Top 10 JavaScript Developer Interview Questions & Answers in 2024

1. Explain the concept of closures in JavaScript. Provide an example and discuss how closures are useful in maintaining data privacy and creating modular code.

Closures in JavaScript allow functions to retain access to variables from their outer (enclosing) scope even after the outer function has finished executing. Example:

function outerFunction() {
  let outerVariable = 'I am from outer function';

  function innerFunction() {
    console.log(outerVariable);
  }

  return innerFunction;
}

const closureExample = outerFunction();
closureExample(); // Outputs: I am from outer function

Closures are useful for creating private variables, implementing data encapsulation, and building modular code.

2. Differentiate between 'undefined' and 'undeclared' variables in JavaScript. How can you check if a variable is 'undefined'?

• Undefined: A variable is declared but has no assigned value. Example: let x; console.log(x); // Outputs: undefined
• Undeclared: A variable is used without being declared using var, let, or const. Example: console.log(y); // Throws a ReferenceError

To check if a variable is 'undefined':

if (typeof variable === 'undefined') {
  // Variable is undefined
}

3. Discuss the event loop in JavaScript and how it enables asynchronous programming. Explain the role of the call stack, callback queue, and event loop in handling asynchronous tasks.

The event loop in JavaScript manages asynchronous operations by using the call stack, callback queue, and event loop. The call stack tracks function calls, the callback queue holds tasks to be executed, and the event loop continuously checks the call stack and callback queue, moving tasks from the queue to the stack when it's empty.

4. Explain the concept of promises in JavaScript. Provide an example of using promises for asynchronous operations, including handling success and failure.

Promises in JavaScript provide a cleaner way to work with asynchronous code. Example:

const fetchData = () => {
  return new Promise((resolve, reject) => {
    // Simulating an asynchronous operation
    setTimeout(() => {
      const data = 'Async data';
      // Simulating success
      resolve(data);
      // Simulating failure
      // reject('Error fetching data');
    }, 1000);
  });
};

fetchData()
  .then((result) => console.log('Success:', result))
  .catch((error) => console.error('Error:', error));

5. What is the 'this' keyword in JavaScript, and how does it behave in different contexts (global, function, and object methods)? Provide examples.

The 'this' keyword refers to the current execution context. In different contexts:

• Global context: 'this' refers to the global object (e.g., window in browsers).
• Function context: 'this' depends on how the function is invoked.
• Object methods: 'this' refers to the object calling the method.

Examples:

console.log(this); // Global context

function exampleFunction() {
  console.log(this); // Function context
}

const exampleObject = {
  method: function() {
    console.log(this); // Object method context
  }
};

6. Discuss the differences between 'let', 'const', and 'var' in JavaScript for variable declarations. How do they differ in terms of hoisting, scoping, and reassignment?

• var: Hoisted to the top of the function scope, can be reassigned, and has function scope.
• let: Hoisted to the top of the block scope, can be reassigned, and has block scope.
• const: Hoisted to the top of the block scope, cannot be reassigned after initialization, and has block scope.

console.log(x); // Outputs: undefined (hoisted)
var x = 10;

console.log(y); // Throws ReferenceError (not hoisted)
let y = 20;

const z = 30;
z = 40; // Throws TypeError (cannot be reassigned)

7. What is the purpose of the 'async' and 'await' keywords in JavaScript? Provide an example of using 'async/await' to handle asynchronous operations more elegantly.

'async' and 'await' are used to simplify asynchronous code, making it look more like synchronous code. Example:

async function fetchData() {
  try {
    const response = await fetch('<https://api.example.com/data>');
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.error('Error fetching data:', error);
  }
}

fetchData();

8. Explain the concept of prototypal inheritance in JavaScript. How does it differ from classical inheritance in other programming languages?

JavaScript uses prototypal inheritance, where objects can inherit properties and methods directly from other objects. Each object has an internal prototype link to another object, and properties not found in the object itself are looked up in its prototype chain. This differs from classical inheritance, where classes are used to create objects and define inheritance hierarchies.

9. Discuss the benefits and drawbacks of using arrow functions in JavaScript. How do they differ from regular functions, especially regarding the 'this' keyword?

Benefits of arrow functions:

• Shorter syntax.
• No binding of 'this', using the lexical scope.

Drawbacks:

• Cannot be used as constructors.
• Lack of their own 'arguments' object.

const regularFunction = function() {
  console.log(this); // Refers to the calling context
};

const arrowFunction = () => {
  console.log(this); // Refers to the lexical scope (not the calling context)
};

10. How can you achieve cross-origin resource sharing (CORS) in JavaScript? Discuss the role of

the 'Access-Control-Allow-Origin' header and common methods to handle CORS-related issues.

CORS allows web pages to make requests to a different domain. Servers can include the 'Access-Control-Allow-Origin' header in their responses to specify which origins are permitted to access the resources. To handle CORS issues, servers should configure the appropriate headers, and client-side solutions like JSONP, CORS proxies, or server-side adjustments can be applied.