25 Advanced JavaScript Questions

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Answer:

JavaScript is a high-level, interpreted programming language primarily used for web development. Key features include:

Dynamically typed.

Supports both procedural and object-oriented programming.

Runs on the client side (web browsers).

Asynchronous programming with callbacks and Promises.

Answer:

var: Function-scoped, can be redeclared, and is hoisted.

let: Block-scoped, can be reassigned, and is hoisted.

const: Block-scoped, cannot be reassigned after declaration, and is hoisted.

Answer:

Closures allow functions to retain access to variables from their outer (enclosing) scope even after the outer function has finished executing. They are crucial for creating private variables and maintaining state.

Answer:

Event delegation is a technique where a single event listener is attached to a common ancestor instead of individual elements. It takes advantage of event bubbling, reducing the number of event listeners and improving performance.

Answer:

this refers to the object that is currently executing the function. Its value is determined by how a function is called, and it allows access to the object's properties and methods.

Answer:

JavaScript uses prototypal inheritance, where objects can inherit properties and methods from other objects via a prototype chain. Each object has a prototype object, and if a property is not found in the object, JavaScript looks up the chain until it finds the property or reaches the end.

Answer:

A closure is a function that has access to variables from its outer (enclosing) scope, even after the outer function has finished executing. Example:

function outer() {

  let outerVar = 10;

  function inner() {

    console.log(outerVar);

  }

  return inner;

}

const closureFunc = outer();

closureFunc(); // Outputs: 10

In this example, inner forms a closure with access to outerVar.

Answer:

null: A deliberate assignment indicating the absence of a value.

undefined: A variable that has been declared but not assigned a value, or a non-existent property in an object.

Answer:

The bind method creates a new function that, when called, has its this keyword set to a specific value. It is often used to create functions with a fixed this value.

Answer:

==: Loose equality operator, only checks for equality of values after type coercion.

===: Strict equality operator, checks for equality of values and types without type coercion.

Answer:

The map function is used to transform each element of an array and create a new array with the results. It does not modify the original array.

const numbers = [1, 2, 3];

const squaredNumbers = numbers.map(num => num * num);

// Result: [1, 4, 9]

Answer:

async is used to declare an asynchronous function, and await is used to pause the execution of an async function until the promise is resolved, returning the resolved value.

async function fetchData() {

  const result = await fetch('https://example.com');

  const data = await result.json();

  console.log(data);

}

Answer:

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during compilation. However, only the declarations are hoisted, not the initializations.

Answer:

The reduce method is used to accumulate values in an array and reduce it to a single value. It takes a callback function that performs the accumulation.

const numbers = [1, 2, 3, 4];

const sum = numbers.reduce((acc, num) => acc + num, 0);

// Result: 10

Answer:

Event delegation involves attaching a single event listener to a common ancestor, rather than attaching multiple listeners to individual elements. It leverages event bubbling, allowing the handling of events on descendant elements through a single listener.

Answer:

localStorage is a web storage object that allows developers to store key/value pairs in a web browser with no expiration time. The stored data persists even when the browser is closed and reopened.

// Storing data

localStorage.setItem('username', 'John');

// Retrieving data

const username = localStorage.getItem('username');

console.log(username); // Outputs: John

Answer:

Errors in JavaScript can be handled using try, catch, finally blocks.

try {

  // Code that might throw an error

  throw new Error('An error occurred');

} catch (error) {

  console.error(error.message);

} finally {

  // Code that always runs

}

Answer:

Promise is an object representing the eventual completion or failure of an asynchronous operation and its resulting value. It allows better handling of asynchronous operations, especially with async/await.

Answer:

The event loop is a mechanism in JavaScript that allows the execution of code to be non-blocking. It consists of a call stack, callback queue, and event loop. The call stack processes synchronous code, while asynchronous code is handled through callback functions pushed to the callback queue by web APIs.

Answer:

Arrow functions provide a concise syntax for writing function expressions. They do not have their own this and arguments and are not suitable for functions that require these features.

const add = (a, b) => a + b;

Answer:

Object.create() is used to create a new object with a specified prototype object. It allows for prototypal inheritance.

const person = {

  greet: function() {

    console.log('Hello!');

  }

};

const john = Object.create(person);

john.greet(); // Outputs: Hello!

Answer:

Event propagation in the DOM occurs in two phases: capturing phase (top-down) and bubbling phase (bottom-up). Event listeners can be placed in either phase to handle events as they propagate through the DOM.

Answer:

A RESTful API (Representational State Transfer) is an architectural style for designing networked applications. It uses standard HTTP methods (GET, POST, PUT, DELETE) for communication and is stateless. In JavaScript, the fetch API is often used to interact with RESTful APIs.

Answer:

setTimeout is used to delay the execution of a function by a specified number of milliseconds.

console.log('Start');

setTimeout(() => {

  console.log('Delayed');

}, 1000);

console.log('End');

Output:

Start

End

Delayed

Answer:

The typeof operator returns a string indicating the type of an operand. It is often used to check the type of a variable.

console.log(typeof 42); // Outputs: 'number'

console.log(typeof 'Hello'); // Outputs: 'string'

console.log(typeof true); // Outputs: 'boolean'