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JAVASCRIPT interview questions and answers for freshers beginners and experienced pdf free download.
JAVASCRIPT Interview Questions and Answers
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| JAVASCRIPT Interview Questions |
What is JavaScript?
A1: JavaScript is a general-purpose programming language designed to let programmers of all skill levels control the behavior of software objects. The language is used most widely today in Web browsers whose software objects tend to represent a variety of HTML elements in a document and the document itself. But the language can be--and is--used with other kinds of objects in other environments. For example, Adobe Acrobat Forms uses JavaScript as its underlying scripting language to glue together objects that are unique to the forms generated by Adobe Acrobat. Therefore, it is important to distinguish JavaScript, the language, from the objects it can communicate with in any particular environment. When used for Web documents, the scripts go directly inside the HTML documents and are downloaded to the browser with the rest of the HTML tags and content.
A2:JavaScript is a platform-independent,event-driven, interpreted client-side scripting and programming language developed by Netscape Communications Corp. and Sun Microsystems.
How is JavaScript different from Java?
JavaScript was developed by Brendan Eich of Netscape; Java was developed at Sun Microsystems. While the two languages share some common syntax, they were developed independently of each other and for different audiences. Java is a full-fledged programming language tailored for network computing; it includes hundreds of its own objects, including objects for creating user interfaces that appear in Java applets (in Web browsers) or standalone Java applications. In contrast, JavaScript relies on whatever environment it's operating in for the user interface, such as a Web document's form elements.
JavaScript was initially called LiveScript at Netscape while it was under development. A licensing deal between Netscape and Sun at the last minute let Netscape plug the "Java" name into the name of its scripting language. Programmers use entirely different tools for Java and JavaScript. It is also not uncommon for a programmer of one language to be ignorant of the other. The two languages don't rely on each other and are intended for different purposes. In some ways, the "Java" name on JavaScript has confused the world's understanding of the differences between the two. On the other hand, JavaScript is much easier to learn than Java and can offer a gentle introduction for newcomers who want to graduate to Java and the kinds of applications you can develop with it.
What’s relationship between JavaScript and ECMAScript?
ECMAScript is yet another name for JavaScript (other names include LiveScript). The current JavaScript that you see supported in browsers is ECMAScript revision 3.
How do you submit a form using Javascript?
Use document.forms[0].submit();
(0 refers to the index of the form – if you have more than one form in a page, then the first one has the index 0, second has index 1 and so on).
How do we get JavaScript onto a web page?
You can use several different methods of placing javascript in you pages.
You can directly add a script element inside the body of page.
1. For example, to add the "last updated line" to your pages, In your page text, add the following:
<p>blah, blah, blah, blah, blah.</p>
<script type="text/javascript" >
<!-- Hiding from old browsers
document.write("Last Updated:" +
document.lastModified);
document.close();
// -->
</script>
<p>yada, yada, yada.</p>
(Note: the first comment, "<--" hides the content of the script from browsers that don't understand javascript. The "// -->" finishes the comment. The "//" tells javascript that this is a comment so javascript doesn't try to interpret the "-->". If your audience has much older browsers, you should put this comments inside your javascript. If most of your audience has newer browsers, the comments can be omitted. For brevity, in most examples here the comments are not shown. )
The above code will look like this on Javascript enabled browsers,
2. Javascript can be placed inside the <head> element
Functions and global variables typically reside inside the <head> element.
<head>
<title>Default Test Page</title>
<script language="JavaScript" type="text/javascript">
var myVar = "";
function timer(){setTimeout('restart()',10);}
document.onload=timer();
</script>
</head>
Javascript can be referenced from a separate file
Javascript may also a placed in a separate file on the server and referenced from an HTML page. (Don't use the shorthand ending "<script ... />). These are typically placed in the <head> element.
<script type="text/javascript" SRC="myStuff.js"></script>
How to read and write a file using javascript?
I/O operations like reading or writing a file is not possible with client-side javascript. However , this can be done by coding a Java applet that reads files for the script.
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.
How can JavaScript make a Web site easier to use? That is, are there certain JavaScript techniques that make it easier for people to use a Web site?
JavaScript's greatest potential gift to a Web site is that scripts can make the page more immediately interactive, that is, interactive without having to submit every little thing to the server for a server program to re-render the page and send it back to the client. For example, consider a top-level navigation panel that has, say, six primary image map links into subsections of the Web site. With only a little bit of scripting, each map area can be instructed to pop up a more detailed list of links to the contents within a subsection whenever the user rolls the cursor atop a map area. With the help of that popup list of links, the user with a scriptable browser can bypass one intermediate menu page. The user without a scriptable browser (or who has disabled JavaScript) will have to drill down through a more traditional and time-consuming path to the desired content.
What are JavaScript types?
Number, String, Boolean, Function, Object, Null, Undefined.
How do you convert numbers between different bases in JavaScript?
Use the parseInt() function, that takes a string as the first parameter, and the base as a second parameter. So to convert hexadecimal 3F to decimal, use parseInt ("3F", 16);
What does "1"+2+3 evaluate to?
Since 1 is a string, everything is a string, so the result is 123.
How about 3+5+"8"?
Since 3 and 5 are integers, this is number arithmetic, since 8 is a string, it’s concatenation, so 88 is the result.
How do you submit a form using Javascript?
Use document.forms[0].submit();
How do you assign object properties?
obj["age"] = 22 or obj.age = 22.
What’s a way to append a value to an array?
arr[arr.length] = value;
What does isNaN function do?
Return true if the argument is not a number.
What’s relationship between JavaScript and ECMAScript?
ECMAScript is yet another name for JavaScript (other names include LiveScript). The current JavaScript that you see supported in browsers is ECMAScript revision 3.
How to read and write a file using javascript?
I/O operations like reading or writing a file is not possible with client-side javascript.
How do you convert numbers between different bases in JavaScript?
Use the parseInt() function, that takes a string as the first parameter, and the base as a second parameter. So to convert hexadecimal FF to decimal, use parseInt ("FF", 16);
What is negative infinity?
It’s a number in JavaScript, derived by dividing negative number by zero.
How to set a HTML document's background color?
document.bgcolor property can be set to any appropriate color.
What boolean operators does JavaScript support?
&&, and !
How to get the contents of an input box using Javascript?
Use the "value" property.
var myValue = window.document.getElementById("textboxID").value;
How to determine the state of a checkbox using Javascript?
var checkedP = window.document.getElementById("CheckBoxID").checked;
How to set the focus in an element using Javascript?
<script> function setFocus() { if(focusElement != null) { document.forms[0].elements["myelementname"].focus(); } } </script>
How to access an external javascript file that is stored externally and not embedded?
This can be achieved by using the following tag between head tags or between body tags.
<script src="raj.js"></script>How to access an external javascript file that is stored externally and not embedded? where abc.js is the external javscript file to be accessed.
What is the difference between an alert box and a confirmation box?
An alert box displays only one button which is the OK button whereas the Confirm box displays two buttons namely OK and cancel.
What is a prompt box?
A prompt box allows the user to enter input by providing a text box.
Can javascript code be broken in different lines?
Breaking is possible within a string statement by using a backslash \ at the end but not within any other javascript statement.
that is ,
document.write("Hello \ world");
is possible but not document.write \
("hello world");
What looping structures are there in JavaScript?
for, while, do-while loops, but no foreach.
How do you create a new object in JavaScript?
var obj = new Object(); or var obj = {};
What is this keyword?
It refers to the current object.
What is the difference between SessionState and ViewState?
ViewState is specific to a page in a session. Session state refers to user specific data that can be accessed across all pages in the web application.
What looping structures are there in JavaScript?
for, while, do-while loops, but no foreach.
To put a "close window" link on a page ?
<a href='javascript:window.close()' class='mainnav'> Close </a>
How to hide javascript code from old browsers that dont run it?
Use the below specified style of comments <script language=javascript> <!-- javascript code goes here // --> or Use the <NOSCRIPT>some html code </NOSCRIPT> tags and code the display html statements between these and this will appear on the page if the browser does not support javascript
How to comment javascript code?
Use // for line comments and
/*
*/ for block comments
Name the numeric constants representing max,min values
Number.MAX_VALUE
Number.MIN_VALUE
What does javascript null mean?
The null value is a unique value representing no value or no object.
It implies no object,or null string,no valid boolean value,no number and no array object.
How do you create a new object in JavaScript?
var obj = new Object(); or var obj = {};
How do you assign object properties?
obj["age"] = 23 or obj.age = 23.
What’s a way to append a value to an array?
arr[arr.length] = value;
To set all checkboxes to true using JavaScript?
//select all input tags
function SelectAll() {
var checkboxes = document.getElementsByTagName("input");
for(i=0;i<checkboxes.length;i++) {
if(checkboxes.item(i).attributes["type"].value == "checkbox") {
checkboxes.item(i).checked = true;
}
}
}
What does undefined value mean in javascript?
Undefined value means the variable used in the code doesn't exist or is not assigned any value or the property doesn't exist.
What is the difference between undefined value and null value?
(i)Undefined value cannot be explicitly stated that is there is no keyword called undefined whereas null value has keyword called null
(ii)typeof undefined variable or property returns undefined whereas typeof null value returns object
What is variable typing in javascript?
It is perfectly legal to assign a number to a variable and then assign a string to the same variable as follows
example
i = 10;
i = "string";
This is called variable typing
Does javascript have the concept level scope?
No. JavaScript does not have block level scope, all the variables declared inside a function possess the same level of scope unlike c,c++,java.
What are undefined and undeclared variables?
Undeclared variables are those that are not declared in the program (do not exist at all),trying to read their values gives runtime error.But if undeclared variables are assigned then implicit declaration is done .
Undefined variables are those that are not assigned any value but are declared in the program.Trying to read such variables gives special value called undefined value.
What is === operator ?
==== is strict equality operator ,it returns true only when the two operands are having the same value without any type conversion.
How to disable an HTML object ?
document.getElementById("myObject").disabled = true;
How to create a popup warning box?
alert('Warning: Please enter an integer between 0 and 1000.');
How to create a confirmation box?
confirm("Do you really want to launch the missile?");
How to create an input box?
prompt("What is your temperature?");
How to force a page to go to another page using JavaScript ?
<script language="JavaScript" type="text/javascript" ><!-- location.href="http://rajeshstutorials.blogspt.com"; //--></script>
What's Math Constants and Functions using JavaScript?
The Math object contains useful constants such as Math.PI, Math.E
Math.abs(value); //absolute value
Math.max(value1, value2); //find the largest
Math.random() //generate a decimal number between 0 and 1
Math.floor(Math.random()*101) //generate a decimal number between 0 and 100
What does the delete operator do?
The delete operator is used to delete all the variables and objects used in the program ,but it does not delete variables declared with var keyword.
How to get value from a textbox?
alert(document.getElementById('txtbox1').value);
How to get value from dropdown (select) control?
alert(document.getElementById('dropdown1').value);
