');

setTimeout(() => {
  console.log('Macrotask: setTimeout');
}, 0);

Promise.resolve()
  .then(() => {
    console.log('Microtask 1');
    Promise.resolve().then(() => {
      console.log('Microtask 2');
    });
  })
  .then(() => {
    console.log('Microtask 3');
  });

console.log('End');

Start
End
Microtask 1
Microtask 2
Microtask 3
Macrotask: setTimeout

function fetchData(callback) {
  setTimeout(() => {
    const data = "some data";
    callback(data);
  }, 1000);
}

fetchData((result) => {
  console.log(result);
});

function fetchData() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      const data = "some data";
      resolve(data);
    }, 1000);
  });
}

fetchData().then((result) => {
  console.log(result);
}).catch((error) => {
  console.error(error);
});

async function fetchData() {
  return new Promise((resolve) => {
    setTimeout(() => {
      const data = "some data";
      resolve(data);
    }, 1000);
  });
}

async function main() {
  try {
    const result = await fetchData();
    console.log(result);
  } catch (error) {
    console.error(error);
  }
}

main();

.element {
    box-sizing: content-box; /* значение по умолчанию */
    width: 200px;
    padding: 20px;
    border: 10px solid black;
}

.element {
    box-sizing: border-box;
    width: 200px;
    padding: 20px;
    border: 10px solid black;
}

<div class="content-box">Content-box</div>
<div class="border-box">Border-box</div>

div {
    margin: 10px;
    padding: 20px;
    border: 10px solid black;
}

.content-box {
    box-sizing: content-box;
    width: 200px;
    height: 100px;
}

.border-box {
    box-sizing: border-box;
    width: 200px;
    height: 100px;
}

<img src="example.jpg" loading="lazy" alt="Пример изображения">

const handler = {
    get(target, property) {
        console.log(`Getting value of ${property}`);
        return target[property];
    },
    set(target, property, value) {
        console.log(`Setting value of ${property} to ${value}`);
        target[property] = value;
        return true;
    }
};
const person = {
    name: 'Alice',
    age: 25
};
const proxyPerson = new Proxy(person, handler);
proxyPerson.name = 'Bob'; // Setting value of name to Bob
console.log(proxyPerson.name); // Getting value of name // Bob

const person = {
    _name: 'Alice',
    _age: 25
};
Object.defineProperty(person, 'name', {
    get() {
        console.log('Getting name');
        return this._name;
    },
    set(value) {
        console.log(`Setting name to ${value}`);
        this._name = value;
    }
});
Object.defineProperty(person, 'age', {
    get() {
        console.log('Getting age');
        return this._age;
    },
    set(value) {
        console.log(`Setting age to ${value}`);
        this._age = value;
    }
});
person.name = 'Bob'; // Setting name to Bob
console.log(person.name); // Getting name // Bob
person.age = 30; // Setting age to 30
console.log(person.age); // Getting age // 30

import { observable, autorun } from 'mobx';
const person = observable({
    name: 'Alice',
    age: 25,
    setName(name) {
        this.name = name;
    },
    setAge(age) {
        this.age = age;
    }
});
// Автоматически вызываемая функция при изменении наблюдаемого состояния
autorun(() => {
    console.log(`Name: ${person.name}, Age: ${person.age}`);
});
person.setName('Bob'); // Name: Bob, Age: 25
person.setAge(30); // Name: Bob, Age: 30

const targetNode = document.getElementById('target');
const config = { attributes: true, childList: true, subtree: true };
const callback = function(mutationsList, observer) {
    for(let mutation of mutationsList) {
        if (mutation.type === 'childList') {
            console.log('A child node has been added or removed.');
        }
        else if (mutation.type === 'attributes') {
            console.log('The ' + mutation.attributeName + ' attribute was modified.');
        }
    }
};
const observer = new MutationObserver(callback);
observer.observe(targetNode, config);
targetNode.setAttribute('data-test', 'value');

const handler = {
    get(target, property) {
        console.log(`Getting value of ${property}`);
        return target[property];
    },
    set(target, property, value) {
        console.log(`Setting value of ${property} to ${value}`);
        target[property] = value;
        return true;
    }
};
const person = {
    name: 'Alice',
    age: 25
};
const proxyPerson = new Proxy(person, handler);
proxyPerson.name = 'Bob'; // Setting value of name to Bob
console.log(proxyPerson.name); // Getting value of name // Bob

npm install mobx mobx-react

import { observable, autorun } from 'mobx';
import { observer } from 'mobx-react';
import React from 'react';
import ReactDOM from 'react-dom';
const appState = observable({
    count: 0,
    increment() {
        this.count++;
    }
});
autorun(() => {
    console.log(`Count: ${appState.count}`);
});
appState.increment(); // Count: 1
appState.increment(); // Count: 2
const Counter = observer(() => (
    <div>
        <p>Count: {appState.count}</p>
        <button onClick={() => appState.increment()}>Increment</button>
    </div>
));
ReactDOM.render(<Counter />, document.getElementById('root'));

const handler = {
    set(target, property, value) {
        console.log(`Setting value of ${property} to ${value}`);
        target[property] = value;
        document.getElementById(property).innerText = value;
        return true;
    }
};
const state = {
    count: 0
};
const proxyState = new Proxy(state, handler);

document.getElementById('increment').addEventListener('click', () => {
    proxyState.count++;
});

<div>
    <p id="count">0</p>
    <button id="increment">Increment</button>
</div>
<script src="path/to/your/script.js"></script>