JavaScript Interview Questions

JavaScript is a fundamental programming language that plays a crucial role in web development.

For job seekers aiming to secure JavaScript positions, it is essential to prepare for interviews that often include a series of challenging questions. In this comprehensive guide, we will delve into the most commonly asked JavaScript interview questions, equipping you with the knowledge and confidence needed to succeed.

What is JavaScript and what are its key features?

JavaScript is a versatile scripting language used for web development. Its key features include support for object-oriented programming, event-driven programming, cross-platform compatibility, dynamic typing, DOM manipulation, asynchronous programming, an extensive library ecosystem, and server-side development with Node.js. These features enable developers to create interactive and dynamic web applications efficiently.

How do you use `this` keyword in JS?

In JavaScript, the this keyword is used to refer to the current execution context. Its value is determined dynamically at runtime. Here are some common use cases:

• Global Scope: In the global scope, this refers to the global object (window in browsers or global in Node.js).
• Object Method: In an object method, this refers to the object itself, allowing access to its properties and methods.
• Constructor Function: When a function is used as a constructor with new, this refers to the newly created object.
• Explicit Binding: You can explicitly set this using functions like call(), apply(), or bind().
• Event Handlers: In event handlers, this refers to the element that triggered the event.
• Arrow Functions: Arrow functions inherit this from the surrounding scope.

How do you create a class in JS?

In JavaScript, you can create a class using the ES6 class syntax. Here’s an example:

class MyClass {
  constructor(property1, property2) {
    this.property1 = property1;
    this.property2 = property2;
  }

  method1() {
    // Method implementation
  }

  method2() {
    // Method implementation
  }
}

In the above code, MyClass is the class name. The constructor method is a special method that is executed when an instance of the class is created. It is used to initialize the object’s properties. Inside the constructor, you can define and assign the class properties using this.propertyName.

The class also contains other methods, such as method1 and method2. These methods define the behavior and functionality of the class.

To create an instance of the class, you can use the new keyword:

What is the difference between call, apply and bind methods in JS?

In JavaScript, the call(), apply(), and bind() methods are used to manipulate the this value and invoke functions in different ways. Although they serve similar purposes, there are subtle differences between them. Here’s an explanation of each method:

• call(): The call() method invokes a function with a specified this value and optional arguments passed individually. It takes the function’s context as the first argument, followed by the function arguments.

  function greet(message) {
    console.log(message + ' ' + this.name);
  }
  
  var person = { name: 'Alice' };
  greet.call(person, 'Hello'); // Output: Hello Alice
  
  

• apply(): The apply() method is similar to call(), but it accepts arguments as an array or an array-like object. It invokes the function with a specified this value and an array of arguments.

  function greet(message) {
    console.log(message + ' ' + this.name);
  }
  
  var person = { name: 'Bob' };
  greet.apply(person, ['Hi']); // Output: Hi Bob
  
  

• bind(): The bind() method creates a new function with a specified this value and optional arguments. It does not immediately invoke the function but returns a new function that can be called later. The bound function retains the specified this value regardless of how it is called.

  function greet(message) {
    console.log(message + ' ' + this.name);
  }
  
  var person = { name: 'Charlie' };
  var greetPerson = greet.bind(person);
  greetPerson('Hey'); // Output: Hey Charlie
  
  

---

What are the differences between JavaScript and other programming languages?

JavaScript stands apart from other programming languages in several ways. Here are the key differences:

• Dynamic Typing: Unlike statically-typed languages such as Java or C++, JavaScript employs dynamic typing. This means that variables can hold values of any type, allowing for more flexibility but also requiring careful handling to prevent unexpected behavior.
• Prototypal Inheritance: JavaScript utilizes prototypal inheritance, where objects can directly inherit properties and methods from other objects. This differs from class-based inheritance found in languages like Java, where objects are instances of classes.
• Execution Environment: JavaScript primarily runs within web browsers, enabling client-side scripting to enhance website interactivity. In contrast, languages like Java or Python can be executed on a wide range of platforms and devices.
• Syntax and Structure: JavaScript has its unique syntax and structure, which may differ from other languages. For example, JavaScript uses curly braces {} to define blocks of code, and semicolons are optional for line termination.
• Application Focus: While JavaScript is commonly associated with front-end web development, it has expanded its reach to encompass full-stack development with the advent of Node.js. In comparison, languages like Java and Python have a broader range of application domains, including desktop applications, mobile development, and server-side operations.

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How does JavaScript differ from Java?

JavaScript and Java are distinct programming languages with notable differences:

• Application Domain: JavaScript is mainly used for web development, while Java has a broader range of applications, including desktop software and enterprise systems.
• Execution Environment: JavaScript runs in web browsers, while Java runs on a Java Virtual Machine (JVM), making it platform-independent.
• Typing: JavaScript is dynamically typed, allowing flexible variable types, while Java is statically typed, requiring explicit type declarations.
• Syntax and Structure: JavaScript has a more flexible syntax with optional semicolons and uses curly braces for code blocks. Java has stricter syntax with mandatory semicolons and follows a class-based structure.
• Inheritance: JavaScript uses prototypal inheritance, while Java employs class-based inheritance.
• Memory Management: JavaScript has automatic memory management through garbage collection, while Java requires more manual memory management.

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Explain the concept of hoisting in JavaScript.

Hoisting in JavaScript is the behavior where variable and function declarations are moved to the top of their respective scopes during the compilation phase.

This means that regardless of where variables and functions are declared in the code, they are effectively “hoisted” to the top and can be used before their actual declarations in the code.

However, it’s important to note that only the declarations are hoisted, not the initializations or assignments.

---

Describe closures in JavaScript and provide use cases.

Closures in JavaScript are functions that have access to variables from their outer (enclosing) scope, even after the outer function has finished executing. They “remember” the environment in which they were created.

Use cases for closures include:

• Data Privacy: Closures enable encapsulation and data privacy by creating private variables and functions that are only accessible within the closure scope.
• Function Factories: Closures can be used to create and return functions with pre-configured settings or behavior, allowing for the creation of function factories.
• Asynchronous Operations: Closures are often used with asynchronous operations, such as event handlers or AJAX requests, to maintain access to relevant variables even when the outer function has completed execution.
• Memoization: Closures can be utilized for caching expensive function results, optimizing performance by remembering and returning previously calculated values.

---

What are the differences between ‘null’ and ‘undefined’?

In JavaScript, ‘null’ and ‘undefined’ are both special values that represent the absence of a value, but they have distinct meanings and use cases:

‘null’: It is a value that represents the intentional absence of an object value. It is a way to explicitly assign an empty or non-existent value to a variable. It is a primitive value and is typically used when a variable needs to be explicitly set to nothing.

var myVariable = null;
console.log(myVariable); // Output: null

undefined’: It represents the absence of an assigned value or an undeclared variable.
When a variable is declared but not assigned a value, or when trying to access a variable that has not been declared at all, it will have the value of ‘undefined’ by default.

var myVariable;
console.log(myVariable); // Output: undefined

console.log(nonExistentVariable); // Output: undefined

‘null’ is a value that can be assigned to a variable explicitly, while ‘undefined’ is a default value assigned to variables that are not assigned a value or variables that are not declared.

‘null’ is of type ‘object’, whereas ‘undefined’ is of type ‘undefined’.

‘null’ is often used to indicate an intentional absence of value, while ‘undefined’ is typically a result of a variable not being assigned or not existing.

 



How does asynchronous programming work in JavaScript? Explain callbacks, Promises, and async/await.

Asynchronous programming in JavaScript allows for non-blocking execution of code, enabling concurrent operations and preventing delays that could negatively impact user experience. There are three common approaches to asynchronous programming in JavaScript: callbacks, Promises, and async/await.

• Callbacks: Callbacks are the traditional approach to handle asynchronous operations in JavaScript. A callback function is passed as an argument to an asynchronous function, which is then invoked once the operation is complete. The callback function handles the result or error.

  function fetchData(callback) {
    // Simulating an asynchronous operation
    setTimeout(function() {
      var data = 'Sample data';
      callback(null, data); // Pass null as the error and data as the result
    }, 1000);
  }
  
  function handleData(error, result) {
    if (error) {
      console.error(error);
    } else {
      console.log(result);
    }
  }
  
  fetchData(handleData);
  

• Promises: Promises provide a more structured and flexible way to handle asynchronous operations. Promises represent the eventual completion (or failure) of an asynchronous operation, allowing chaining and easier error handling.

  function fetchData() {
    return new Promise(function(resolve, reject) {
      // Simulating an asynchronous operation
      setTimeout(function() {
        var data = 'Sample data';
        resolve(data); // Resolve the Promise with the data
        // Or reject with an error: reject(new Error('Something went wrong'));
      }, 1000);
    });
  }
  
  fetchData()
    .then(function(result) {
      console.log(result);
    })
    .catch(function(error) {
      console.error(error);
    });
  

• async/await: The async/await syntax provides a more concise and readable way to write asynchronous code. It allows writing asynchronous operations in a synchronous style, making the code easier to understand and maintain. It is built on top of Promises.

  async function fetchData() {
    return new Promise(function(resolve, reject) {
      // Simulating an asynchronous operation
      setTimeout(function() {
        var data = 'Sample data';
        resolve(data); // Resolve the Promise with the data
        // Or reject with an error: reject(new Error('Something went wrong'));
      }, 1000);
    });
  }
  
  async function handleData() {
    try {
      var result = await fetchData();
      console.log(result);
    } catch (error) {
      console.error(error);
    }
  }
  
  handleData();
  
  

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What is event delegation in JavaScript?

Event delegation in JavaScript is a technique where instead of attaching event listeners to individual elements, you attach a single event listener to a parent element that listens for events on its child elements. When an event occurs, it “bubbles up” through the DOM hierarchy, triggering the event listener on the parent element. The event object contains information about the event target, allowing you to identify the specific child element that triggered the event.

Event delegation offers several benefits:

• Efficiency: By attaching a single event listener to a parent element, you eliminate the need to attach listeners to multiple child elements. This can improve performance, especially when dealing with a large number of dynamically created elements.
• Simplified Code: Event delegation reduces the amount of repetitive code needed to attach listeners to individual elements. You only need to handle events in one place, making the codebase more concise and maintainable.
• Dynamic Elements: When new child elements are added dynamically, they automatically inherit the event listener from the parent element. This eliminates the need to reattach listeners to newly added elements.
• Memory Management: With event delegation, you avoid potential memory leaks caused by attaching event listeners directly to individual elements. When child elements are removed, their associated event listeners are automatically cleaned up by the browser.

// HTML structure
// HTML structure
<ul id="parentList">
 <li>Item 1</li>
  <li>Item 2</li>
  <l>Item 3</li>
</ul>;

// JavaScript event delegation
document.getElementById("parentList").addEventListener("click", function(event) {
  if (event.target.nodeName === "LI") {
    console.log("Clicked on:", event.target.textContent);
  }
});

In the above example, a click event listener is attached to the parent

element. When a list item (
• ) is clicked, the event bubbles up to the parent element, and the listener checks if the target element’s nodeName is “LI”. If so, it logs the text content of the clicked list item.

  Event delegation simplifies event handling by leveraging event bubbling and reducing the number of event listeners needed, resulting in more efficient and scalable JavaScript code.

  ---

  How does JavaScript handle prototypal inheritance?

  JavaScript uses prototypal inheritance, which is a unique approach to object-oriented programming. Objects in JavaScript have a prototype, which serves as a blueprint for inheriting properties and methods.

  Here’s how prototypal inheritance works in JavaScript:

  • Prototype Chain: Each object in JavaScript has an internal property called [[Prototype]], which refers to its prototype object. When accessing a property or method on an object, JavaScript first looks for it in the object itself. If not found, it continues the search in the object’s prototype, and this process continues up the prototype chain until the property is found or until the end of the chain (where the prototype is null).
  • Prototype Object: The prototype object is a regular object that is assigned to the [[Prototype]] property of another object. It acts as a fallback for property and method lookups. Every object (except the root object) has a prototype, which can be another object or null.
  • Inheritance and Property Lookup: When accessing a property or method on an object, JavaScript looks for it first in the object itself. If not found, it traverses up the prototype chain until it finds the property or reaches the end of the chain. This allows objects to inherit properties and methods from their prototypes.
  • prototype Property: Functions in JavaScript have a special property called prototype, which is an object that becomes the prototype of objects created using that function as a constructor. It defines the properties and methods that objects created from that constructor will inherit.
  • Constructor Functions: Constructor functions are used to create objects with shared properties and methods. When a constructor function is invoked with the new keyword, a new object is created with its prototype set to the constructor’s prototype property.

  // Constructor function
  function Person(name) {
    this.name = name;
  }
  
  // Adding a method to the prototype
  Person.prototype.sayHello = function() {
    console.log("Hello, my name is " + this.name);
  };
  
  // Creating objects using the constructor
  var person1 = new Person("Alice");
  var person2 = new Person("Bob");
  
  // Calling the inherited method
  person1.sayHello(); // Output: Hello, my name is Alice
  person2.sayHello(); // Output: Hello, my name is Bob
  
  

  In the above example, the Person constructor function is defined with a name property. The sayHello method is added to the Person.prototype. When objects (person1 and person2) are created using the new keyword, they inherit the sayHello method from the prototype, allowing them to call the method.

  This is a simplified explanation of how JavaScript handles prototypal inheritance. It provides a flexible and powerful mechanism for object-oriented programming, allowing objects to share and inherit properties and methods from their prototypes.

  ---

  How would you optimize JavaScript code for performance?

  • Minimize DOM manipulation and batch changes.
  • Use efficient DOM access by caching element references.
  • Limit variable scope and use let and const.
  • Optimize algorithms and data structures.
  • Use asynchronous operations to avoid blocking the main thread.
  • Profile code using browser developer tools and optimize critical sections.
  • Bundle and minify JavaScript files to reduce network latency.
  • Properly manage memory to avoid leaks.
  • Optimize loops and minimize work inside them.
  • Use well-optimized libraries and stay updated with the latest versions.

  ---

  What are differences between ES5 and ES6?

  ES5 (ECMAScript 5) and ES6 (ECMAScript 2015) are different versions of the ECMAScript standard, which is the specification that defines the JavaScript language. Here are some key differences between ES5 and ES6:

  1. Syntax and Features: ES6 introduced several new syntax features and enhancements over ES5. Some notable additions in ES6 include arrow functions, classes, template literals, destructuring assignment, default parameters, and rest/spread operators. These new features provide more concise and expressive ways to write JavaScript code.
  2. Arrow Functions: ES6 introduced arrow functions, which provide a shorter syntax for writing function expressions. Arrow functions have lexical scoping for this, meaning they inherit the this value from the surrounding code.
  3. Classes: ES6 introduced a class syntax that simplifies the creation of objects and inheritance. It provides a more familiar and structured way of defining classes, constructor functions, and prototype-based inheritance.
  4. Modules: ES6 introduced native support for modules, allowing developers to organize and share code between files. Modules provide a standardized way to define dependencies and export/import functionality.
  5. Let and Const: ES6 introduced block-scoped variables with let and constants with const. Unlike var in ES5, let and const are not hoisted and have block-level scope, which helps in avoiding unintended variable redeclarations and enhances code clarity.
  6. Promises: ES6 introduced native promises for handling asynchronous operations. Promises simplify working with asynchronous code and provide a cleaner alternative to callback-based patterns.
  7. Enhanced Object Literal: ES6 introduced enhancements to object literals, such as shorthand property and method definitions, computed property names, and the ability to define methods using concise function syntax.
  8. Iterators and Generators: ES6 introduced the concept of iterators and generators, which provide an iterable protocol for traversing data structures. Iterators allow custom objects to define their iteration behavior, while generators simplify the creation of iterators using a special function* syntax.

  ---

  What are arrow functions in JS?

  Arrow functions, introduced in ES6 (ECMAScript 2015), are a more concise syntax for writing function expressions in JavaScript.
  They provide a shorter and more readable way to define functions compared to traditional function expressions. Here’s an example of an arrow function:

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

  ---

  What is template literal in JS?

  In JavaScript, a template literal is a string literal that supports embedded expressions. It provides a more convenient and flexible way to concatenate strings and embed dynamic values or expressions within them. Template literals are denoted by backticks ( ) instead of single or double quotes used for regular strings. Here’s an example:

  const name = 'John';
  const greeting = `Hello, ${name}!`;
  
  console.log(greeting); // Output: Hello, John!
  
  

  ---

  What is the difference between `==` and `===` in JS?

  In JavaScript, the == (loose equality) and === (strict equality) are comparison operators used to compare values for equality. Here’s the difference between them:

  == (Loose Equality):
  The == operator compares values for equality after performing type coercion if necessary. It allows for type conversion between different types before making the comparison. For example, if you compare a string and a number using ==, JavaScript will attempt to convert one of the operands to the type of the other operand.

  === (Strict Equality):
  The === operator compares values for equality without performing type coercion. It checks if the values are of the same type and have the same value. It requires both the type and the value to match for the comparison to be considered true.

  ---

  What are different data types in JS?

  JavaScript has several built-in data types that are used to represent different kinds of values. The primary data types in JavaScript are:

  1. Number: Represents numeric values, including integers and floating-point numbers. For example: 42, 3.14.
  2. String: Represents sequences of characters enclosed in single quotes (') or double quotes ("). For example: 'Hello', "JavaScript".
  3. Boolean: Represents logical values that can be either true or false. Used for logical operations and conditional statements.
  4. Undefined: Represents a variable that has been declared but has not been assigned a value. Variables that are not assigned a value are automatically assigned undefined.
  5. Null: Represents the intentional absence of any object value. It is a special value denoting the absence of an object reference.
  6. Object: Represents a collection of key-value pairs and is one of the most fundamental data types in JavaScript. Objects can be created using object literals or through the new keyword.
  7. Array: Represents an ordered list of values enclosed in square brackets ([]). Arrays can store multiple values of any type and can be accessed using numeric indices.
  8. Symbol: Represents a unique identifier. Symbols are often used as keys in objects to avoid naming conflicts.

  In addition to these primary data types, JavaScript also provides some additional data types that are considered special cases or extensions of the primary types:

  • Function: Functions are a subtype of objects. They can be assigned to variables, passed as arguments to other functions, and returned from functions.
  • BigInt: Represents arbitrary-precision integers. It is used when working with numbers beyond the range of the Number type.

  ---

  What are the differences between let const and var in JS?

  The differences between let, const, and var in JavaScript are related to scoping, reassignment, and hoisting. Here’s a breakdown of the key differences:

  1. var: Variables declared with var are function-scoped. They are accessible within the function in which they are declared, or if not declared within a function, they become global variables accessible throughout the entire program.
  2. let and const: Variables declared with let and const are block-scoped. They are limited to the block in which they are declared, such as within a loop or an if statement. They are not accessible outside of the block.
  3. Reassignment:
  4. var and let: Variables declared with var and let can be reassigned with new values.
  5. const: Variables declared with const are constants and cannot be reassigned once they are defined. However, for objects and arrays declared with const, the properties or elements within them can be modified.
  6. Hoisting:
  7. var: Variables declared with var are hoisted to the top of their scope. This means that they can be accessed and used before they are declared. However, their assignment is not hoisted, so they will have an initial value of undefined until assigned a value.
  8. let and const: Variables declared with let and const are also hoisted, but they are not initialized. They cannot be accessed before they are declared, resulting in a ReferenceError.

In summary, let and const provide block scoping, allowing for better control over variable visibility and reducing the likelihood of variable conflicts. const is used for variables that should not be reassigned, while let allows for variable reassignment. var is function-scoped and has hoisting behavior, making it behave differently from let and const.

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What is destructuring in JS?

Destructuring is a feature in JavaScript that allows you to extract values from arrays or objects into distinct variables. It provides a concise and convenient way to unpack values, making it easier to work with complex data structures. Destructuring can be applied to both arrays and objects, and here’s how it works:

1. Array Destructuring:

With array destructuring, you can assign array elements to individual variables by matching their positions.

const numbers = [1, 2, 3];
const [a, b, c] = numbers;

console.log(a); // Output: 1
console.log(b); // Output: 2
console.log(c); // Output: 3

2.Object Destructuring:

• With object destructuring, you can extract values from an object by using their corresponding property names.

 
const person = {
  name: 'John',
  age: 30,
  country: 'USA'
};

const { name, age, country } = person;

console.log(name); // Output: John
console.log(age); // Output: 30
console.log(country); // Output: USA

Destructuring can also be used in function parameters to extract values directly from objects or arrays passed as arguments.

Destructuring is particularly useful when you need to extract specific values from complex data structures quickly.

It promotes cleaner and more readable code, especially when dealing with large objects or arrays.

Additionally, destructuring can provide default values and handle nested structures, allowing for more flexibility in data extraction.

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Conclusion:

Preparing for a JavaScript interview can be intimidating, but with the right knowledge and practice, you can approach it with confidence. By familiarizing yourself with the commonly asked questions outlined in this blog post, you’ll be well-equipped to showcase your understanding of JavaScript’s concepts and principles. Remember, interviewers are