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.
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 is the difference between window and document in Javascript?
Javascript window is a global object which holds variables, functions, history, location; the document also comes under the window and can be considered as the property of the window.
What is the difference between innerHTML and innerText?
innerHTML will process an HTML tag if found in a string, whereas innerText will not. For Example document.querySelector('p').innerHTML='one <br> two' gives the output one and two in two lines as <br> in html is a new line. Whereas document.querySelector('p').innerText='one <br> two' gives the output of the text as it is in one line.
What is the difference between textContent and innerText?
Let us have a paragraph element and a span element in it as a child element.
<p>some text and a <span style="visibility: hidden">span tag hidden <\span>in it</p>
Now, if the following two steps would result in the following-
console.log(document.querySelector('p').textContent); gives some text and a span tag hidden in it.
console.log(document.querySelector('p').innerText); gives some text and a in it.
What is the difference between HTMLCollection and NodeList?
The functions querySelectorAll() returns NodeList in which the forEach can be used directly to traverse the elements. Whereas, the getElementsByClassName() or getElementsByTagName() returns an HTMLCollection, which does not have a forEach by default.
How can an HTMLCollection be traversed?
The HTMLCollection by default does not have forEach, it needs to be converted into an array (eleName = Array.from(eleName)) and then the traversal is possible using forEach.
What is the difference between childNode and children?
A childNode, when used returns a NodeList. Whereas, children, when used; returns an HTMLCollection.
What is the difference between firstChild and firstElementChild?
A firstChild when used returns the first node. It could be an HTML element or even a space, or a new line. Whereas, firstElementChild, when used returns the first HTML element only.
Name the two functions that are used to create an HTML element dynamically.
To generate an html element dynamically, we need to use two functions:
(i) var ele = createElement ('elementName'), which creates an element and
(ii) parentElement.appendChild(ele)
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 are the protocols used by Ajax?
HTTP’s GET or POST
XMLHttpRequest for placing a request with the web server
Uses JSON to communicate between the client and server
UED or URL encoded data
What are all the security issues of Ajax?
Security issues that can be encountered
When Ajax calls are sent through plain text and it may lead to know the database details
Inserting scripts can also be possible and attackers can easily penetrate into the system
How can we handle concurrent requests?
Javascript functions should be written to handle concurrent requests and call back function can be passed as a parameter. Those parameters are passed to AjaxInteraction(URL, callback) object.
Define the role of the Update Panel?
Update Panel is used to add functionality to the existing ASP.NET applications. By using partial page rendering, it can be used to update the content. Refresh can be made for the partial page instead of whole page.
Can we use nested update panel in Ajax?
Yes, we can use nested update panel in Ajax. Update panels can be nested to have more control over the Page Refresh.
What are the types of post back in Ajax?
There are two types of post backs:
Synchronous Postback
Asynchronous Postback.
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.
 |
| Daum JavaScript Most Frequently Asked Latest Interview Questions Answers |
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.
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 is the difference between window and document in Javascript?
Javascript window is a global object which holds variables, functions, history, location; the document also comes under the window and can be considered as the property of the window.
What is the difference between innerHTML and innerText?
innerHTML will process an HTML tag if found in a string, whereas innerText will not. For Example document.querySelector('p').innerHTML='one <br> two' gives the output one and two in two lines as <br> in html is a new line. Whereas document.querySelector('p').innerText='one <br> two' gives the output of the text as it is in one line.
What is the difference between textContent and innerText?
Let us have a paragraph element and a span element in it as a child element.
<p>some text and a <span style="visibility: hidden">span tag hidden <\span>in it</p>
Now, if the following two steps would result in the following-
console.log(document.querySelector('p').textContent); gives some text and a span tag hidden in it.
console.log(document.querySelector('p').innerText); gives some text and a in it.
What is the difference between HTMLCollection and NodeList?
The functions querySelectorAll() returns NodeList in which the forEach can be used directly to traverse the elements. Whereas, the getElementsByClassName() or getElementsByTagName() returns an HTMLCollection, which does not have a forEach by default.
How can an HTMLCollection be traversed?
The HTMLCollection by default does not have forEach, it needs to be converted into an array (eleName = Array.from(eleName)) and then the traversal is possible using forEach.
What is the difference between childNode and children?
A childNode, when used returns a NodeList. Whereas, children, when used; returns an HTMLCollection.
What is the difference between firstChild and firstElementChild?
A firstChild when used returns the first node. It could be an HTML element or even a space, or a new line. Whereas, firstElementChild, when used returns the first HTML element only.
Name the two functions that are used to create an HTML element dynamically.
To generate an html element dynamically, we need to use two functions:
(i) var ele = createElement ('elementName'), which creates an element and
(ii) parentElement.appendChild(ele)
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 are the protocols used by Ajax?
HTTP’s GET or POST
XMLHttpRequest for placing a request with the web server
Uses JSON to communicate between the client and server
UED or URL encoded data
What are all the security issues of Ajax?
Security issues that can be encountered
When Ajax calls are sent through plain text and it may lead to know the database details
Inserting scripts can also be possible and attackers can easily penetrate into the system
How can we handle concurrent requests?
Javascript functions should be written to handle concurrent requests and call back function can be passed as a parameter. Those parameters are passed to AjaxInteraction(URL, callback) object.
Define the role of the Update Panel?
Update Panel is used to add functionality to the existing ASP.NET applications. By using partial page rendering, it can be used to update the content. Refresh can be made for the partial page instead of whole page.
Can we use nested update panel in Ajax?
Yes, we can use nested update panel in Ajax. Update panels can be nested to have more control over the Page Refresh.
What are the types of post back in Ajax?
There are two types of post backs:
Synchronous Postback
Asynchronous Postback.
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