10 most asked JavaScript interview questions in 2023!
JavaScript is one of the most popular programming languages in the world, and it’s no surprise that many developers are looking to master it in order to advance their careers. When it comes to interviews, knowing the most frequently asked questions can help you prepare and increase your chances of impressing your interviewer.
Here are ten of the most commonly asked JavaScript interview questions:
- What is the difference between let and var?
The main difference between let and var is that var is function-scoped, while let is block-scoped. This means that variables declared with var can be accessed throughout the entire function, while variables declared with let can only be accessed within the block they are defined in.
2. Explain closure in JavaScript.
Closure in JavaScript refers to the ability of a function to remember and access variables in its parent scope, even after the parent function has returned. Closures are often used to create private variables and methods, and to preserve state across multiple function calls.
3. How do you define a class in JavaScript?
In JavaScript, classes are created using the class keyword, followed by a class name and a set of curly braces. The class definition includes a constructor function, which is used to initialize the class’s properties and methods.
4. What is the difference between == and ===?
The difference between == and === is that == compares values without considering their data types, while === compares both values and data types. This means that if you use ==, JavaScript will try to convert the values to the same data type before making the comparison, while === will not.
5. How does JavaScript handle asynchronous code?
JavaScript uses callbacks, promises, and async/await to handle asynchronous code. Callbacks are functions that are passed as arguments to other functions and are executed when a certain event happens. Promises are objects that represent the eventual completion or failure of an asynchronous operation, and async/await is a more modern way of handling asynchronous code that makes it look more like synchronous code.
6. What is the difference between null and undefined?
Both null and undefined represent non-existent values in JavaScript, but they are used in slightly different ways. Undefined is the default value of uninitialized variables, while null is used to explicitly indicate that a variable has no value.
7. How do you create an object in JavaScript?
Objects in JavaScript are created using the object literal notation ({}), the object constructor (new Object()), or the object.create() method. They can be used to store key-value pairs, and their properties can be accessed using dot notation or square bracket notation.
8. Explain the event loop in JavaScript.
The event loop in JavaScript is a mechanism that allows the execution of code to be managed in a non-blocking way. It constantly checks the message queue and runs any messages that are waiting in the queue, allowing the JavaScript engine to handle multiple tasks at the same time.
9. How do you check if an object is an array?
To check if an object is an array in JavaScript, you can use the Array.isArray() method or the “instanceof” operator. The Array.isArray() method returns true if the object is an array, and false if it is not. The “instanceof” operator checks if an object is an instance of a particular constructor or class.
10. How do you create a promise in JavaScript?
A promise in JavaScript can be created using the Promise constructor. The constructor takes a single argument, which is a function called the "executor." The executor function takes two arguments, a "resolve" function and a "reject" function, which are used to signal the completion or failure of the promise, respectively.
These are some of the most asked JavaScript interview questions and I can asure you if not all most of them will be asked in an interview.
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