JavaScript (JS) is the most popular lightweight scripting and interpreted programming language. It was developed by Brendan Eich in 1995. It is well-known as a scripting language for web pages, mobile apps, web servers, and many more. It is an important language for aspiring front-end and back-end developers. some job roles demand individuals to be fluent in JavaScript.
Javascript is one of the most popular programming languages in the world currently, and it is basically the language of the web. It is a lightweight, cross-platform, and interpreted programming language, primarily used for web development and backend development. Javascript is a language that is used widely on both the client and server sides of development. Javascript offers a vast standard library that has a wide variety of functions and methods available to help in the process of development, making the entire process easier and hassle-free.
Javascript can be added to our HTML code in mostly 2 ways:
Internal Javascript: Javascript in this case is directly added to the HTML code by adding the JS code inside the <script> tag, which can be placed either inside the <head> or the <body> tags based on the requirement of the user.
External Javascript: In this case, the Javascript code is written inside an external file with the extension file_name.js, and then the file is linked to the HTML file inside the <head> tag.
Features of Javascript
Javascript is a lightweight, cross-platform, and interpreted language.
It was developed with the main intention of DOM manipulation, bringing forth the era of dynamic websites.
Javascript functions are objects and can be passed in other functions as parameters.
Can handle date and time manipulation.
Can perform Form Validation.
A compiler is not needed.
JavaScript Interview Questions
Let’s discuss some common questions that you should prepare for the interviews. These questions will be helpful in clearing the interviews specially for the frontend development role.
1. What are the differences between Java and JavaScript?
JavaScript is a client-side scripting language and Java is object Oriented Programming language. Both of them are totally different from each other.
JavaScript: It is a light-weighted programming language (“scripting language”) for developing interactive web pages. It can insert dynamic text into the HTML elements. JavaScript is also known as the browser’s language.
Java: Java is one of the most popular programming languages. It is an object-oriented programming language and has a virtual machine platform that allows you to create compiled programs that run on nearly every platform. Java promised, “Write Once, Run Anywhere”.
2. What are JavaScript Data Types?
There are three major Data types in JavaScript.
Primitive
- Numbers
- Strings
- Boolean
- Symbol
Trivial
- Undefined
- Null
Composite
- Objects
- Functions
- Arrays
3. Which symbol is used for comments in JavaScript?
Comments prevent the execution of statements. Comments are ignored while the compiler executes the code. There are two type of symbols to represent comments in JavaScript:
Double slash: It is known as a single-line comment.
// Single line comment
Slash with Asterisk: It is known as a multi-line comment.
/*
Multi-line comments
...
*/
4. What would be the result of 3+2+”7″?
Here, 3 and 2 behave like an integer, and “7” behaves like a string. So 3 plus 2 will be 5. Then the output will be 5+”7″ = 57.
5. What is the use of the isNaN function?
The number isNan function determines whether the passed value is NaN (Not a number) and is of the type “Number”. In JavaScript, the value NaN is considered a type of number. It returns true if the argument is not a number, else it returns false.
6. Which is faster in JavaScript and ASP script?
JavaScript is faster compared to ASP Script. JavaScript is a client-side scripting language and does not depend on the server to execute. The ASP script is a server-side scripting language always dependable on the server.
7. What is negative infinity?
The negative infinity is a constant value represents the lowest available value. It means that no other number is lesser than this value. It can be generate using a self-made function or by an arithmetic operation. JavaScript shows the NEGATIVE_INFINITY value as -Infinity.
8. Is it possible to break JavaScript Code into several lines?
Yes, it is possible to break the JavaScript code into several lines in a string statement. It can be broken by using the backslash n ‘\n’.
For example:
console.log("A Online Computer Science Portal\n for Interview goal")let gfg= 10, GFG = 5,
abc =
gfg + GFG;
let gfg= 10, GFG = 5,
abc =
gfg + GFG;
The code-breaking line is avoid by JavaScript which is not preferable.
9. Which company developed JavaScript?
Netscape developed JavaScript and was created by Brenden Eich in the year of 1995.
10. What are undeclared and undefined variables?
- Undefined: It occurs when a variable is declare but not assign any value. Undefined is not a keyword.
- Undeclared: It occurs when we try to access any variable which is not initialize or declare earlier using the var or const keyword. If we use ‘typeof’ operator to get the value of an undeclare variable, we will face the runtime error with the return value as “undefined”. The scope of the undeclare variables is always global.
11. Write a JavaScript code for adding new elements dynamically.
<!DOCTYPE html>
<html lang="en">
<head>
<title>Document</title>
</head>
<body>
<button onclick="create()">
Click Here!
</button>
<script>
function create() {
let abc = document.createElement('abc');
geeks.textContent = "InterviewGoal";
geeks.setAttribute('class', 'note');
document.body.appendChild(abc);
}
</script>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
<title>Document</title>
</head>
<body>
<button onclick="create()">
Click Here!
</button>
<script>
function create() {
let abc = document.createElement('abc');
geeks.textContent = "InterviewGoal";
geeks.setAttribute('class', 'note');
document.body.appendChild(abc);
}
</script>
</body>
</html>
13. What do you mean by NULL in JavaScript?
The NULL value represents that no value or no object. It is known as empty value/object.
14. How to delete property-specific values?
The delete keyword deletes the whole property and all the values at once like
let gfg={Course: "DSA", Duration:30};delete gfg.Course;
15. What is a prompt box?
The prompt box is a dialog box with an optional message prompting the user to input some text. It is often used if the user wants to input a value before entering a page. It returns a string containing the text entered by the user, or null.
16. What is the ‘this’ keyword in JavaScript?
Functions in JavaScript are essential objects. Like objects, it can be assign to variables, pass to other functions, and return from functions. And much like objects, they have their own properties. ‘this’ stores the current execution context of the JavaScript program. Thus, when it use inside a function, the value of ‘this’ will change depending on how the function is defined, how it is invoked, and the default execution context.
17. Explain the working of timers in JavaScript. Also elucidate the drawbacks of using the timer, if any.
The timer executes some specific code at a specific time or any small amount of code in repetition to do that you need to use the functions setTimout, setInterval, and clearInterval. If the JavaScript code sets the timer to 2 minutes and when the times are up then the page displays an alert message “times up”. The setTimeout() method calls a function or evaluates an expression after a specified number of milliseconds.
18. What is the difference between ViewState and SessionState?
ViewState: It is specific to a single page in a session.
SessionState: It is user specific that can access all the data on the web pages.
19. How to submit a form using JavaScript?
You can use document.form[0].submit() method to submit the form in JavaScript.
20. Does JavaScript support automatic type conversion?
Yes, JavaScript supports automatic type conversion.
JavaScript Intermediate Interview Questions
21. What are all the looping structures in JavaScript ?
while loop: A while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. The while loop can be thought of as a repeating if statement.
for loop: A for loop provides a concise way of writing the loop structure. Unlike a while loop, for statement consumes the initialization, condition and increment/decrement in one line thereby providing a shorter, easy to debug structure of looping.
do while: A do-while loop is similar to while loop with the only difference that it checks the condition after executing the statements, and therefore is an example of Exit Control Loop.
22. How can the style/class of an element be changed?
To change the style/class of an element there are two possible ways. We use document.getElementByID method
document.getElementById("myText").style.fontSize = "16px;document.getElementById("myText").className = "class";
23. Explain how to read and write a file using JavaScript?
The readFile() functions is used for reading operation.
readFile( Path, Options, Callback)
The writeFile() functions is used for writing operation.
writeFile( Path, Data, Callback)
24. What is called Variable typing in JavaScript ?
The variable typing is the type of variable used to store a number and using that same variable to assign a “string”.
Geeks = 42;Geeks = "GeeksforGeeks";
25. How to convert the string of any base to integer in JavaScript?
In JavaScript, parseInt() function is used to convert the string to an integer. This function returns an integer of base which is specified in second argument of parseInt() function. The parseInt() function returns Nan (not a number) when the string doesn’t contain number.
26. Explain how to detect the operating system on the client machine?
To detect the operating system on the client machine, one can simply use navigator.appVersion or navigator.userAgent property. The Navigator appVersion property is a read-only property and it returns the string that represents the version information of the browser.
27. What are the types of Pop up boxes available in JavaScript?
There are three types of pop boxes available in JavaScript.
- Alert
- Confirm
- Prompt
28. What is the difference between an alert box and a confirmation box?
An alert box will display only one button which is the OK button. It is used to inform the user about the agreement has to agree. But a Confirmation box displays two buttons OK and cancel, where the user can decide to agree or not.
29. What is the disadvantage of using innerHTML in JavaScript?
There are lots of disadvantages of using the innerHTML in JavaScript as the content will replace everywhere. If you use += like “innerHTML = innerHTML + ‘html’” still the old content is replaced by HTML. It preserves event handlers attached to any DOM elements.
30. What is the use of void(0) ?
The void(0) is used to call another method without refreshing the page during the calling time parameter “zero” will be passed.
source :[Internet]
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