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Javascript Interview Questions and Answers
What does the term sticky session mean in
a web-farm scenario? Why would you use a sticky session? What is the
potential disadvantage of using a sticky session?
Sticky session refers to the feature of many commercial load balancing
solutions for web-farms to route the requests for a particular session
to the same physical machine that serviced the first request for that
session. This is mainly used to ensure that a in-proc session is not
lost as a result of requests for a session being routed to different
servers. Since requests for a user are always routed to the same machine
that first served the request for that session, sticky sessions can
cause uneven load distribution across servers.
You have an ASP.NET web application running on a web-farm that does not
use sticky sessions - so the requests for a session are not guaranteed
to be served the same machine. Occasionally, the users get error message
Validation of viewstate MAC failed. What could be one reason that is
causing this error?
The most common reason for this error is that the machinekey value
in machine.config is different for each server. As a result, viewstate
encoded by one machine cannot be decoded by another. To rectify this,
edit the machine.config file on each server in the web-farm to have the
same value for machinekey.
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;
}
}
}
How to select an element by id and swapping an image ? ...
<script language="JavaScript" type="text/javascript" >
function setBeerIcon() {
var beerIcon = document.getElementById("beerIcon");
beerIcon.src = "images/"+getSelectValue("beer")+".jpg";
}
</script>
...
<img border="0" src="" id="brandIcon" alt="brand" />
<select name="beer" id="beer" onChange="setButton();setBeerIcon();">
<option value="--Select--">Select beer</option>
<option value="heineken">heineken</option>
<option value="sol">sol</option>
<option value="amstellight">amstellight</option>
<option value="coronalight">coronalight</option>
<option value="coronaextra">coronaextra</option>
<option value=""></option>
</select>
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.
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