1. What is JavaScript?
JavaScript is a high-level programming language used to create interactive and dynamic web pages. It is a scripting language that is interpreted by web browsers and allows developers to create dynamic content and perform various tasks on the client-side of a website.
2. What are the features of JavaScript?
Some features of JavaScript are:
3. What is the difference between JavaScript and Java?
JavaScript and Java are two different programming languages. While Java is a general-purpose language used for building desktop and mobile applications, JavaScript is a client-side scripting language used for creating interactive web pages. JavaScript is not related to Java in any way.
4. What is a closure in JavaScript?
A closure is a function that has access to its outer function's variables and parameters, even after the outer function has returned. It allows a function to access and manipulate variables outside its own scope, which is useful for creating private variables and functions.
5. What is the difference between == and === operators in JavaScript?
The == operator compares the values of two operands after converting their types, whereas the === operator compares the values of two operands without converting their types. For example, 5 == "5" returns true because JavaScript converts the string "5" to the number 5, but 5 === "5" returns false because the two operands have different types.
6. What is an event in JavaScript?
A: An event is an action or occurrence that happens on a web page, such as a mouse click, a button press, or a page load. JavaScript can detect these events and respond to them by executing code, such as updating the page content or displaying a message.
7. What is the use of the let keyword in JavaScript?
The let keyword is used to declare block-scoped variables in JavaScript. Block-scoped variables are only accessible within the block of code where they are defined. This is different from var, which declares variables with function scope or global scope.
8. What is the difference between null and undefined in JavaScript?
In JavaScript, null is a value that represents the intentional absence of any object value, whereas undefined is a value that represents an uninitialized, missing, or non-existent value. If a variable is declared but not assigned a value, its value is undefined. If a variable is intentionally set to no value, its value is null.
9. What is a callback function in JavaScript?
A callback function is a function that is passed as an argument to another function and is executed after the first function has completed. It allows asynchronous programming and is commonly used in event handling and AJAX calls.
10. What is the difference between synchronous and asynchronous programming in JavaScript?
In synchronous programming, each line of code is executed one after the other, and the program waits for each line to complete before moving on to the next. In asynchronous programming, multiple lines of code can be executed simultaneously, and the program does not wait for each line to complete before moving on to the next. Asynchronous programming is useful for tasks that involve waiting for external resources, such as network requests or user input.
11. What is the difference between a for loop and a forEach loop in JavaScript?
A for loop and a forEach loop are both used to iterate over an array in JavaScript. However, a for loop provides more flexibility in controlling the iteration process, while a forEach loop is simpler and easier to read. A for loop allows you to specify the starting and ending points of the loop, as well as the increment or decrement value, whereas a forEach loop simply executes a function for each element in the array.
12. What is hoisting in JavaScript?
Hoisting is a JavaScript mechanism that allows variables and function declarations to be moved to the top of their respective scopes, regardless of where they are defined in the code. This means that a variable or function can be used before it is declared, although it is recommended to declare them before using them to avoid confusion and potential errors.
13. What is the use of the this keyword in JavaScript?
The this keyword refers to the current object in a JavaScript function or method. It can be used to access and manipulate properties and methods of the current object, or to pass the current object as a parameter to another function.
14. What is a promise in JavaScript?
A: A promise is an object that represents the eventual completion or failure of an asynchronous operation and its resulting value. It allows you to write asynchronous code in a more readable and manageable way, by using the then() and catch() methods to handle the resolved or rejected value of the promise.
15. What is the difference between a regular function and an arrow function in JavaScript?
The main difference between a regular function and an arrow function in JavaScript is the way they handle the this keyword. In a regular function, the value of this is determined by how the function is called, whereas in an arrow function, the value of this is inherited from the surrounding scope. Arrow functions are also more concise and easier to read, but they cannot be used as constructors and do not have their own this, arguments, and super keywords.
16. What is the use of the bind() method in JavaScript?
The bind() method is used to create a new function that has the same body as the original function, but with a specific value for the this keyword. It allows you to bind a specific object to a function and use that object as the context for the function.
17. What is the difference between var, let, and const in JavaScript?
var declares variables with function scope or global scope, let declares block-scoped variables, and const declares block-scoped variables that cannot be reassigned. The value of a variable declared with var can be changed throughout the program, whereas the value of a variable declared with const cannot be changed once it is assigned. The value of a variable declared with let can be changed within the block of code where it is defined.
18. What is the difference between a local and a global variable in JavaScript?
A local variable is a variable that is declared inside a function or a block of code, and is only accessible within that function or block. A global variable is a variable that is declared outside of any function or block, and is accessible throughout the entire program. It is generally recommended to use local variables whenever possible to avoid naming conflicts and improve code readability and maintainability.
19. What is the difference between an alert box, a confirm box, and a prompt box in JavaScript?
An alert box is a dialog box that displays a message to the user and requires them to click "OK" to dismiss it. A confirm box is a dialog box that displays a message to the user and requires them to click "OK" or "Cancel" to proceed. A prompt box is a dialog box that displays a message to the user and requires them to enter a value before proceeding.
20. What is the difference between the document and window objects in JavaScript?
The document object represents the web page loaded in the browser window, and provides methods and properties for accessing and manipulating the contents of the page. The window object represents the browser window itself, and provides methods and properties for controlling the behavior of the browser, such as opening and closing windows, and setting the size and position of the window.