JavaScript Questions and Answers

Question 1: Explain Event Delegation in JavaScript.

Answer:

Event delegation is a technique involving adding an event listener to a parent element instead of multiple child elements. When an event occurs on a child element, it bubbles up to the parent, which handles the event through a single listener. This is efficient for memory usage and performance, especially when dealing with a large number of elements.

Explanation:

In JavaScript, events bubble up through the DOM. By leveraging this, event delegation allows us to avoid adding event listeners to specific nodes; instead, we add a single event listener to a parent element that can catch all events coming from its children. It’s particularly useful in cases where new elements are added dynamically.

Question 2: What is a Closure and how is it used?

Answer:

A closure is a function that has access to its own scope, the outer function’s scope, and the global scope. Closures are used to encapsulate variables and functions, keeping them private and protecting them from the global scope.

Explanation:

Closures are a fundamental concept in JavaScript, allowing for data encapsulation. A closure is created when a function is defined within another function, taking advantage of JavaScript’s lexical scoping. This lets the inner function access the outer function’s variables even after the outer function has executed.

Question 3: Describe the difference between var, let, and const.

Answer:

var is function-scoped, and it can be redeclared and updated.

let is block-scoped, and it can be updated but not redeclared within the same scope.

const is also block-scoped, but it cannot be updated or redeclared. It requires an initial value.

Explanation:

var was the standard before ES6 and is function-scoped, which can lead to unexpected behavior in block statements. let and const are block-scoped, making them more predictable and safer to use in loops or if statements. const is for constants whose values shouldn’t change, while let is for variables whose values can change.

Question 4: Explain the this keyword in JavaScript.

Answer:

The this keyword refers to the object that is executing the current function. In global scope, this refers to the global object. In a method, this refers to the owner object. In a function, this refers to the global object (or undefined in strict mode).

Explanation:

this is context-dependent. Its value is determined based on how a function is called. With new instances (new keyword), this refers to the newly created object. Arrow functions don’t have their own this and inherit it from the surrounding scope.

Question 5: What is Prototypal Inheritance in JavaScript?

Answer:

Prototypal Inheritance is a feature in JavaScript where an object can inherit properties and methods from another object. Traditionally, this is done using the prototype property to add methods and properties to a constructor function, which can then be inherited.

Explanation:

Every JavaScript object has a prototype property that is a reference to another object. When a property or method is called on an object, JavaScript will look up the property on the object, and if not found, on its prototype, and so on up the prototype chain until it’s found or the chain ends. This mechanism is known as prototypal inheritance.