5 commonly asked frontend interview questions in product-based companies

Implement a debounce function

Debouncing is a technique to limit the rate at which a function can fire.

function debounce(func, delay) {
  let timeoutId;
  return function (...args) {
    clearTimeout(timeoutId);
    timeoutId = setTimeout(() => {
      func.apply(this, args);
    }, delay);
  };
}

// Usage
const debouncedSearch = debounce((query) => {
  console.log('Searching for:', query);
}, 300);

Create a simple todo list component

This question tests your ability to manage state and handle user interactions.

import React, { useState } from 'react';

function TodoList() {
  const [todos, setTodos] = useState([]);
  const [input, setInput] = useState('');

  const addTodo = () => {
    if (input) {
      setTodos([...todos, input]);
      setInput('');
    }
  };

  return (
    <div>
      <input
        value={input}
        onChange={(e) => setInput(e.target.value)}
      />
      <button onClick={addTodo}>Add Todo</button>
      <ul>
        {todos.map((todo, index) => (
          <li key={index}>{todo}</li>
        ))}
      </ul>
    </div>
  );
}

Implement a basic Promise

Understanding Promises is crucial for handling asynchronous operations.

class MyPromise {
  constructor(executor) {
    this.state = 'pending';
    this.value = undefined;
    this.callbacks = [];

    const resolve = (value) => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
        this.callbacks.forEach(callback => callback(value));
      }
    };

    executor(resolve);
  }

  then(onFulfilled) {
    if (this.state === 'fulfilled') {
      onFulfilled(this.value);
    } else {
      this.callbacks.push(onFulfilled);
    }
  }
}

// Usage
const promise = new MyPromise((resolve) => {
  setTimeout(() => resolve('Done'), 1000);
});

promise.then(console.log);

Implement a basic event emitter

Event emitters are useful for building pub-sub systems.

class EventEmitter {
  constructor() {
    this.events = {};
  }

  on(eventName, callback) {
    if (!this.events[eventName]) {
      this.events[eventName] = [];
    }
    this.events[eventName].push(callback);
  }

  emit(eventName, data) {
    const eventCallbacks = this.events[eventName];
    if (eventCallbacks) {
      eventCallbacks.forEach(callback => callback(data));
    }
  }
}

// Usage
const emitter = new EventEmitter();
emitter.on('userLoggedIn', (user) => console.log('User logged in:', user));
emitter.emit('userLoggedIn', { name: 'John' });

Flatten an array of arrays

This tests your ability to work with nested data structures.

function flattenArray(arr) {
  return arr.reduce((flat, toFlatten) => {
    return flat.concat(Array.isArray(toFlatten) ? flattenArray(toFlatten) : toFlatten);
  }, []);
}

// Usage
const nestedArray = [1, [2, 3], [4, [5, 6]]];
console.log(flattenArray(nestedArray)); // [1, 2, 3, 4, 5, 6]

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