Top 10 JavaScript Engineer Interview Questions & Answers in 2024

Get ready for your JavaScript Engineer interview by familiarizing yourself with required skills, anticipating questions, and studying our sample answers.

1. Explain the concept of event delegation in JavaScript. How does it improve performance, and what considerations should be taken into account when implementing it?

Event delegation involves assigning a single event listener to a common ancestor for multiple elements, instead of attaching listeners to each individual element. This improves performance by reducing the number of event handlers. Considerations include ensuring that the common ancestor is present when the event occurs and using event.target to identify the specific element triggering the event.

2. Discuss the differences between 'apply', 'call', and 'bind' in JavaScript. Provide examples of each and explain when you might use one over the others.

apply: Calls a function with a given 'this' value and an array or array-like object of arguments. Example:

function exampleFunction(arg1, arg2) {
  console.log(this, arg1, arg2);
}

const context = { name: 'Example' };
exampleFunction.apply(context, [1, 2]);

call: Similar to apply but accepts individual arguments instead of an array. Example:
```
exampleFunction.call(context, 1, 2);
```
bind: Returns a new function with a specified 'this' value and partially applied arguments. Example:
```
const boundFunction = exampleFunction.bind(context, 1);
boundFunction(2);
```

Choose 'apply' or 'call' when the number of arguments is dynamic, and 'bind' when creating a function with a fixed context and partially applied arguments.

3. What is the difference between 'let', 'const', and 'var' regarding block scoping in JavaScript? Provide an example demonstrating their behavior.

var: Has function-level scope and is hoisted to the top of the function. Example:

function exampleFunction() {
  if (true) {
    var x = 10;
  }
  console.log(x); // Outputs: 10
}

let and const: Have block-level scope and are hoisted to the top of the block. Example:

function exampleFunction() {
  if (true) {
    let y = 20;
    const z = 30;
  }
  console.log(y); // ReferenceError: y is not defined
  console.log(z); // ReferenceError: z is not defined
}

Use 'let' and 'const' for block-scoped variables, and prefer 'const' for variables that should not be reassigned.

4. Explain the concept of closures in JavaScript and provide an example. How can closures be used to create private variables?

Closures occur when a function is defined inside another function, allowing the inner function to access variables from the outer function even after it 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 can be used to create private variables by encapsulating them within the outer function, making them inaccessible from outside the scope.

5. Discuss the differences between '==' and '===' in JavaScript. Provide examples and explain when to use one over the other.

'==' (loose equality): Compares values after type coercion. Example:
```
5 == '5'; // true
```
'===' (strict equality): Compares values without type coercion. Example:
```
5 === '5'; // false
```

Use '===' for strict equality comparisons to avoid unexpected type coercion and ensure both value and type match.

6. Explain the principles of Object-Oriented Programming (OOP) in JavaScript. How can you implement inheritance, encapsulation, and polymorphism using prototype-based inheritance?

Inheritance: Objects can inherit properties and methods from other objects using prototype chains.
Encapsulation: Grouping related properties and methods within an object, and using closures to create private variables.
Polymorphism: Objects can take on multiple forms through shared interfaces.

Example of prototype-based inheritance:

function Animal(name) {
  this.name = name;
}

Animal.prototype.sayName = function() {
  console.log(this.name);
};

function Dog(name, breed) {
  Animal.call(this, name);
  this.breed = breed;
}

Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;

Dog.prototype.bark = function() {
  console.log('Woof!');
};

7. How does the event loop work in JavaScript, and what is the role of the call stack, callback queue, and event loop in handling asynchronous tasks?

The event loop is the mechanism that allows asynchronous operations to be handled in JavaScript. 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. Asynchronous tasks are moved from the queue to the stack when it is empty, allowing for non-blocking execution.

8. Discuss the principles of Functional Programming (FP) in JavaScript. How can you use concepts like immutability, higher-order functions, and pure functions to write more maintainable and predictable code?

Immutability: Avoiding the mutation of data, creating new objects instead.
Higher-order functions: Functions that take other functions as arguments or return functions.
Pure functions: Functions that produce the same output for the same input and have no side effects.

Example:

const numbers = [1, 2, 3];

// Immutability
const doubledNumbers = numbers.map(num => num * 2);

// Higher-order functions
const square = num => num * num;
const squaredNumbers = numbers.map(square);

// Pure function
const add = (a, b) => a + b;

9. How can you handle asynchronous operations in JavaScript, and what are the differences between callbacks, Promises, and async/await?

Callbacks: Functions passed as arguments to be executed after an asynchronous operation completes.
```
fetchData((data) => {
  console.log(data);
});
```
Promises: Objects representing the eventual completion or failure of an asynchronous operation.
```
fetchData()
```

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

Async/await: Syntactic sugar for working with Promises, making asynchronous code appear more synchronous.

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

Choose the approach that aligns with the project's requirements and improves code readability.

10. What is the purpose of the 'this' keyword in JavaScript, and how does it behave in different contexts? Provide examples of how to control the value of 'this' using methods like 'call,' 'apply,' 'bind,' and arrow functions.

The 'this' keyword refers to the execution context and can behave differently based on how a function is called:

In a function declaration, 'this' refers to the global object (e.g., 'window' in a browser).
In a method (a function within an object), 'this' refers to the object calling the method.
In an event handler, 'this' refers to the element that triggered the event.
In arrow functions, 'this' retains the value of the enclosing execution context.

Examples:

function exampleFunction() {
  console.log(this);
}

const obj = { method: exampleFunction };

// Controlling 'this' using call
exampleFunction.call(obj);

// Controlling 'this' using bind
const boundFunction = exampleFunction.bind(obj);
boundFunction();

// Arrow function preserving 'this'
const arrowFunction = () => {
  console.log(this);
};
arrowFunction.call(obj); // 'this' remains the same as the outer context (lexical scoping)

Understanding the different scenarios and controlling 'this' appropriately is crucial for writing robust JavaScript code.