JavaScript Browser BOM — Interview Questions & Answers

Ques: What is the Browser Object Model (BOM)?

Ans: The BOM (Browser Object Model) allows JavaScript to interact with the browser environment — not just the webpage (DOM).

It gives access to browser-specific objects like window, navigator, screen, location, history, and more.

Example:

console.log(window.innerWidth); // Get browser width

Ques: What is the window object in JavaScript?

Ans: The window object is the top-level global object in browsers.

All global JavaScript variables, functions, and objects automatically become properties of window.

Example:

window.alert("Hello!");
console.log(window.innerHeight);

Ques: What are some commonly used window properties?

• window.innerHeight, window.innerWidth → browser’s viewport size
• window.location → URL info
• window.navigator → browser info
• window.screen → device screen details
• window.history → navigation history
• window.document → access DOM

Ques: What are some important window methods?

• window.alert() – Displays an alert box
• window.confirm() – Asks for confirmation
• window.prompt() – Takes user input
• window.open() – Opens a new window or tab
• window.close() – Closes the current window

Example:

if (confirm("Are you sure?")) {
  alert("Confirmed!");
}

Ques: What is the screen object in JavaScript?

Ans: The screen object provides information about the user’s display monitor.

Example:

console.log(screen.width);
console.log(screen.height);

Common properties:

• screen.width, screen.height → screen resolution
• screen.availWidth, screen.availHeight → available space excluding OS bars
• screen.colorDepth → bit depth for color display

Ques: What is the location object used for?

Ans: The location object represents the current URL and allows redirection or reloading of pages.

Example:

console.log(location.href); // Full URL
location.href = "<https://google.com>"; // Redirects

Common properties & methods:

• location.href → full URL
• location.hostname → domain name
• location.pathname → file path
• location.protocol → http / https
• location.reload() → reloads page

Ques: How can you reload or redirect a webpage using JavaScript?

location.reload(); // Reloads current page
location.href = "<https://example.com>"; // Redirects to another page

Ques: What is the history object in JavaScript?

Ans: The history object allows navigation through the browser’s session history.

Example:

history.back();  // Go to previous page
history.forward(); // Go to next page

Methods:

• history.back() – Same as browser back button
• history.forward() – Same as next button
• history.go(n) – Moves n steps in history (negative for back)

Ques: What is the navigator object?

Ans: The navigator object gives information about the browser and system environment.

Example:

console.log(navigator.userAgent);
console.log(navigator.language);

Common properties:

• navigator.userAgent → browser details
• navigator.language → user’s preferred language
• navigator.onLine → check if online
• navigator.platform → OS info
• navigator.geolocation → access location (with permission)

Ques: How do you detect the browser type in JavaScript?

console.log(navigator.userAgent);

Example Output:

Mozilla/5.0 (Windows NT 10.0; Win64; x64)

Ques: How can you display popups in JavaScript?

1. Alert box

   alert("Welcome!");
   

2. Confirm box

   if (confirm("Are you sure?")) console.log("Yes");
   

3. Prompt box

   let name = prompt("Enter your name:");
   alert("Hello " + name);
   

Ques: What is the difference between alert(), confirm(), and prompt()?

Method	Description	Returns
alert()	Displays a message	undefined
confirm()	Asks user to confirm	true / false
prompt()	Takes user input	text / null

Ques: What are JavaScript timing events?

Ans: Timing events execute code after a delay or repeatedly.

• setTimeout() → executes once after delay
• setInterval() → repeats at intervals
• clearTimeout() / clearInterval() → stop timers

Example:

setTimeout(() => console.log("Hello after 2s"), 2000);

Ques: What is the difference between setTimeout() and setInterval()?

Method	Description
setTimeout(fn, delay)	Runs code once after delay
setInterval(fn, delay)	Runs code repeatedly every interval

Example:

setInterval(() => console.log("Repeating..."), 1000);

Ques: How can you stop a timer in JavaScript?

let id = setInterval(() => console.log("Running"), 1000);
clearInterval(id); // stops repeating

Ques: What are cookies in JavaScript?

Ans: Cookies are small pieces of data stored on the user’s browser to remember information (like login or preferences).

Example:

document.cookie = "username=John; expires=Fri, 31 Dec 2025 12:00:00 UTC; path=/";

Ques: How can you read cookies using JavaScript?

console.log(document.cookie);

Ques: How can you delete a cookie?

Ans: Set the cookie’s expiry date to the past:

document.cookie = "username=; expires=Thu, 01 Jan 1970 00:00:00 UTC;";

Ques: What are the limitations of cookies?

• Limited to ~4KB per cookie
• Stored as plain text (less secure)
• Sent with every request → increases load
• Can be disabled by users

Ques: What are alternatives to cookies in modern web apps?

• LocalStorage – Persistent storage (no expiry)
• SessionStorage – Temporary per-session data
• IndexedDB – Structured large storage

Ques: What is the difference between BOM and DOM?

Feature	DOM	BOM
Full Form	Document Object Model	Browser Object Model
Purpose	Interacts with page content	Interacts with browser environment
Main Object	document	window
Example	document.getElementById()	window.alert()

Ques; How can you detect if the user is online or offline?

if (navigator.onLine) {
  console.log("Online");
} else {
  console.log("Offline");
}

Ques: How can you get the user’s location using BOM?

navigator.geolocation.getCurrentPosition(
  (pos) => console.log(pos.coords.latitude, pos.coords.longitude),
  (err) => console.error(err)
);

Ques: What is the window.open() method used for?

Ans: Opens a new browser tab or window:

window.open("<https://example.com>", "_blank");

Ques: What are common use cases of the BOM?

• Detecting browser or screen details
• Redirecting users
• Managing cookies or sessions
• Handling browser navigation
• Displaying popups or alerts
• Setting timeouts and intervals