Free 030-100 practice test: 30 questions with answers

const prevents reassigning the variable itself, but the object it refers to remains mutable, so the property update succeeds. The other options confuse const with Object.freeze, which is a separate mechanism that makes an object's properties read-only.

typeof null returns the string object, a long-standing quirk of JavaScript that cannot be changed without breaking existing code. Null is checked with an explicit comparison such as value === null, because undefined and boolean are not what typeof reports for it.

The loose equality operator converts the string '0' to a number before comparing, so 0 == '0' is true. Strict equality also compares types, so 0 === '0' is false, null and undefined are different values, and NaN is never equal to itself.

filter builds and returns a new array containing only the elements that pass the test, and the source array is untouched. splice, push and sort all modify the original array in place, so they do not produce a filtered copy without changing the source.

A closure is a function bundled with references to the variables of its enclosing scope, so it can use them after the outer function returns. Hoisting moves declarations up in their scope, destructuring unpacks values from structures, and debouncing delays repeated calls.

A Set rejects duplicate values, and a Map iterates its entries in the order they were inserted. Map keys can be of any type, and a Set holds only values with no key-value pairs and no index access.

The title element in the head provides the document title that browsers show in tabs and use in bookmarks. h1 is a visible heading in the body, meta supplies machine-readable metadata such as the character set, and header is a body-level element for introductory content.

nav, article and footer are semantic elements that describe the role of each block, which assistive technology and search engines can use. Generic div and span elements carry no meaning and need classes to convey structure, and paragraphs with bold and italic tags describe appearance rather than role.

The alt attribute supplies the text alternative that screen readers announce when the image cannot be seen or loaded. src gives the image location, href belongs to anchor and link elements, and title creates a tooltip that is not a reliable replacement for a text alternative.

defer downloads the script in parallel and runs it after parsing finishes, in document order. async runs each script as soon as it downloads, so the order is not guaranteed, and the other two options do not change when the script runs.

The viewport meta element with width=device-width sets the layout width to the device width, which is the basis for responsive design. The charset meta element declares the text encoding, a stylesheet link loads CSS, and a title element only sets the document title.

A label's for attribute matches the id of the control, which gives the input an accessible name. A placeholder is not a reliable label, wrapping the input in a div creates no association, and a value attribute pre-fills the field rather than naming it.

#hero p has one id and one element (1,0,1), while p.intro has one class and one element (0,1,1), and an id outranks any number of classes, so red wins. Source order only decides between rules of equal specificity, and conflicting rules never cancel each other out.

border-box makes width and height include padding and border, while content-box, the default, adds them on top of the declared width. Neither padding-box nor margin-box is a box-sizing value, since padding-box belongs to background properties.

Adjacent vertical margins of block boxes collapse into one margin equal to the larger value, so the gap is 30px. A 50px gap is what happens when margins do not collapse, such as horizontal margins, and the other options either ignore the second margin or invent a rule that margins cancel.

rem is relative to the root element's font size, so it scales consistently across the page. em is relative to the font size of the current element or its parent, vw is a percentage of the viewport width, and px is a fixed absolute length.

display: flex on the list makes its direct children flex items, which line up in a row by default. display: block keeps the stacked layout, display: none removes the list from rendering, and visibility: hidden hides it while keeping its space.

color and font-family are inherited, so children take them unless they declare their own values. margin, border-width and padding are not inherited, so each element sets its own box spacing and borders.

module.exports publishes a value from a CommonJS module, and require loads it where it is needed. Copying code duplicates it and makes changes hard to maintain, while window and script tags belong to the browser and are not part of Node.js module loading.

package.json holds the project's name, version, scripts and dependencies, which npm reads during npm install. index.js is typically an entry-point script, node_modules holds installed packages, and README.md documents the project for people.

The fs module offers file system operations such as readFile and writeFile. The http module creates servers and clients, the path module manipulates file path strings, and the os module reports information about the operating system.

readFileSync does not return until the file is fully read, so the single JavaScript thread cannot handle other events in the meantime. The other options invent limits or threading behavior that Node.js does not have, and strings can hold file contents without problems.

Node.js executes JavaScript on the V8 engine outside the browser, and npm install installs the dependencies declared in package.json. Node.js is a server-side runtime rather than a browser extension, it does not require TypeScript, and it runs on Linux, macOS and Windows.

Values named in a route path with a colon are placed on req.params, so req.params.id holds the string '42'. req.query holds values after the question mark, req.body holds parsed request payloads, and res.locals holds data for the response and its templates.

POST is used to submit data that creates or changes server-side state, and it is neither safe nor idempotent, so repeating it can create duplicates. GET is safe and idempotent, while PUT and DELETE are idempotent, so repeating them leaves the same final state.

201 Created signals success and that a new resource now exists, often with a Location header pointing to it. 200 OK only reports success without naming creation, 204 No Content means success with no response body, and 301 means the resource has moved to another address.

Three-tier architecture separates the presentation, application logic and data layers. Peer-to-peer networks have equal nodes rather than layers, a single-file script has no separation at all, and batch processing describes how work is scheduled rather than how the system is layered.

Small, well-described commits and release tags both preserve the history of changes, so earlier states can be inspected or restored. Renaming files and keeping only the latest copy lose the record of earlier versions, and deleting history removes the ability to roll back.

Secrets should be supplied at runtime from the environment or a secrets store so they never sit in version control. Base64 is an encoding that anyone can reverse, and moving or renaming the code only hides the value from casual reading without protecting it.

Cookies or session identifiers sent with each request allow the server to associate later requests with the same visitor and stored cart. Longer paths, larger documents and faster connections do not give the server any way to identify a returning client.