Javascript

Question 1

What are the 4 ways we can write messages in Javascript?

Answer 1

1) Writing into an HTML element, using innerHTML.
2) Writing into the HTML output using document.write().
3) Writing into an alert box, using window.alert().
4) Writing into the browser console, using console.log()

Question 2

How to get an element by ID?

Answer 2

document.getElementById(id)

Question 3

What is the disadvantage of document.write()?

Answer 3

Never call document.write after the document has finished loading. It will overwrite the whole document. The document.write() method should only be used for testing.

Question 4

Find the type of a JavaScript variable with…

Answer 4

typeof

Question 5

Example o JavaScript object

Answer 5

var person = {firstName:”John”, lastName:”Doe”, age:50, eyeColor:”blue”};

Question 6

How can I set a value to empty in JS?

Answer 6

By setting the value to undefined. car = undefined; // Value is undefined, type is undefined; or by suing NULL. person = null; // Now value is null, but type is still an object

Question 7

Example of complex object

Answer 7

var person = {
  firstName: "John",
  lastName : "Doe",
  id       : 5566,
  fullName : function() {
    return this.firstName + " " + this.lastName;
  }
};

Question 8

Example of event handling

Question 9

Common JS events

Answer 9

onchange: An HTML element has been changed
onclick: The user clicks an HTML element
onmouseover: The user moves the mouse over an HTML element
onmouseout The user moves the mouse away from an HTML element
onkeydown: The user pushes a keyboard key
onload: The browser has finished loading the page

Question 10

Comparing two JavaScript objects will always return false

Answer 10

because they are different objects in memory

Question 11

What does the === do?

Answer 11

operator expects equality in both type and value

Question 12

What does the method lastIndexOf() do?

Answer 12

The lastIndexOf() method returns the index of the last occurrence of a specified text in a string

Question 13

What does the method search() do?

Answer 13

The search() method searches a string for a specified value and returns the position of the match

Question 14

What is the difference between substring() and slice() methods?

Answer 14

substring() is similar to slice(). The difference is that substring() cannot accept negative indexes

Question 15

How to replace values (case insensitive mode)?

Answer 15

To replace case insensitive, use a regular expression with an /i flag (insensitive):
str = "Please visit Microsoft!";
var n = str.replace(/MICROSOFT/i, "W3Schools");

Question 16

How to replace all matches?

Answer 16

use a regular expression with a /g flag (global match).
str = "Please visit Microsoft and Microsoft!";
var n = str.replace(/Microsoft/g, "W3Schools");

Question 17

How to concatenate strings?

Answer 17

concat()
var text1 = "Hello";
var text2 = "World";
var text3 = text1.concat(" ", text2);

Question 18

JS rounding method

Answer 18

var x = 9.656;

x. toFixed(0); // returns 10
x. toFixed(2); // returns 9.66
x. toFixed(4); // returns 9.6560
x. toFixed(6); // returns 9.656000

Question 19

Example of forEach loop

Answer 19

var fruits, text;
fruits = [“Banana”, “Orange”, “Apple”, “Mango”];

text = “<ul>”;
fruits.forEach(myFunction);
text += “</ul>”;

function myFunction(value) {
  text += "<li>" + value + "</li>";
}

Question 20

shift()

Answer 20

removes the first array element and “shifts” all other elements to a lower index

Question 21

unshift()

Answer 21

adds a new element to an array (at the beginning), and “unshifts” older elements

Question 22

splice()

Answer 22

used to add new items to an array
var fruits = [“Banana”, “Orange”, “Apple”, “Mango”];
fruits.splice(2, 0, “Lemon”, “Kiwi”);

Question 23

splice() with no extra parameters

Answer 23

remove elements without leaving “holes” in the array.
var fruits = [“Banana”, “Orange”, “Apple”, “Mango”];
fruits.splice(0, 1); // Removes the first element of fruits

Question 24

JS sorting

Answer 24

var fruits = [“Banana”, “Orange”, “Apple”, “Mango”];

fruits. sort(); // First sort the elements of fruits
fruits. reverse(); // Then reverse the order of the elements

Question 25

custom sorting

Answer 25

cars.sort(function(a, b){return a.year - b.year});

Question 26

map()

Answer 26

var numbers1 = [45, 4, 9, 16, 25];
var numbers2 = numbers1.map(myFunction);

function myFunction(value, index, array) {
  return value * 2;
}

Question 27

filter()

Answer 27

var numbers = [45, 4, 9, 16, 25];
var over18 = numbers.filter(myFunction);

function myFunction(value, index, array) {
  return value > 18;
}

Question 28

Write the code for adding new elements dynamically

Answer 28

t1

	function addNode() { var newP = document.createElement("p"); 
	var textNode = document.createTextNode(" This is a new text node"); 
	newP.appendChild(textNode); document.getElementById("firstP").appendChild(newP); } 

<p>firstP</p>

<p>
</p>

Question 29

What are JavaScript Data Types?

Answer 29

Number
String
Boolean
Object
Undefined

Question 30

Explain the working of timers in JavaScript? Also elucidate the drawbacks of using the timer, if any?

Answer 30

Timers are used to execute a piece of code at a set time or also to repeat the code in a given interval of time. This is done by using the functions setTimeout, setInterval and clearInterval.

The setTimeout(function, delay) function is used to start a timer that calls a particular function after the mentioned delay. The setInterval(function, delay) function is used to repeatedly execute the given function in the mentioned delay and only halts when cancelled. The clearInterval(id) function instructs the timer to stop.

Timers are operated within a single thread, and thus events might queue up, waiting to be executed.

Question 31

What is the difference between ViewState and SessionState

Answer 31

‘ViewState’ is specific to a page in a session.

‘SessionState’ is specific to user specific data that can be accessed across all pages in the web application.

Question 32

how can you submit a form using JavaScript?

Answer 32

document.form[0].submit();

Question 33

How can the style/class of an element be changed?

Answer 33

document.getElementById(“myText”).style.fontSize = “20”;
or
document.getElementById(“myText”).className = “anyclass”;

Question 34

Explain how to detect the operating system on the client machine?

Answer 34

In order to detect the operating system on the client machine, the navigator.platform string (property) should be used.
or
The ‘Navigator.appversion’ is used to find the name of the operating system in the client machine.

Question 35

What are all the types of Pop up boxes available in JavaScript?

Answer 35

Alert
Confirm and
Prompt

Question 36

What is the use of Void(0)?

Answer 36

Void(0) is used to prevent the page from refreshing and parameter “zero” is passed while calling.

Void(0) is used to call another method without refreshing the page.

Question 37

How can a page be forced to load another page in JavaScript?

Answer 37

<!-- location.href="http://newhost/newpath/newfile.html"; //-->

Question 38

What is the difference between an alert box and a confirmation box?

Answer 38

An alert box displays only one button which is the OK button.

But a Confirmation box displays two buttons namely OK and cancel.

Question 39

What are JavaScript Cookies?

Answer 39

Cookies are the small test files stored in a computer and it gets created when the user visits the websites to store information that they need. Example could be User Name details and shopping cart information from the previous visits.

Question 40

Mention what is the disadvantage of using innerHTML in JavaScript?

Answer 40

Content is replaced everywhere
We cannot use like “appending to innerHTML”
Even if you use +=like “innerHTML = innerHTML + ‘html’” still the old content is replaced by html
The entire innerHTML content is re-parsed and build into elements, therefore its much slower
The innerHTML does not provide validation and therefore we can potentially insert valid and broken HTML in the document and break it

Question 41

Which keyword is used to print the text in the screen?

Answer 41

document.write(“Welcome”) is used to print the text – Welcome in the screen.

Question 42

What is the ‘Strict’ mode in JavaScript and how can it be enabled?

Answer 42

Strict Mode adds certain compulsions to JavaScript. Under the strict mode, JavaScript shows errors for a piece of codes, which did not show an error before, but might be problematic and potentially unsafe. Strict mode also solves some mistakes that hamper the JavaScript engines to work efficiently.

Strict mode can be enabled by adding the string literal “use strict” above the file. This can be illustrated by the given example:
function myfunction() {
“use strict”;
var v = “This is a strict mode function”;
}

Question 43

What is the way to get the status of a CheckBox?

Answer 43

alert(document.getElementById(‘checkbox1’).checked);

Question 44

Explain window.onload and onDocumentReady?

Answer 44

The onload function is not run until all the information on the page is loaded. This leads to a substantial delay before any code is executed.

onDocumentReady loads the code just after the DOM is loaded. This allows early manipulation of the code.

Question 45

How will you explain closures in JavaScript? When are they used?

Answer 45

Return a function and save it in a variable then call te variable

Question 46

How can a particular frame be targeted, from a hyperlink, in JavaScript?

Answer 46

<a>>New Page</a>