October 11, 2018

Sreekanth B

Hewlett-Packard JavaScript Most Frequently Asked Latest Interview Questions Answers

What is the use of the ‘this’ keyword?

The keyword ‘this’ refers to the current instance of the object when used inside a function. But, when used outside a function, it refers to the window object.

Is Exception handling possible in Javascript?

With the latest version of Javascript, exception handling is possible; and this can be achieved using the following keywords try, catch and finally.

How is it possible to get the total number of arguments that are passed to a function?

The arguments.length property helps in getting the total number of arguments that are passed to a function.

What is the difference between typeof and instanceof operators in Javascript?

The typeof operator returns a string of what type the operand is. Whereas, the instanceof operator does not work with primitive data types; but works with objects and checks on what type the object is.

What is DOM?

DOM (Document Object Model) is an object-oriented representation of the HTML elements. All the elements (or nodes) are part of window.document.
Hewlett-Packard JavaScript Most Frequently Asked Latest Interview Questions Answers
Hewlett-Packard JavaScript Most Frequently Asked Latest Interview Questions Answers

Expand BOM and explain it.

BOM stands for Browser Object Model. Using BOM interaction with a browser is possible. Default object of the browser is a window.

What will the code below output to the console and why?

(function(){
  var a = b = 3;
})();

console.log("a defined? " + (typeof a !== 'undefined'));
console.log("b defined? " + (typeof b !== 'undefined'));

Since both a and b are defined within the enclosing scope of the function, and since the line they are on begins with the var keyword, most JavaScript developers would expect typeof a and typeof b to both be undefined in the above example.

However, that is not the case. The issue here is that most developers incorrectly understand the statement var a = b = 3; to be shorthand for:

var b = 3;
var a = b;
But in fact, var a = b = 3; is actually shorthand for:

b = 3;
var a = b;
As a result (if you are not using strict mode), the output of the code snippet would be:

a defined? false
b defined? true

But how can b be defined outside of the scope of the enclosing function? Well, since the statement var a = b = 3; is shorthand for the statements b = 3; and var a = b;, b ends up being a global variable (since it is not preceded by the var keyword) and is therefore still in scope even outside of the enclosing function.

Note that, in strict mode (i.e., with use strict), the statement var a = b = 3; will generate a runtime error of ReferenceError: b is not defined, thereby avoiding any headfakes/bugs that might othewise result. (Yet another prime example of why you should use use strict as a matter of course in your code!)

What is the significance of, and reason for, wrapping the entire content of a JavaScript source file in a function block?

This is an increasingly common practice, employed by many popular JavaScript libraries (jQuery, Node.js, etc.). This technique creates a closure around the entire contents of the file which, perhaps most importantly, creates a private namespace and thereby helps avoid potential name clashes between different JavaScript modules and libraries.

Another feature of this technique is to allow for an easily referenceable (presumably shorter) alias for a global variable. This is often used, for example, in jQuery plugins. jQuery allows you to disable the $ reference to the jQuery namespace, using jQuery.noConflict(). If this has been done, your code can still use $ employing this closure technique, as follows:

(function($) { /* jQuery plugin code referencing $ */ } )(jQuery);

What is the significance, and what are the benefits, of including 'use strict' at the beginning of a JavaScript source file?

The short and most important answer here is that use strict is a way to voluntarily enforce stricter parsing and error handling on your JavaScript code at runtime. Code errors that would otherwise have been ignored or would have failed silently will now generate errors or throw exceptions. In general, it is a good practice.

Some of the key benefits of strict mode include:

Makes debugging easier. Code errors that would otherwise have been ignored or would have failed silently will now generate errors or throw exceptions, alerting you sooner to problems in your code and directing you more quickly to their source.
Prevents accidental globals. Without strict mode, assigning a value to an undeclared variable automatically creates a global variable with that name. This is one of the most common errors in JavaScript. In strict mode, attempting to do so throws an error.
Eliminates this coercion. Without strict mode, a reference to a this value of null or undefined is automatically coerced to the global. This can cause many headfakes and pull-out-your-hair kind of bugs. In strict mode, referencing a a this value of null or undefined throws an error.
Disallows duplicate parameter values. Strict mode throws an error when it detects a duplicate named argument for a function (e.g., function foo(val1, val2, val1){}), thereby catching what is almost certainly a bug in your code that you might otherwise have wasted lots of time tracking down.
Note: It used to be (in ECMAScript 5) that strict mode would disallow duplicate property names (e.g. var object = {foo: "bar", foo: "baz"};) but as of ECMAScript 2015 this is no longer the case.
Makes eval() safer. There are some differences in the way eval() behaves in strict mode and in non-strict mode. Most significantly, in strict mode, variables and functions declared inside of an eval() statement are not created in the containing scope (they are created in the containing scope in non-strict mode, which can also be a common source of problems).
Throws error on invalid usage of delete. The delete operator (used to remove properties from objects) cannot be used on non-configurable properties of the object. Non-strict code will fail silently when an attempt is made to delete a non-configurable property, whereas strict mode will throw an error in such a case.

Consider the two functions below. Will they both return the same thing? Why or why not?

function foo1()
{
  return {
      bar: "hello"
  };
}

function foo2()
{
  return
  {
      bar: "hello"
  };
}

Surprisingly, these two functions will not return the same thing. Rather:

console.log("foo1 returns:");
console.log(foo1());
console.log("foo2 returns:");
console.log(foo2());
will yield:

foo1 returns:
Object {bar: "hello"}
foo2 returns:
undefined
Not only is this surprising, but what makes this particularly gnarly is that foo2() returns undefined without any error being thrown.

The reason for this has to do with the fact that semicolons are technically optional in JavaScript (although omitting them is generally really bad form). As a result, when the line containing the return statement (with nothing else on the line) is encountered in foo2(), a semicolon is automatically inserted immediately after the return statement.

No error is thrown since the remainder of the code is perfectly valid, even though it doesn’t ever get invoked or do anything (it is simply an unused code block that defines a property bar which is equal to the string "hello").

This behavior also argues for following the convention of placing an opening curly brace at the end of a line in JavaScript, rather than on the beginning of a new line. As shown here, this becomes more than just a stylistic preference in JavaScript.

What is update panel?

Update panel  is a server control used to update the specified portion of a web page. Script Manager needs to be used whenever update panel is used. Using update panel, user cannot handle outside controls.

Which are the two methods used for cross domain Ajax calls?

There are two methods used to transfer data between the two more more security domains:

CORS – Cross Origin Resource Sharing and it works with the HTTP web browsers
JSONP – JSON with Padding which works with the HTTP GET and on legacy browsers

What are all the technologies used by Ajax?

AJAX uses following technologies:

JavaScript
XMLHttpRequest
Document Object Model (DOM)
Extensible HTML (XHTML)
Cascading Style Sheets (CSS)

What are all the features of Ajax?

Following are the features of Ajax and they are as follows:

Live data binding
Client-side template rendering
Declarative instantiation of client components
Observer pattern on JavaScript objects and arrays
Invoking ADO.NET data services and data contexts
DataView control

What are Ajax applications?

Browser based applications and platform independent applications are used by Ajax.

How many types of triggers are present in update panel?

There are two types of triggers used in update panel:

PostBackTrigger – This works as full postback and it cannot work asynchronously
AsyncPostBackTrigger – Partial post back asynchronously

What are all the controls of Ajax?

Following are the controls of Ajax:

ScriptManager
ScriptManagerProxy
UpdatePanel
UpdateProgress
Timer

What is the name of the DLL that contains Ajax control tool kit?

Ajaxcontroltoolkit.dll is the DLL used for Ajax control tool kit and it can be downloaded from the internet. It can be added in the tool box or copied directly in the bin folder.

What role of #&& in querystring?

# is treated as fragment delimiter to delimit the history state and && precedes is used to check on the information in the query string.

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