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Browser Environment Specification

Browser Document: Browser Environment Specs

What is the Browser Environment Specification?

View Answer:
Interview Response: It's a set of standards defining how JavaScript interacts with web browsers, including the DOM, events, and communication with servers via AJAX.

What was the original purpose of JavaScript?

View Answer:
Interview Response: The original purpose of JavaScript was to add interactivity and dynamic content to web pages, enabling user interactions, form validation, and manipulation of HTML elements in the browser environment.

JavaScript can operate on what kinds of technological platforms?

View Answer:
Interview Response: JavaScript operates on various platforms, including web browsers, server-side with Node.js, desktop and mobile applications using frameworks like Electron and React Native, and even IoT devices through specialized libraries.

Technical Response: The JavaScript language can run on a browser, or a webserver or another host, even a “smart” coffee machine, if it can run JavaScript. Each of them provides platform-specific functionality. The JavaScript specification calls that a host environment. A host environment provides its own objects and functions additional to the language core. Web browsers give a means to control web pages. Node.js provides server-side features, and so on.

Can you briefly explain the general structure of the browser window environment?

View Answer:
Interview Response: A browser at its root consists of a window (root object), DOM, BOM, and the JavaScript Object. The DOM is the document object model, and the BOM is the browser object model. JavaScript is the language used to interact with the window, DOM, and BOM.

What is the Document Object Model (DOM) in JavaScript?

View Answer:
Interview Response: The DOM is a tree-like structure representing an HTML document, allowing scripts to manipulate the content, structure, and styles of a webpage. The Document Object Model, or DOM, represents all page content as editable objects. The principal "entry point" to the page is the document object, and we may use it to edit or create anything on the website.

Code Example:

// change the background color to red = 'red';

// change it back after 1 second
setTimeout(() => ( = ''), 1000);

Can you name a good reference document to find information about the document object model?

View Answer:
Interview Response: Two good reference documents used to reference information about the DOM include the MDN Web Docs and the DOM Living Standard. The DOM Living Standard provides you with detailed information about the specification, and the MDN is a detailed guide expressed in an easy way to learn.

What is the DOM Living Standard?

View Answer:
Interview Response: The DOM Living Standard is a constantly evolving specification that defines the structure, behavior, and APIs of the Document Object Model, ensuring cross-browser consistency and addressing modern web development requirements.

Is the DOM only used in Browsers, or can it be found on other platforms?

View Answer:
Interview Response: While the DOM is primarily used in browsers, it can also be found on other platforms, such as server-side environments like Node.js, using libraries like JSDOM to simulate browser-like DOM interactions.

What is the CSS Object Model (CSSOM) used for in a browser?

View Answer:
Interview Response: The CSS Object Model (CSSOM) is a browser API that represents stylesheets, allowing JavaScript to interact with and manipulate CSS rules and styles applied to HTML elements.

Technical Response: The CSS Object Model is a set of APIs that allow JavaScript to manipulate CSS. It is similar to the DOM, but for CSS rather than HTML. It enables users to view and alter CSS styles in real-time. It is independent of the DOM, yet they collaborate when we change the document's style rules. CSSOM, on the other hand, is rarely a necessity in reality.

What is the Browser Object Model in JavaScript?

View Answer:
Interview Response: The Browser Object Model (BOM) in JavaScript represents browser components, such as 'window', 'navigator', and 'location', enabling developers to interact with the browser environment and its properties.

Technical Response: The Browser Object Model (BOM) represents additional objects provided by the browser (host environment) for working with everything except the document.

For Example:

  1. The navigator object gives context about the browser and the operating system. There are various characteristics, but the navigator and location are the most well-known. userAgent tells us about the current browser, and navigator.platform tells us about the platform (which varies depending on whether it's Windows, Linux, or Mac).
  2. We can read the current URL and redirect the browser to a new URL using the location object.

Code Example: Here’s how we can use the location object

// Here’s how we can use the location object:
console.log(location.href); // shows current URL

if (confirm('Go to Wikipedia?')) {
location.href = ''; // redirect the browser to another URL

Are the alert, confirm, and prompt functions part of the DOM or BOM?

View Answer:
Interview Response: The alert, confirm, and prompt functions are part of the Browser Object Model (BOM), specifically the 'window' object, and are used for displaying simple dialogs to interact with users.

Under what specification does the BOM fall?

View Answer:
Interview Response: The BOM falls under the HTML Living Standard specification, which defines the browser environment, including the 'window', 'navigator', and other related objects, in addition to the DOM and HTML elements.

Technical Response: The BOM is part of the HTML standard. Yes, you read that correctly. The HTML standard, available at, encompasses more than just the "HTML language" (tags, attributes) and a slew of objects, methods, and browser-specific DOM extensions. That's "HTML in broad strokes." Additionally, certain parts have supplementary specifications given at

What is the HTML Living Standard?

View Answer:
Interview Response: The HTML Living Standard is an evolving specification that defines HTML, DOM, and browser-related APIs, ensuring consistent behavior across browsers and addressing modern web development needs.

View Answer:
Interview Response: Key manuals related to the browser are the HTML Living Standard, Document Object Model (DOM) specification, CSS Object Model (CSSOM), JavaScript ECMAScript specification, and Web API interfaces documentation.

  1. HTML Living Standard: Covers HTML, DOM, and browser-related APIs.
  2. CSS Specifications: Defines the behavior and styling of CSS, including CSSOM.
  3. ECMAScript (JavaScript): Specifies the core JavaScript language features.
  4. Web APIs: Defines additional APIs for web development, like Fetch, Web Storage, and Web Workers.
  5. WebRTC: Covers real-time communication between browsers.
  6. WebAssembly: Describes a binary instruction format for secure and efficient code execution in web browsers.

What are JavaScript events?

View Answer:
Interview Response: Events are actions or occurrences, like clicks or keypresses, that trigger functions or event listeners to execute code in response to user interactions.

What is the purpose of the 'window' object?

View Answer:
Interview Response: The window object represents the browser window and provides properties, methods, and events for interacting with the global environment and document content.

What is the same-origin policy?

View Answer:
Interview Response: The same-origin policy (SOP) is a crucial concept in the web application security model. Under this policy, a web browser permits scripts contained in a first web page to access data in a second web page, but only if both web pages have the same origin. An origin is defined as a combination of URI scheme (also known as protocol, such as HTTP or HTTPS), host name (domain), and port number.

What is AJAX?

View Answer:
Interview Response: AJAX stands for Asynchronous JavaScript and XML. It is a set of web development techniques that uses various web technologies together on the client side to create asynchronous web applications.

Code Example:

fetch('', {
method: 'GET', // or 'POST'
headers: {
'Content-Type': 'application/json',
// 'Authorization': 'Bearer your-token(optional)'
.then(response => response.json()) // parse the response as JSON
.then(data => console.log(data)) // Here's where you handle the result
.catch(error => console.error('Error:', error)); // Handle any errors

What is event bubbling?

View Answer:
Interview Response: Event bubbling is the propagation of an event from a child to parent elements in the DOM tree, triggering event listeners on each element along the way.

Technical Response: Event bubbling is a type of event propagation in the HTML DOM API when an event occurs in an element inside another element, and both elements have registered a handle for that event. The event propagation mode determines the order in which elements receive the event. With bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements. In other words, it starts from the element that triggered the event, and then bubbles up to its parents, and its parents' parents, and so on, until it reaches the root element, typically the document object or the window.

Code Example:

<div id="parent">
<div id="child">

And this JavaScript:

document.getElementById('parent').addEventListener('click', () => console.log('parent clicked'));
document.getElementById('child').addEventListener('click', () => console.log('child clicked'));

If you click on the element with id "child", you'll see both "child clicked" and "parent clicked" logged to the console, because the click event starts at the child, then bubbles up to the parent.

Sometimes, you might not want an event to bubble. In such cases, you can use event.stopPropagation() in the event handler to prevent further propagation. For instance, modifying the child's click event listener like so would stop the event from bubbling up to the parent:

document.getElementById('child').addEventListener('click', (event) => {
console.log('child clicked');

With this change, if you click on the "child" element, only "child clicked" will be logged. The "parent clicked" will not be logged, because the event's propagation is stopped at the child.

What are Web Storage APIs?

View Answer:
Interview Response: Web Storage APIs (localStorage and sessionStorage) are used to store key-value pairs in the browser, providing a simple way to persist data across page reloads and sessions.

Code Example:

// Store data
localStorage.setItem('key', 'value');

// Get data
let data = localStorage.getItem('key');

// Remove data

// Clear all data

It's important to note that while Web Storage is useful for storing smaller amounts of data, it's not intended to be a replacement for a database. It's also worth noting that these APIs are synchronous and can cause performance issues if you're trying to store larger amounts of data.


As of March 2023, for larger amounts of data, IndexedDB is a good choice. IndexedDB is a low-level API for client-side storage of significant amounts of structured data, including files/blobs.

What is the difference between localStorage and sessionStorage?

View Answer:
Interview Response: localStorage stores data with no expiration, while sessionStorage stores data for the duration of the page session, deleting the data when the browser is closed.

What is feature detection?

View Answer:
Interview Response: Feature detection is a technique used to identify browser capabilities, allowing developers to provide fallback solutions or enhancements for unsupported or partially supported features.

Code Example:

Let's say you want to use localStorage, but you want to make sure the browser supports it first. Here's how you might do that:

if (typeof(Storage) !== "undefined") {
// Code for localStorage/sessionStorage.
} else {
// Sorry, no web storage support..

Similarly, you can use feature detection for checking other HTML5 features like Geolocation, Audio, Video, Canvas, etc. For example:

if ("geolocation" in navigator) {
/* geolocation is available */
} else {
/* geolocation IS NOT available */


Modernizr is a JavaScript library that automates this process for many features, allowing you to test for support with a simple conditional statement.

Please note that feature detection doesn't tell you what browser or version the user is using, it only tells you whether a particular feature is available or not.

What are browser cookies?

View Answer:
Interview Response: Cookies are small pieces of data stored by the browser, used to maintain state between requests or visits, often for user authentication, preferences, or tracking.

Code Example:

Here is a simple example of setting a cookie in JavaScript...

document.cookie = "username=John Doe; expires=Thu, 18 Dec 2023 12:00:00 UTC; path=/";

In this example, a cookie named "username" is set with a value of "John Doe". The cookie expires on December 18, 2023, and it's accessible to all pages in the same domain.

And here's how you can read cookies:

let allCookies = document.cookie;

This will return a string containing all cookies in the format key=value; key2=value2;.


You should note that cookies have several limitations and implications regarding security and privacy, so they need to be used responsibly and appropriately. For example, sensitive data should never be stored in cookies, and they should always be transmitted over secure (HTTPS) connections when possible.

What is CORS?

View Answer:

Interview Response: CORS (Cross-Origin Resource Sharing) is a mechanism that enables many resources, like fonts, images, and scripts, to be requested across origins, while maintaining security restrictions in the browser.

What are Service Workers?

View Answer:

Interview Response: Service Workers are scripts that run in the background, independent of a web page, enabling features like offline support, push notifications, and background data updates.

Technical Response: Service Workers are a type of web worker. They're JavaScript files that can control the web page/site it is associated with, intercepting and modifying navigation and resource requests, and caching resources in a very granular fashion to complete offline experiences, or to boost performance. Service Workers run in the background, separate from the web page, and they don't have access to the Document Object Model (DOM). This design allows them to be fully asynchronous, meaning they're non-blocking, and can be efficient and effective for features needing good performance.

Code Example:

Service Workers have a lifecycle which includes events like 'install', 'activate', and 'fetch'.

Here's a very basic Service Worker registration example:

if ('serviceWorker' in navigator) {
window.addEventListener('load', function() {
navigator.serviceWorker.register('/sw.js').then(function(registration) {
// Registration was successful
console.log('ServiceWorker registration successful with scope: ', registration.scope);
}, function(err) {
// registration failed :(
console.log('ServiceWorker registration failed: ', err);

This script checks if the service worker API is available, and if it is, the service worker at /sw.js is registered once the page is loaded.


Remember that service workers require HTTPS, because the level of control they have over network requests could be dangerous if intercepted or altered. During development, localhost is considered a secure origin so you can develop your service worker locally.

What is the 'navigator' object in relation to the browser?

View Answer:

Interview Response: The 'navigator' object provides information about the user's browser, OS, and device, helping developers optimize their applications for different environments.

Code Example:

Here's an example of using the navigator object:

if (navigator.onLine) {
console.log('You are online!');
} else {
console.log('You are offline.');

This code checks the navigator.onLine property to determine whether the user is online or offline, and logs a message to the console accordingly.