Ashutosh Sarangi

Posted on Aug 3 • Updated on Aug 7

HTML Interview Preparation

#webdev #html #programming #interview

V24.08.03

1. What is the difference between defer And async loading?

defer:-

download the JS file
And Done execution at the end of HTML parsing

async:-

download the file
continue the parsing
once it has the downloaded file it halts the parsing and starts executing it.

2. What is the difference between addEventListener vs onClick (DOM event handler)?

DOM Event Handlers:-

They are simple like functions
if I added same event handlers 2 times the last one will replace the 1st one.

EventListner

It follows the Observable / Subscribe pattern
Let's say the same element is attached to the same events multiple times, it will broadcast.

Advance concept addEventListener

const options= {
     once: true,
     passive: true
};

ele.addEventListener('load', callBack, options);

once:- It will remove the event listener after the first time the event is triggered.
passive:- It will not prevent the default behavior of the event.

3. Why we need removeEventListener?

ele.removeEventListener('load', callBack);

1. When we have a single page application where we add/remove elements from the DOM.
2. better to remove in this case.
3. Other wise it will keep listening for the event and will keep the memory.

When we close the browser it will clean everything

4. How to dispatch a Dispatch Custome Event?

Create a new event

const event = new Event('Ashu-Event');
ele.dispatchEvent(event);

ele.addEventlistner('Ashu-Event', callback);

Note

it should not be a shadow DOM; other wise we will not get the document object, as it has its own document and style property.
If it is a shadow dom, we can bind and listen the events from the window.

5. what is the Difference between href attribute and getAttribute()

Note: The href property returns the full URL while the getAttribute method will only return the pathname if that's what's in the attribute.

6. What are the standards of Web Component

Web Components

it has set of standards

Custom Elements
HTML Templates
Shadow DOM
Declarative Shadow DOM (New)

7. Overview of Custome Element

Syntax for web Component

class MyComponent extends HTMLElement {
    constructor() {
        super();
        this.dataset.languagen = 'en';
    }
}

customElements.define('my-component', MyComponent);

<body>
<my-component data-language='en'></my-component>
</body>

8. What is the life cycle of the Custome Element?

class MyComponent extends HTMLElement {
    constructor() {
        super();
    }

    connectedCallback() {
        console.log('Connected to the DOM');
    }

    disconnectedCallback() {
        console.log('Disconnected from the DOM');
    }

    adoptedCallback() {
        console.log('Adopted by a new parent');
        //
    }

    attributeChangedCallback(name, oldValue, newValue) {
        console.log('Attribute Changed', name, oldValue, newValue);
    }
}

9. What is HTML Template?

<!DOCTYPE HTML>
...
<template id='template1'>
    <span> Some Random String</span>
</template>

by default content inside the template are not rendered
We can use it only after cloning it, then we can use it

10. What are the problems with CustomElement?

Problem with template tag let's say we have some style added to the template tag

<template id="my-template">
    <style>
        h1 {
            color: red;
        }
    </style>
    <h1>Hello World</h1>
</template>

Here it will affect the red color all across the page, but if we want to restrict the style to only the template tag, we can use Shadow dom

11. What is Shadow DOM & Its Advantages?

A private, isolated DOM tree within a web component that is separated from the main document's DOM tree.'

Advantages of Shadow DOM:

Allows more control over stylling and encapsulation of functionality of a custom Element;
By default, css declared in the main DOM won't be applied to the shadow DOM;'
It has its own dom
CSS declared in shadow dom applies only there
There are new Pseudo-classes and pseudo element to allow communication between Host and shadow DOM in styleSheet
It can be opened / closed defining visibility from the outer DOM. (Kind of Private / Public) private :- close

this.attachShadow({mode: 'open'}) // Open Shadow DOM

https://developer.mozilla.org/en-US/docs/Web/API/DOMParser

12. What is Declarative Shadow DOM?

A Way to define shadow DOM directly in HTML markup using a new set of attributes. and tags

13. What are Proxies With Reactive Programming

A Wrapper Object that lets you intercept and modify operations performed on the wrapped(object)
Allowing you t0 add custom behavior or validation to the object('s properties and methods)

Proxy is a kind of decorator pattern

const obj = {
     name: 'Ashu',
     age: 30;
}

const handler = {
     get: function(target, prop) {
          if (property === 'age') {
               return target[prop] + ' years old';
          } else {
               return target[prop];
          }
     }
};
const s = new Proxy(obj, handler);

console.log(s.age); //30 years old