JavaScript Related Questions & Answers
1.What is
JavaScript?
JavaScript
is a client-side as well as server side scripting language that can be inserted
into HTML pages and is understood by web browsers. JavaScript is also an Object
based Programming language
2.Enumerate
the differences between Java and JavaScript?
Java
is a complete programming language. In contrast, JavaScript is a coded program
that can be introduced to HTML pages. These two languages are not at all
inter-dependent and are designed for the different intent. Java is an
object – oriented programming (OOPS) or structured programming language like
C++ or C whereas JavaScript is a client-side scripting language.
3.What
are JavaScript Data Types?
Following
are the JavaScript Data types:
·
Number
·
String
·
Boolean
·
Null
·
Object
·
Undefined
·
Array
4.What
is the use of isNaN function?
isNaN
function returns true if the argument is not a number otherwise it is false.
5.Between
JavaScript and an ASP script, which is faster?
JavaScript
is faster. JavaScript is a client-side language and thus it does not need the
assistance of the web server to execute. On the other hand, ASP is a
server-side language and hence is always slower than JavaScript.
Javascript now is also a server side language (nodejs).
6.Which
company developed JavaScript?
Netscape
is the software company who developed JavaScript.
7.What
are undeclared and undefined variables?
Undeclared
variables are those that do not exist in a program and are not declared. If the
program tries to read the value of an undeclared variable, then a runtime error
is encountered.
Undefined
variables are those that are declared in the program but have not been given
any value. If the program tries to read the value of an undefined variable, an
undefined value is returned.
8.What
is callback?
A
callback is a plain JavaScript function passed to some method as an argument or
option. Some callbacks are just events, called to give the user a chance to
react when a certain state is triggered.
9.How
to create an object?
An
object in JavaScript can be created using two was:
New Key word:
To create a student object from the above student class we can call the Student function using new keyword.
var student1 = new Student(‘santosh’,2)
New Key word:
To create a student object from the above student class we can call the Student function using new keyword.
var student1 = new Student(‘santosh’,2)
Anonymous Object:
Anonymous objects can be created using pair of curls’ braces containing property name and value pairs.
Var rose = {‘color’: ‘red’}
Anonymous objects can be created using pair of curls’ braces containing property name and value pairs.
Var rose = {‘color’: ‘red’}
10.
What are global variables? How are these variable declared and what are the
problems associated with using them?
Global
variables are those that are available throughout the length of the code, that
is, these have no scope. The var keyword is used to declare a local variable or
object. If the var keyword is omitted, a global variable is declared.
Example:
//
Declare a global globalVariable = “Test”;
The
problems that are faced by using global variables are the clash of variable
names of local and global scope. Also, it is difficult to debug and test the
code that relies on global variables.
11. What is a
prompt box?
A
prompt box is a box which allows the user to enter input by providing a text
box. Label and box will be provided to enter the text or number.
12.
What is ‘this’ keyword in JavaScript?
‘This’
keyword refers to the object from where it was called.
13.
Explain the working of timers in JavaScript? Also elucidate the drawbacks of
using the timer, if any?
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.
14.
Which symbol is used for comments in Javascript?
//
for Single line comments and
/*
Multi
Line
Comment
*/
15.
What is === operator?
===
is called as strict equality operator which returns true when the two operands
are having the same value without any type conversion.
16.
Explain how can you submit a form using JavaScript?
To
submit a form using JavaScript use document.form[0].submit();
document.form[0].submit();
17.
How can the style/class of an element be changed?
It
can be done in the following way:
document.getElementById(“myText”).style.fontSize
= “20″;
or
document.getElementById(“myText”).className
= “anyclass”;
18.
Explain the difference between “==” and “===”?
“==”
checks only for equality in value whereas “===” is a stricter equality test and
returns false if either the value or the type of the two variables are
different.
19.
What would be the result of 3+2+”7″?
Since
3 and 2 are integers, they will be added numerically. And since 7 is a string,
its concatenation will be done. So the result would be 57.
20.
Explain how to detect the operating system on the client machine?
In
order to detect the operating system on the client machine, the
navigator.appVersion string (property) should be used.
21. What do
mean by NULL in Javascript?
The
NULL value is used to represent no value or no object. It implies no
object or null string, no valid boolean value, no number and no array object.
22.
What is the function of delete operator?
The
functionality of delete operator is used to delete all variables and objects in
a program but it cannot delete variables declared with VAR keyword.
23.
What is an undefined value in JavaScript?
Undefined
value means the
·
Variable used in the code doesn’t exist
·
Variable is not assigned to any value
·
Property doesn’t exist
24.
Explain what is pop()method in JavaScript?
The
pop() method is similar as the shift() method but the difference is that the
Shift method works at the start of the array. Also the pop() method take
the last element off of the given array and returns it. The array on which is
called is then altered.
Example:
var cloths = [“Shirt”, “Pant”, “TShirt”];
cloths.pop();
//Now cloth becomes Shirt,Pant
Example:
var cloths = [“Shirt”, “Pant”, “TShirt”];
cloths.pop();
//Now cloth becomes Shirt,Pant
25.
What is break and continue statements?
Break
statement exits from the current loop.
Continue
statement continues with next statement of the loop.
26.
What is the use of type of operator?
‘Typeof’
is an operator which is used to return a string description of the type of a
variable.
27.
What is the use of Push method in JavaScript?
The
push method is used to add or append one or more elements to the end of an
Array. Using this method, we can append multiple elements by passing multiple
arguments.
28.
What is unshift method in JavaScript?
Unshift
method is like push method which works at the beginning of the array.
This method is used to prepend one or more elements to the beginning of the
array.
29.
What is the ‘Strict’ mode in JavaScript and how can it be enabled?
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.
30.
What is the way to get the status of a CheckBox?
The
status can be acquired as follows –
alert(document.getElementById(‘checkbox1’).checked);
If
the CheckBox will be checked, this alert will return TRUE.
31. Describe
the properties of an anonymous function in JavaScript?
A
function that is declared without any named identifier is known as an anonymous
function. In general, an anonymous function is inaccessible after its
declaration.
32.
Is JavaScript case sensitive? Give an example?
Yes, JavaScript
is case sensitive. For example, a function parseInt is not same as the function
Parseint.
33.
What boolean operators can be used in JavaScript?
The
‘And’ Operator (&&), ‘Or’ Operator (||) and the
‘Not’ Operator (!) can be used in JavaScript.
*Operators
are without the parenthesis.
34.
What is the role of break and continue statements?
Break
statement is used to come out of the current loop while the continue statement
continues the current loop with a new recurrence.
35.
How are DOM utilized in JavaScript?
DOM
stands for Document Object Model and is responsible for how various objects in
a document interact with each other. DOM is required for developing web pages,
which includes objects like paragraph, links, etc. These objects can be
operated to include actions like add or delete. DOM is also required to add
extra capabilities to a web page. On top of that, the use of API gives an
advantage over other existing models.
36.
Explain the unshift() method ?
This
method is functional at the starting of the array, unlike the push(). It adds
the desired number of elements to the top of an array.
37.
How to use external JavaScript file?
I
am assuming that js file name is message.js, place the following script tag
inside the head tag.
<script type=”text/javascript” src=”message.js”></script>
38.
What is DOM? What is the use of document object?
DOM stands
for Document
Object Model. A document object represent the html document. It can
be used to access and change the content of html.
39.
What is the use of window object?
The
window object is automatically created by the browser that represents a window
of a browser.
It
is used to display the popup dialog box such as alert dialog box, confirm
dialog box, input dialog box etc.
40.
What is the use of history object?
The
history object of browser can be used to switch to history pages such as back
and forward from current page or another page. There are three methods of
history object.
1.
back()
2.
forward()
3.
go(number): number may be positive for forward, negative for
backward.
41. How to
write comment in JavaScript?
There are two types of comments
in JavaScript.
1.
Single Line Comment: It is represented by // (double forward
slash)
2.
Multi Line Comment: It is represented by slash with asterisk symbol
as /* write comment here */
42. What is the difference between == and
===?
The == operator checks equality
only whereas === checks equality and data type i.e. value must be of same type.
43. How to write html code dynamically using
JavaScript?
The innerHTML property is used to
write the HTML code using JavaScript dynamically. Let’s see a simple example:
document.getElementById(‘mylocation’).innerHTML=”<h2>This is heading using JavaScript</h2>”;
44. How to write normal text code using
JavaScript dynamically?
The innerText property is used to
write the simple text using JavaScript dynamically. Let’s see a simple example:
document.getElementById(‘mylocation’).innerText=”This is text using JavaScript”;
45. How to create objects in JavaScript?
There are 3 ways to create object
in JavaScript.
1.
By object literal
2.
By creating instance of Object
3.
By Object Constructor
Let’s see a simple code to create
object using object literal.
emp={id:102,name:”Rahul Kumar”,salary:50000}
46. What does the isNaN() function?
The isNaN() function returns true
if the variable value is not a number.
47. What is the output of 10+20+”30″ in
JavaScript?
3030 because 10+20 will be 30. If
there is numeric value before and after +, it is treated is binary +
(arithmetic operator).
48. What is the output of “10”+20+30 in
JavaScript?
102030 because after a string all
the + will be treated as string concatenation operator (not binary +).
49. Difference between Client side JavaScript
and Server side JavaScript?
Client side
JavaScript comprises the basic language and predefined objects which are
relevant to running java script in a browser. The client side JavaScript is
embedded directly by in the HTML pages. This script is interpreted by the
browser at run time.
Server side JavaScript also
resembles like client side java script. It has relevant java script which is to
run in a server. The server side JavaScript are deployed only after
compilation.
50. What is the real name of JavaScript?
The original name was Mocha,
a name chosen by Marc Andreessen, founder of Netscape. In September of 1995,
the name was changed to LiveScript. In December 1995, after receiving a
trademark license from Sun, the name JavaScript was adopted.
51. What is
the difference between undefined value and null value?
Undefined value: A
value that is not defined and has no keyword is known as undefined value. For
example:
var number;//Here, number has undefined value.
Null value: A
value that is explicitly specified by the keyword “null” is known as null
value. For example:
var str=null;//Here, str has a null value.
52.
How to submit a form using JavaScript by clicking a link?
Let’s
see the JavaScript code to submit form on clicking the link.
<form name=”myform” action=”index.php”>
Search: <input type=’text’ name=’query’ />
<a href=”javascript: submitform()”>Search</a>
</form>
<script type=”text/javascript”>
function submitform()
{
document.myform.submit();
}
</script>
53.
How to change the background color of HTML document using JavaScript?
<script type=”text/javascript”>
document.body.bgColor=”pink”;
</script>
54.
How can you read properties of an Object in JavaScript?
You
can write and read properties of an object using the dot notation as follows −
Getting
object properties
emp.name // ==> Zara
emp.age // ==> 10
Setting
object properties
emp.name = “Daisy” //
<== Daisy
emp.age =
20 // <== 20
55.
How can you create an Array in JavaScript?
You
can define arrays using the array literal as follows −
var x =
[];
var y =
[1, 2, 3, 4, 5];
56.
How to read elements of an array in JavaScript?
An
array has a length property that is useful for iteration. We can read elements
of an array as follows −
var x =
[1, 2, 3, 4, 5];
for (var
i = 0; i < x.length; i++) {
// Do something with x[i]
}
57.
What is a named function in JavaScript? How to define a named function?
A
named function has a name when it is defined. A named function can be defined
using function keyword as follows −
function
named(){
// do some stuff here
}
58.
How to define a anonymous function?
An
anonymous function can be defined in similar way as a normal function but it
would not have any name.
59.
Can you assign a anonymous function to a variable?
Yes!
An anonymous function can be assigned to a variable.
60.
What is the purpose of ‘this’ operator in JavaScript?
JavaScript
famous keyword this always refers to the current context.
61. What are
the valid scopes of a variable in JavaScript?
The
scope of a variable is the region of your program in which it is defined.
JavaScript variable will have only two scopes.
Global Variables − A
global variable has global scope which means it is visible everywhere in your
JavaScript code.
Local Variables − A
local variable will be visible only within a function where it is defined.
Function parameters are always local to that function.
62.
What is callback?
A
callback is a plain JavaScript function passed to some method as an argument or
option. Some callbacks are just events, called to give the user a chance to
react when a certain state is triggered.
63.
Which built-in method returns the character at the specified index?
charAt()
method returns the character at the specified index.
64.
How typeof operator works?
The
typeof is a unary operator that is placed before its single operand, which can
be of any type. Its value is a string indicating the data type of the operand.
The
typeof operator evaluates to “number”, “string”, or “boolean” if its operand is
a number, string, or boolean value and returns true or false based on the
evaluation.
65.
How to redirect a url using JavaScript?
This
is very simple to do a page redirect using JavaScript at client side. To
redirect your site visitors to a new page, you just need to add a line in your
head section as follows −
<head>
<script
type=”text/javascript”>
<!–
window.location=”http://www.newlocation.com”;
//–>
</script>
</head>
66.
What is Date object in JavaScript?
The
Date object is a datatype built into the JavaScript language. Date objects are
created with the new Date( ).
Once
a Date object is created, a number of methods allow you to operate on it. Most
methods simply allow you to get and set the year, month, day, hour, minute,
second, and millisecond fields of the object, using either local time or UTC
(universal, or GMT) time.
67.
What is Number object in JavaScript?
The
Number object represents numerical date, either integers or floating-point
numbers. In general, you do not need to worry about Number objects because the
browser automatically converts number literals to instances of the number
class.
Syntax
−
Creating
a number object −
var
val = new Number(number);
If
the argument cannot be converted into a number, it returns NaN (Not-a-Number).Enq
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