JavaScript is a scripting language that enables you to create dynamically updating content, control multimedia, animate images, and pretty much everything else. JavaScript is a programming language used to make web pages interactive. It provides functionality to websites that HTML and CSS can’t provide. JavaScript can be used to make webpages more interactive, dynamic, and functional.
Q 1. What is an event loop in JavaScript?
An event loop is a process that waits for and responds to events or messages in a program. In JavaScript, it is a mechanism by which asynchronous tasks can be scheduled by an event-driven programming language. The event loop allows the program to continuously run and respond to events, such as user input or network requests.
Q 2. What is a closure in JavaScript?
A closure is a feature in JavaScript which allows a function to access variables from its enclosing scope, even after the outer function has returned. Closures are a way to maintain state and are often used in object-oriented programming to create private properties and methods.
Q 3. What is the difference between == and === in JavaScript?
In JavaScript, the == operator checks for value equality, while the === operator checks for both value and type equality. For example, if you compare the number 1 with the string “1”, == will return true, but === will return false.
Q 4. What is the difference between a global variable and a local variable in JavaScript?
A global variable is one that is declared outside of any function, and is available throughout the entire document. A local variable is one that is declared inside of a function, and is only available within that function.
Q 5. What is the purpose of ‘use strict’ in JavaScript?
The purpose of ‘use strict’ in JavaScript is to indicate that the code should be executed in “strict mode”. This means that certain coding practices which are considered bad or potentially dangerous will be disabled, such as implicit globals and other activities that can cause errors or unexpected behavior. Strict mode also helps to prevent accidental or malicious code execution.
Q 6. What are the different data types in JavaScript?
The different data types in JavaScript include:
-String
-Number
-Boolean
-Object
-Null
-Array
-Function
-Symbol
Q 7. What is closure in JavaScript?
Closure in JavaScript is when a function is able to remember and access its lexical scope even when that function is executed outside of its original scope. Closures are created every time a function is declared and can be used to create private variables and functions. Closures are a powerful feature of JavaScript which allow us to create more sophisticated functions and programs.
Q 8. What is the difference between let, var and const in JavaScript?
Let and var are both used for declaring variables in JavaScript. The main difference between the two is that variables declared with let are block scoped, meaning they are only accessible within the block they are declared in, while variables declared with var are function scoped, meaning they are accessible anywhere within the function they are declared.
Const is used to declare a constant variable, which means that its value cannot be changed once it is declared. This is useful for values that you want to remain the same, such as configuration settings.
Q 9. What is the difference between asynchronous and synchronous code in JavaScript?
Synchronous code is code that executes one instruction at a time in the order it is written. Asynchronous code is code that allows the program to move on to other instructions while waiting for a time-consuming task to complete. Asynchronous code can be used to run multiple tasks simultaneously, while synchronous code will execute each task in order.
Q 10. What is the purpose of event loop in JavaScript?
The event loop in JavaScript is used to continuously execute code in response to events, such as user input, network requests, and timer events. The event loop allows the JavaScript code to run in a single thread, making it easier to manage asynchronous tasks. It makes sure that all pending tasks are handled in the correct order and that no tasks are missed.
Q 11. What is the purpose of JSON.parse in JavaScript?
JSON.parse is used to parse a string of JSON data into a JavaScript object. It takes a JSON string and returns a JavaScript object or array containing the data from the string. It is useful for accessing data stored in JSON format.
Q 12. What is the purpose of the spread operator in JavaScript?
The spread operator (…) is used to spread out the elements of an array or object into a list of arguments. It is also used to combine two or more arrays or objects into a single array or object.
Q 13. What is the difference between null and undefined in JavaScript?
The main difference between null and undefined is that null is an assignment value and undefined is a type and indicates that a variable has been declared but has not yet been assigned a value. Null is an object with no value, while undefined is an object with no value assigned to it. Null is an assigned value, while undefined is a type.
Q 14. Explain event bubbling and capture in JavaScript?
Event bubbling is a way of handling events in a hierarchical structure, where an event can bubble up through parent elements. This means that when an event occurs on a certain element, it will bubble up to its parent elements until it is handled. This allows events to propagate from the element where they originated to higher levels in the DOM tree.
Event capture is the opposite of event bubbling. It works in a top-down approach, where the event starts at the root element and is then captured and handled at each level of the DOM tree until it reaches the target element. Event capture allows you to specify that an event should be handled at the exact element where it originated, instead of propagating up to the parent elements.
Q 15. What is an asynchronous callback in JavaScript?
An asynchronous callback in JavaScript is a function that is passed as an argument to another function that is then invoked after the first function has completed its operations. This allows you to run code after an asynchronous operation has finished, without blocking the main thread of execution. Asynchronous callbacks are commonly used in AJAX requests and other asynchronous operations.
Q 16. What is an anonymous function in JavaScript?
An anonymous function in JavaScript is a function that is declared without any named identifier. Anonymous functions are often arguments being passed to higher-order functions, or used for constructing function objects on the fly. They are usually used when the function body is short and can be defined inline.
Q 17. Explain the concept of hoisting in JavaScript?
Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. This means that no matter where functions and variables are declared, they are moved to the top of their scope regardless of whether their scope is global or local. This means that all variables and functions can be accessed and called from anywhere in the code, regardless of where the actual declaration has been made. Hoisting helps to make JavaScript code less error-prone and more readable.
Q 18. How does the ‘this’ keyword work in JavaScript?
The ‘this’ keyword in JavaScript refers to the object that is executing the current function. It can be used to access properties of the object and to call other methods of the object. In most cases, ‘this’ is the object that the function is a method of. However, ‘this’ can be bound to a different value when the function is called, using the ‘call’, ‘apply’, or ‘bind’ methods.
Q 19. What is the purpose of the ‘new’ keyword in JavaScript?
The ‘new’ keyword is used in JavaScript to create a new instance of an object. This keyword creates a new object, and sets the value of ‘this’ to the new created object. It also calls the constructor method of the object, which initializes the new object properties.
Q 20. What is the difference between a variable that is: null, undefined, and undeclared?
Null is a variable that has been intentionally set to no value. It is an assignment value.
Undefined is a variable that has been declared, but has not been assigned a value.
Undeclared is a variable that has not been declared at all, so it does not exist in the current scope.
Q21. what is rxJs?
RxJS (Reactive Extensions for JavaScript) is a library for reactive programming using observables that makes it easier to compose asynchronous or callback-based code. It provides one core type, the Observable, satellite types (Observer, Schedulers, Subjects) and operators inspired by Array#extras (map, filter, reduce, every, etc.) to allow handling asynchronous events as collections.
Q21. What are the primitive and non primitive data types in javaScripts?
Primitive data types are simple data types that are predefined by the programming language and are used to store basic values.
-Number
-String
-Boolean
-Null
-Undefined
-Symbol
Non-primitive data types are more complex data types and are not predefined by the programming language.
-Object
-Function
-Array
-Classes
Q13. What are the polyfills in javascript?
Polyfills are pieces of code that provide functionality to older browsers that do not natively support newer web technologies. Examples of polyfills include web components, HTML5 forms, and ES6 features such as Promises and Map/Set. Polyfills are essential for ensuring that code runs properly on all browsers, regardless of their level of support for modern web technologies.
Q14. what are the http parameters?
HTTP parameters are variables that are passed via a client’s HTTP request, such as a GET or POST, from the browser to the server. These parameters can be used by the server to identify the client, access data, and return specific content. Common HTTP parameters include cookies, query strings, and form data.
Q15. OOPS consepts in javascript?
- Closures
- Prototypal Inheritance
- Modules
- Event Loop
- Promises
- Asynchronous programming
- Hoisting
- Scope
- Context
- Currying
Q16. what is no conflicts in jQuery?
No Conflicts in jQuery is a built-in method that allows multiple versions of jQuery to safely coexist on the same page. It ensures that the variables and functions from one version of jQuery don’t conflict with the variables and functions from another version. This is especially important when working with plugins that are written with a specific version of jQuery in mind.
Q17. What Is Currying in JavaScript?
Currying is a technique in functional programming that allows a function with multiple arguments to be transformed into a sequence of functions with a single argument each.
In JavaScript, currying is typically achieved by creating a function that returns another function, where the returned function takes the first argument and then returns another function that takes the second argument, and so on until all the arguments have been provided.
Q18. What is polyfill in JS.
A polyfill is a piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it.