Web Development Rapid-Fire Questions &amp; Answers

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var is function-scoped, is hoisted and initialised as undefined, and can be redeclared, which causes bugs in loops and blocks. let and const are block-scoped and sit in the temporal dead zone until their declaration line runs. let allows reassignment, while const does not, although objects declared with const can still have their properties changed. Modern code uses const by default and let only when the value must change.

What are PHP superglobals?

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Superglobals are built-in arrays available in every scope without the global keyword. Common ones are $_GET and $_POST for request data, $_REQUEST combining them, $_SESSION, $_COOKIE, $_FILES for uploads, $_SERVER for headers and paths, and $_ENV for environment variables. All user-supplied superglobals must be treated as untrusted input and validated, for example using filter_input(INPUT_POST, 'email', FILTER_VALIDATE_EMAIL).

Should you use PDO or mysqli to connect to MySQL?

PHP ProgrammingMedium

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Both support prepared statements and are secure when used correctly. PDO supports around a dozen databases with the same API, has named placeholders like :email, and throws exceptions cleanly, so switching from MySQL to PostgreSQL is easier. mysqli works only with MySQL but exposes some MySQL-specific features. Most modern codebases and frameworks like Laravel use PDO under the hood, so it is the usual recommendation.

What is the difference between id and class attributes?

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An id must be unique within a page and identifies one element, so it is used for anchor links, label for attributes and getElementById. A class can be reused on many elements and is the normal hook for CSS styling and grouping in JavaScript. Ids also have much higher CSS specificity, so styling with ids makes overrides harder; most teams style only with classes.

When would you choose Flexbox and when CSS Grid?

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Flexbox is one-dimensional: it arranges items in a row or a column and is great for navbars, button groups and centering. Grid is two-dimensional: it controls rows and columns together, which suits page layouts, dashboards and card galleries. A common pattern is Grid for the overall page structure and Flexbox inside each component. Both are fully supported in all modern browsers.

What is AJAX and how is it done today?

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AJAX means updating parts of a page by exchanging data with the server in the background, without a full page reload. Originally it used XMLHttpRequest and XML, but today it is done with the fetch API and JSON. Examples include live search suggestions, adding to cart without reload, and infinite scroll. The server returns data, and JavaScript updates the DOM with the result.

What is the difference between == and ===?

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== compares values after type coercion, so 0 == '0' and null == undefined are both true, which can hide bugs. === is strict equality and returns true only when both type and value match, so 0 === '0' is false. Most style guides require === everywhere. One accepted exception is value == null, which conveniently checks for both null and undefined in one condition.

What is the difference between WHERE and HAVING?

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WHERE filters individual rows before grouping and cannot use aggregate functions. HAVING filters groups after GROUP BY and can use aggregates. For example, SELECT city, COUNT(*) FROM students WHERE status = 'active' GROUP BY city HAVING COUNT(*) > 50 first keeps only active students, then shows cities with more than 50 of them. Filtering in WHERE when possible is faster because fewer rows are grouped.

What is the difference between UNION and UNION ALL?

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Both combine the results of two SELECT queries with the same number and type of columns. UNION removes duplicate rows, which requires extra sorting or hashing work. UNION ALL keeps all rows, including duplicates, and is therefore faster. Use UNION ALL when you know results cannot overlap or duplicates are acceptable, for example combining this year's and last year's order tables.

What is the difference between InnoDB and MyISAM?

MySQL DatabaseMedium

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InnoDB is the default MySQL storage engine. It supports transactions, row-level locking, foreign keys and crash recovery, making it suitable for almost all applications. MyISAM uses table-level locking, has no transactions or foreign keys, and can corrupt on crashes. MyISAM was once faster for read-only workloads, but modern InnoDB matches or beats it. Old tables still on MyISAM should usually be converted.

What is the difference between DELETE, TRUNCATE and DROP?

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DELETE removes selected rows using a WHERE clause, can be rolled back inside a transaction and fires triggers, but is slow on large tables. TRUNCATE removes all rows quickly by recreating the table, resets AUTO_INCREMENT and cannot be rolled back in MySQL. DROP removes the table structure and data completely. Always take a backup and double-check the WHERE clause before running DELETE on production.

What is the difference between GET and POST methods in an HTML form?

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GET sends form data in the URL query string, so it is visible, bookmarkable, cached and limited in length; it suits search and filter forms. POST sends data in the request body, so it is not shown in the URL and can carry large or binary content such as file uploads. Any action that changes data on the server, like login, registration or payment, should use POST.

What are custom events and when would you use them?

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Custom events let components communicate without knowing about each other. You create one with new CustomEvent('cart:updated', { detail: { count: 3 }, bubbles: true }) and fire it with element.dispatchEvent. Any part of the page can listen with addEventListener('cart:updated'). For example, a product card can announce an add-to-cart action, and the header badge updates itself, keeping code loosely coupled.

Compare shared hosting, VPS and cloud hosting.

Hosting and DeploymentEasy

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Shared hosting puts many sites on one server with a control panel like cPanel; it is cheap and easy but offers limited resources and control. A VPS gives you a virtual machine with root access, dedicated RAM and CPU, so you install and secure the stack yourself. Cloud platforms like AWS, Azure and Google Cloud add auto-scaling, managed databases and pay-per-use pricing, suited to growing applications.

When should you use CHAR, VARCHAR and TEXT?

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CHAR(n) stores a fixed length and pads shorter values, which suits fixed codes like a 6-digit PIN code or two-letter state code. VARCHAR(n) stores variable length up to n characters with a small length prefix, ideal for names and emails. TEXT is for long content like blog bodies but cannot have a default value and is indexed only by prefix. Using right-sized types saves memory and improves index efficiency.

Why is the meta viewport tag needed for mobile pages?

HTML FundamentalsMedium

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Without a viewport meta tag, mobile browsers render the page at a virtual width of around 980 pixels and then zoom out, so text looks tiny. Adding <meta name='viewport' content='width=device-width, initial-scale=1'> tells the browser to use the real device width, which makes media queries work as expected. Avoid disabling user zoom with maximum-scale=1 because it hurts accessibility for users with low vision.

What is event delegation and why is it useful?

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Event delegation attaches one listener to a common parent instead of many listeners on child elements, then uses event.target.closest() to find which child was clicked. Because of bubbling, it works even for children added later. For example, one listener on a ul can handle delete buttons for hundreds of todo items. It saves memory and avoids re-binding listeners after dynamic updates.

Explain closures with an example.

JavaScript CoreMedium

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A closure is a function that remembers variables from the scope where it was created, even after that outer function has finished. For example, function counter() { let count = 0; return () => ++count; } returns a function that keeps its own private count. Closures are used for data privacy, function factories, debounce helpers and event handlers. Their trade-off is that captured variables stay in memory as long as the function exists.

Explain the values of the CSS position property.

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static is the default and follows normal flow. relative keeps the element in flow but lets you offset it, and makes it a reference for absolute children. absolute removes the element from flow and positions it against the nearest positioned ancestor. fixed positions against the viewport and stays during scroll. sticky behaves like relative until a scroll threshold, then sticks like fixed within its parent, which is ideal for table headers.

What is CORS and why does the browser block some API calls?

Web SecurityMedium

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The same-origin policy stops scripts on one origin from reading responses from another origin. CORS is how a server opts in: it returns headers like Access-Control-Allow-Origin naming allowed origins. For non-simple requests, such as JSON with custom headers, the browser first sends an OPTIONS preflight. CORS is enforced only by browsers, so it is not a security control for the server itself, and Allow-Origin * should never be combined with credentials.

What is the difference between echo and print in PHP?

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Both output strings. echo is a language construct with no return value and can take multiple comma-separated arguments, like echo $a, $b. print also is a construct but takes only one argument and returns 1, so it can be used inside expressions. echo is marginally faster and far more common. For formatted output, printf or sprintf are used, for example printf('%.2f', $price).

What is the difference between DOMContentLoaded and the load event?

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DOMContentLoaded fires when the HTML has been fully parsed and the DOM is built, without waiting for images, stylesheets or iframes. load fires on window only after every resource, including images, has finished loading. Most scripts that attach listeners should run on DOMContentLoaded, or simply use defer. load is useful when you need image dimensions or must start something after everything is ready.

What is Composer and how does PSR-4 autoloading work?

PHP ProgrammingMedium

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Composer is PHP's dependency manager. composer.json lists packages, composer.lock pins exact versions, and the vendor folder holds installed code. PSR-4 maps a namespace prefix to a folder, for example App\ to src/, so App\Models\User loads from src/Models/User.php automatically after you require vendor/autoload.php. This removes manual require statements. composer.lock should be committed so every environment installs identical versions.

What is the difference between sessions and cookies in PHP?

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A cookie stores data in the browser and is sent with every request, so users can see and edit it. A session stores data on the server and keeps only a session id in a cookie, usually PHPSESSID. Use $_SESSION for login state and cart data, after calling session_start(). Use cookies only for non-sensitive preferences, and mark session cookies HttpOnly, Secure and SameSite.

What are the differences between Apache and Nginx?

Hosting and DeploymentMedium

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Apache uses a process or thread based model and supports per-directory .htaccess files, which is convenient on shared hosting. Nginx uses an event-driven model that handles many concurrent connections with low memory, making it excellent for static files and as a reverse proxy. Nginx runs PHP through PHP-FPM and has no .htaccess, so rules go in central config. Many setups put Nginx in front of Apache or application servers.

What is the difference between px, em and rem units?

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px is a fixed unit. em is relative to the font size of the current element, so nested em values compound and can grow unexpectedly. rem is relative to the root html font size, usually 16 pixels, so 1.5rem is always 24 pixels unless the user changes browser settings. Using rem for font sizes and spacing respects user preferences and keeps scaling consistent across the site.

Explain the CSS box model.

CSS Layout and Responsive DesignEasy

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Every element is a rectangular box made of content, padding, border and margin from inside to outside. With the default box-sizing: content-box, the width you set applies only to the content, so padding and border increase the final size. Setting box-sizing: border-box makes width include padding and border, which makes layouts predictable. Most projects apply border-box to all elements in a global reset.

How did string and number comparison with == change in PHP 8?

PHP ProgrammingMedium

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Before PHP 8, comparing a number with a non-numeric string converted the string to a number, so 0 == 'abc' was true, which caused security bugs. Since PHP 8, if the string is not numeric, the number is converted to a string and compared as strings, so 0 == 'abc' is now false. Using === is still recommended because it checks type and value without any conversion.

Explain include, require, include_once and require_once.

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All four insert another PHP file. If the file is missing, include gives a warning and continues, while require throws a fatal error and stops the script. The _once versions skip the file if it was already loaded, which prevents 'cannot redeclare function' errors. Use require_once for essential files like config and database connection. Modern projects rely on Composer autoloading instead of manual includes for classes.

How is querySelector different from getElementById and getElementsByClassName?

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querySelector and querySelectorAll accept any CSS selector and return the first match or a static NodeList. getElementById accepts only an id and is slightly faster. getElementsByClassName returns a live HTMLCollection that updates automatically when the DOM changes, which can cause surprises inside loops. In practice querySelector is used for flexibility, and the speed difference rarely matters.

What is the difference between block-level and inline elements?

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Block-level elements such as div, p, h1 and section start on a new line and take the full available width by default. Inline elements such as span, a, strong and img sit inside a line of text and only take as much width as their content. Inline elements ignore vertical width and height settings, which is why developers often switch them to inline-block or block when they need sizing.

What does mobile-first CSS mean?

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Mobile-first means writing base styles for small screens and adding min-width media queries to enhance the layout for larger screens. Small devices then download and apply only simple rules, and the design is forced to prioritise essential content. For example, cards stack in one column by default, and @media (min-width: 768px) switches them to a two or three column grid.

What is the DOM?

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The Document Object Model is a tree representation of an HTML page that the browser builds after parsing. Each element, attribute and piece of text becomes a node that JavaScript can read and change through APIs like document.querySelector, element.classList and element.append. When the DOM changes, the browser updates what users see. The DOM is not the HTML source itself; scripts can change it after the page loads.

What is hoisting in JavaScript?

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Hoisting means declarations are processed before code runs, so they behave as if moved to the top of their scope. Function declarations are hoisted completely and can be called before they appear. var variables are hoisted but set to undefined until assignment. let, const and class are hoisted too, but accessing them before declaration throws a ReferenceError because of the temporal dead zone.

What are truthy and falsy values?

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In a boolean context, JavaScript converts values to true or false. The falsy values are false, 0, -0, 0n, empty string, null, undefined and NaN; everything else is truthy, including '0', empty arrays and empty objects. This matters in conditions: if (items.length) works, but if (count) fails when count is legitimately 0. The nullish coalescing operator ?? helps by only falling back for null or undefined.

What are the spread and rest operators?

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Both use three dots but do opposite jobs. Spread expands an iterable or object into individual elements, for example [...a, ...b] to merge arrays or {...user, city: 'Pune'} to copy and update an object. Rest collects multiple values into one, for example function sum(...nums) gathers all arguments into an array, or const { id, ...others } = user separates one property from the rest.

Explain the common DNS record types.

Hosting and DeploymentEasy

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An A record maps a domain to an IPv4 address and AAAA to IPv6. CNAME points one name to another, such as www to example.com, but cannot sit on the root domain. MX records route email to mail servers. TXT records hold verification and email security data like SPF, DKIM and DMARC. NS records name the authoritative nameservers. TTL controls how long resolvers cache each record.

What is the difference between null and undefined?

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undefined means a variable was declared but not assigned, a missing function argument, or a property that does not exist. null is an intentional empty value set by the developer, like clearing a selected user. typeof undefined is 'undefined', while typeof null is 'object', a long-standing quirk. APIs often return null for 'no data', and optional chaining like user?.address handles both safely.

What does the DOCTYPE declaration do in an HTML page?

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The DOCTYPE tells the browser which rendering mode to use. Writing <!DOCTYPE html> at the very top puts the browser in standards mode, so CSS box model and layout rules follow the modern specification. Without it, browsers fall back to quirks mode, which imitates old browser bugs and can make widths, margins and table layouts behave inconsistently across browsers.

Compare localStorage, sessionStorage and cookies.

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localStorage stores about 5 MB per origin and persists until cleared. sessionStorage has similar size but is cleared when the tab closes and is separate per tab. Cookies are small, around 4 KB, are sent to the server with every request, and can be HttpOnly, Secure and SameSite. Use cookies for session identifiers, and storage for UI preferences. Never store tokens or passwords in localStorage, since any XSS can read it.

How do srcset and sizes help with responsive images?

HTML FundamentalsMedium

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srcset lists several versions of the same image with their widths, for example hero-480.jpg 480w and hero-1200.jpg 1200w. sizes tells the browser how wide the image will be shown at different viewport widths. The browser then picks the smallest file that still looks sharp for that screen and pixel density. This saves a lot of mobile data compared to sending one large image to every device.

Explain INNER JOIN, LEFT JOIN and RIGHT JOIN.

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INNER JOIN returns only rows with matching values in both tables, such as students who have enrolled. LEFT JOIN returns all rows from the left table and matching rows from the right, filling NULL where there is no match, which is useful to find students with no enrollment. RIGHT JOIN is the mirror of LEFT JOIN. MySQL has no FULL OUTER JOIN, but you can simulate it with a UNION of LEFT and RIGHT joins.

How does CSS specificity work?

CSS Layout and Responsive DesignEasy

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Specificity decides which rule wins when several match the same element. It is calculated as a score: inline styles highest, then id selectors, then classes, attributes and pseudo-classes, then element and pseudo-element selectors. For example, #menu a beats .nav a. When specificity is equal, the rule written later wins. !important overrides normal rules, but overusing it creates maintenance problems, so keeping selectors flat with classes is preferred.

How do arrow functions differ from regular functions?

JavaScript CoreMedium

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Arrow functions have shorter syntax, an implicit return for single expressions, and no own this, arguments or super; they inherit these from the outer scope. They cannot be used as constructors with new and do not have a prototype property. This makes them ideal for callbacks like array.map, but unsuitable for object methods that need this to refer to the object, or for event handlers that rely on this as the element.

What is a CDN and how does it improve a website?

Hosting and DeploymentMedium

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A content delivery network caches static files such as images, CSS and JavaScript on servers in many locations, so a user in Chennai is served from a nearby edge rather than a distant origin. This reduces latency, lowers load on your server and absorbs traffic spikes and some DDoS attacks. Cache-Control headers decide how long files stay cached, and versioned filenames let you deploy updates without stale content.

What is the difference between a primary key and a foreign key?

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A primary key uniquely identifies each row in a table and cannot be NULL, for example students.id. A foreign key is a column in another table that refers to that primary key, such as enrollments.student_id, and enforces referential integrity so you cannot add an enrollment for a student who does not exist. Foreign keys can define ON DELETE CASCADE or RESTRICT behaviour. InnoDB supports foreign keys.

What is the difference between innerHTML, textContent and innerText?

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innerHTML reads or writes HTML markup, so inserting user input through it can run malicious scripts and cause XSS. textContent reads or writes raw text, including hidden elements, and is fast because it does not trigger layout. innerText returns only visible text as rendered, respects CSS, and triggers a reflow, so it is slower. For displaying user data, textContent is the safe default.

What is semantic HTML and why does it matter?