Javascript And Jquery Interview Questions
Commonly Asked Javascript and JQuery Interview Questions
JavaScript and jQuery are two popular programming languages used for developing interactive and dynamic web applications. During an interview, one can expect questions related to basic concepts of JavaScript such as data types, functions, objects, event handling, and debugging techniques. Questions related to jQuery might focus on its features, advantages, and common methods used for front-end development. Employers may also inquire about the differences between JavaScript and jQuery, the role of AJAX and DOM manipulation, and how to incorporate jQuery into web projects. Strong knowledge and understanding of these concepts can help in acing JavaScript and jQuery interviews.
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1 - What is the difference between Javascript and JQuery?
Javascript is a programming language used to add interactivity to web pages. It is used to manipulate the content and behavior of HTML elements on a webpage. Whereas, JQuery is a library built on top of Javascript, making it easier to manipulate HTML elements, handle events and create animations. It simplifies common tasks in Javascript and helps to improve the overall user experience.
2) What is a closure in Javascript?
A closure is a function that has access to the variables in its outer scope, even after the outer function has returned. In simpler terms, it is an inner function that has access to the parent function's variables. Closures are commonly used to create private variables and functions in Javascript.
3) What is the Document Object Model (DOM)?
The Document Object Model (DOM) is a platform and language independent interface that allows programs and scripts to access and update the content, structure, and style of a webpage. It provides a tree like representation of the HTML elements on a webpage, with each element being a node in the tree.
4) How do you select elements in JQuery?
JQuery uses CSS selectors to select elements on a webpage. It uses the $() function to select elements based on their id, class, tag name, attribute, etc. For example, $('#myId') will select the element with the id ‘myId’, and $('.myClass') will select all elements with the class ‘myClass’.
5) What is event bubbling in Javascript?
Event bubbling is a process in which an event is handled by its parent elements first and then propagated to its child elements. When an event is triggered on a child element, it will bubble up through its parent elements until it reaches the document object, and then to the window object.
6) How do you prevent the default behavior of an event in Javascript?
To prevent the default behavior of an event, we can use the event.preventDefault() method. This method stops the default action associated with the event from occurring, such as submitting a form or following a link.
7) What is the difference between == and === in Javascript?
The == operator compares two values and returns true if they are equal, but it does not check the data type. On the other hand, the === operator not only compares the values but also checks if they are of the same data type. For example, 1 == “1” would return true, but 1 === “1” would return false.
8) How do you debug a Javascript code?
There are various ways to debug a Javascript code, such as using the console.log() method to log values and messages to the console, using a browser's built in developer tools, setting breakpoints in the code, and using a debugger statement within the code.
9) What is the difference between let, var, and const in Javascript?
let and var are used to declare variables, while const is used to declare constants. Variables declared with let and var can be reassigned new values, while the value of a constant cannot be changed once it is assigned. Also, let and const have block level scope, while var has function level scope.
10) What is the difference between synchronous and asynchronous code?
Synchronous code is executed in a sequential order, meaning one statement is executed at a time, and the program waits for each statement to finish before moving on to the next one. On the other hand, asynchronous code allows multiple tasks to be executed simultaneously, and the program does not wait for each task to finish before moving on to the next one.
11) Name some methods for iterating over an array in JavaScript.
Some of the methods for iterating over an array in JavaScript are for loop, forEach, map, filter, reduce, and while loop. Each method has a different purpose, such as filtering, transforming, or reducing the array elements.
12) How do you handle errors in Javascript?
Errors in JavaScript can be handled using try catch blocks. The try block contains the code that we want to execute, and the catch block handles any errors that occur in the try block. We can also use the throw statement to create custom errors and the finally block to execute code after the try catch block, regardless of the result.
13) What is a promise in Javascript?
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A promise in Javascript is an object that represents the eventual completion or failure of an asynchronous operation. It is used to handle asynchronous operations and provides a more structured way of handling callbacks. Promises have three states: pending, fulfilled, and rejected.
14) What is JSON and how do you parse it in Javascript?
JSON (JavaScript Object Notation) is a data format used to send and store data in a structured way. It is based on the JavaScript syntax and is often used to send data between a server and a web application. The JSON.parse() method can be used in Javascript to convert a string of JSON into a Javascript object.
15) How do you add and remove classes in Javascript?
To add a class to an element, we can use the element.classList.add() method. This method takes the name of the class as its parameter and adds it to the element's class attribute. To remove a class, we can use the element.classList.remove() method, which takes the class name as its parameter and removes it from the element's class attribute.
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