October 11, 2018

Sreekanth B

HCL Technologies 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.
HCL Technologies JavaScript Most Frequently Asked Latest Interview Questions Answers
HCL Technologies JavaScript Most Frequently Asked Latest Interview Questions Answers

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 is the difference between remove() and removeChild()?

The remove() function just removes the element. Whereas, the removeChild() returns the deleted element.

List out the Mouse Events.

There are altogether 9 mouse events. Here is the list:

1.    click: A single click

2.    dblclick: A double click

3.    mousedown: When the mouse button is clicked down.

4.    mouseup: When the mouse button is released up.

5.    mouseenter: When the mouse cursor enters an external element.

6.    mouseleave: When the mouse cursor leaves an external element.

7.    mouseover: When the mouse cursor enters an internal and external element.

8.    mouseout: When the mouse cursor leaves an internal and external element.

9.    mousemove: When the mouse cursor is moved.

What will the following code output to the console and why:

var hero = {
    _name: 'John Doe',
    getSecretIdentity: function (){
        return this._name;
    }
};

var stoleSecretIdentity = hero.getSecretIdentity;

console.log(stoleSecretIdentity());
console.log(hero.getSecretIdentity());

What is the issue with this code and how can it be fixed.

The code will output:

undefined
John Doe
The first console.log prints undefined because we are extracting the method from the hero object, so stoleSecretIdentity() is being invoked in the global context (i.e., the window object) where the _name property does not exist.

One way to fix the stoleSecretIdentity() function is as follows:

var stoleSecretIdentity = hero.getSecretIdentity.bind(hero);

Create a function that, given a DOM Element on the page, will visit the element itself and all of its descendents (not just its immediate children). For each element visited, the function should pass that element to a provided callback function.

The arguments to the function should be:

a DOM element
a callback function (that takes a DOM element as its argument)

Visiting all elements in a tree (DOM) is a classic Depth-First-Search algorithm application. Here’s an example solution:

function Traverse(p_element,p_callback) {
   p_callback(p_element);
   var list = p_element.children;
   for (var i = 0; i < list.length; i++) {
       Traverse(list[i],p_callback);  // recursive call
   }
}

Testing your this knowledge in JavaScript: What is the output of the following code?

var length = 10;
function fn() {
console.log(this.length);
}

var obj = {
  length: 5,
  method: function(fn) {
    fn();
    arguments[0]();
  }
};

obj.method(fn, 1);

Output:

10
2
Why isn’t it 10 and 5?

In the first place, as fn is passed as a parameter to the function method, the scope (this) of the function fn is window. var length = 10; is declared at the window level. It also can be accessed as window.length or length or this.length (when this === window.)

method is bound to Object obj, and obj.method is called with parameters fn and 1. Though method is accepting only one parameter, while invoking it has passed two parameters; the first is a function callback and other is just a number.

When fn() is called inside method, which was passed the function as a parameter at the global level, this.length will have access to var length = 10 (declared globally) not length = 5 as defined in Object obj.

Now, we know that we can access any number of arguments in a JavaScript function using the arguments[] array.

Hence arguments[0]() is nothing but calling fn(). Inside fn now, the scope of this function becomes the arguments array, and logging the length of arguments[] will return 2.

Hence the output will be as above.

Consider the following code. What will the output be, and why?

(function () {
    try {
        throw new Error();
    } catch (x) {
        var x = 1, y = 2;
        console.log(x);
    }
    console.log(x);
    console.log(y);
})();

1
undefined
2
var statements are hoisted (without their value initialization) to the top of the global or function scope it belongs to, even when it’s inside a with or catch block. However, the error’s identifier is only visible inside the catch block. It is equivalent to:

(function () {
    var x, y; // outer and hoisted
    try {
        throw new Error();
    } catch (x /* inner */) {
        x = 1; // inner x, not the outer one
        y = 2; // there is only one y, which is in the outer scope
        console.log(x /* inner */);
    }
    console.log(x);
    console.log(y);
})();.

What are the components of the ASP.NET AJAX architecture?

There are two components of AJAX Architecture:

AJAX client architecture
AJAX server architecture

What are the extender controls?

The extender controls uses a block of JavaScript code to add new and enhanced capabilities to ASP.NET.

What are the pre-requisites to execute AJAX applications on a server?

AJAX is a built-in functionality of .NET Framework 4.0 and AJAX application can be executed by just installing Microsoft Visual Studio 2010. To use extenders in your applications, you are required to install AJAX Control Toolkit and copy the AjaxControlToolkit.dll file to the Bin directory of your application.

Which request is better, Get or Post?

AJAX requests should use an HTTP GET request where the data does not change for a given URL requested.

An HTTP POST should be used when state is updated on the server. This is highly recommended for a consistent web application architecture.

What are the limitations of Ajax?

An Ajax Web Application tends to confuse end users if the network bandwidth is slow and there is no full postback running.

What is AJAX Framework?

ASP.NET AJAX is a free framework to implement Ajax in asp.net web applications. It is used to quickly creating efficient and interactive Web applications that work across all browsers.

How can we cancel the XMLHttpRequest in AJAX?

Abort() method can be called to cancel the XMLHttpRequest in Ajax.

Is AJAX code cross browser compatible?

No, it is supporting cross browser compatible. If the browsers supports native XMLHttpRequest JavaScript object, then this can be used.

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