Javascript Interview Questions with answers

33. Temporal Dead Zone (TDZ): In JavaScript, the Temporal Dead Zone is a term used to describe the time from the start of execution until the point where variables declared with let or const are in scope and can be accessed within their containing block. Variables declared with var are hoisted to the top of their scope but their values are undefined until they are actually assigned. The TDZ is not present for var declarations, meaning you can reference variables declared with var before declaration without encountering a ReferenceError, which can lead to subtle bugs.

34. let vs var: Both let and var are used to declare variables in JavaScript, but they have different scopes and behaviors. A let declaration creates a block-scoped variable, meaning the variable is only accessible within the block (denoted by curly braces) it was defined in. A var declaration creates a function-scoped or globally-scoped variable (depending on where it's declared). The key differences are:

• Block scope vs. Function/Global scope
• Temporal Dead Zone for let and not for var
• let cannot be redeclared in the same scope, while var can be.

35. String Concatenation and Arithmetic puzzle: In JavaScript, strings and numbers are different types, so you cannot directly concatenate a string with a number without converting the number to a string first. Here's an example of how you might solve this puzzle:

let greeting = "Hello, ";
let count = 5;
greeting += (count + 1); // Converts count to a string and concatenates
console.log(greeting); // Outputs: "Hello, 6"

In the above code, count is converted to a string "6" and concatenated with the existing string greeting.

36. What is class in ES6?: In ECMAScript 2015 (ES6), the class keyword provides a more conventional and clean syntax for defining object types (which are still prototypical objects under the hood) and their methods. A class declaration groups related code together, improves readability, and makes inherance between user-defined types easier to write and understand.

37. So with class Keyword does it imply JavaScript is a OOP language?: Yes, JavaScript has always been an object-oriented programming (OOP) language. However, prior to ES6, JavaScript used prototypal inheritance as its object creation mechanism. The class syntax introduced in ES6 simply provides a more familiar interface for developers accustomed to classical OOP languages like Java or C++. Under the hood, JavaScript still relies on its prototypal inheritance model.

38. Differentiate between class and normal function?: The main differences between a class and a function in ES6 are:

• Syntax: A class provides a more structured way to create an object, with properties and methods grouped together. A function, on the other hand, is a standalone block of code that can be invoked to perform actions or return values.
• Inheritance: Classes support classical inheritance patterns, making it easier to understand where features are coming from. Functions define their own scope and create new objects based on their prototype.
• Syntax Sugar: The class syntax is syntactic sugar over the Object.create() method and Function.prototype.bind(). Under the hood, a class-defined object is still an instance of an object with its methods defined as properties.

39. What is an Arrow function?: An arrow function is a more concise way to write a function in ES6. It's syntax shorthand that can make code more succinct and easier to read, especially for functions with a single expression. Arrow functions capture context (this) lexically, which means they do not have their own this context, unlike traditional function expressions.

40. Why do we use Arrow functions?: We use arrow functions for several reasons:

• Conciseness: They provide a shorter syntax compared to regular function expressions, especially when used as callbacks or inline functions.
• Lexical this: They inherit the this value from the parent scope at definition time, which avoids unexpected context issues in certain use cases, like inside array methods.
• Immutability: They do not have their own this, so they cannot be bound to an object and thus do not change the value of this.

48. Concurrency vs Parallelism: Concurrency and parallelism are often used interchangeably, but they have distinct meanings:

• Concurrency is about handling multiple operations within a program without waiting for any single operation to complete. It involves scheduling tasks such that executions of concurrent tasks have some overlap (interleaving) and might run on the same or different processors. Concurrency focuses on dealing with time, especially when processes are waiting for I/O operations to complete.
• Parallelism is about performing multiple operations at the exact same time, hence in true parallel. Parallelism typically requires multiple processing units (such as multiple CPU cores) to execute simultaneously.

49. Asynch vs Threads: Asynchronous programming and multi-threading are two different approaches to handling concurrency:

• Asynchronous programming is a programming technique that allows for non-blocking execution of code. It's useful for I/O-bound or high-level structured networking systems. JavaScript uses callbacks, promises, and async/await syntax for asynchronous operations.
• Multi-threading involves having multiple threads of execution running in parallel within the same process space. Each thread can execute independently of others, with its own stack and program counter, but shares the same memory and resources. This is typically used in languages that support multi-threading out of the box, like Java or C#.

50. WebWorker vs Promises: A WebWorker is a lightweight thread for performing long or expensive calculations away from the main execution thread (the UI thread) and without blocking JavaScript's event loop in web applications. A Promise, on the other hand, represents the eventual completion (or failure) of an asynchronous operation, and its resulting value. The key differences are:

• Environment: WebWorkers run in a separate thread within the browser environment, while Promises work within the single-threaded JavaScript runtime model.
• Usage: Use a WebWorker for CPU-intensive tasks that should not block the main thread (e.g., image processing, video decoding). Use Promises for handling asynchronous operations like network requests, file reading/writing, or any asynchronous task where you want to handle the result later.

51. WebWorker vs Promises: A WebWorker is suitable for tasks that are computationally heavy and do not require UI updates until they complete, as it runs in a separate thread and thus does not block the main thread. Promises, on the other hand, are used to handle asynchronous operations within a single thread, allowing for cleaner and more maintainable asynchronous code through their then() and catch() methods (or async/await syntax). The choice between a WebWorker and a Promise depends on the context of the task at hand and whether it needs to be run concurrently with other tasks or not.