A common cause of `getElementById()` returning null is that the script executes before the HTML element with that ID has been parsed and added to the DOM. Placing scripts in the without waiting for the DOM to be fully loaded (e.g., using `window.onload` or placing scripts at the end of the ) is a classic source of this error. The `getElementById()` method itself is very widely and consistently supported.

The selector `’#content p’` is the most efficient and correct. The `#` symbol signifies an ID selector. Since IDs must be unique in the DOM, there is no need to prefix it with the element type (`div`). The `>` child combinator in option D would only select direct child paragraphs, not all descendant paragraphs, which may not be the intended behavior. Option B is concise and targets all

elements within the element with the ID ‘content’.

The core issue is that mobile browsers simulate mouse events (like `click`) from touch events, but there is often a 300-350ms delay and the behavior can be inconsistent. The most professional and robust solution is to use a library (like Hammer.js) or write code that listens to both touch (`touchstart`, `touchmove`) and mouse (`mousedown`, `mousemove`) events, abstracting them into a unified ‘drag’ or ‘swipe’ logic. This provides a consistent user experience across all input methods.

The Fetch API is a modern, native browser API for making HTTP requests. It uses Promises, which makes asynchronous code cleaner and easier to manage than the callback-based `XMLHttpRequest`. While Axios is a popular promise-based library and jQuery.ajax is a legacy method, the Fetch API is the current standard built into JavaScript and is a core topic for the CIW JavaScript Specialist exam.

The `Array.reduce()` method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value. It is the perfect tool for operations like summing numbers, concatenating strings, or flattening arrays. `map()` transforms each element, `filter()` creates a new array with elements that pass a test, and `forEach()` simply executes a function for each element but does not inherently combine results.

An Immediately Invoked Function Expression (IIFE) is a function that is defined and executed immediately. Variables declared inside an IIFE are scoped to that function and are not added to the global object, effectively creating a private namespace. This was a very common pattern before the widespread adoption of native modules (ES6 Modules) to prevent variable collisions and is a fundamental concept for managing scope.

A very common syntax error is writing `addEventListener(‘click’, myFunction())`. The parentheses `()` immediately invoke the function and pass its *return value* (often `undefined`) as the handler, instead of passing the function reference itself. The correct syntax is `addEventListener(‘click’, myFunction)` (no parentheses). Event names for `addEventListener` do not use the ‘on’ prefix, so `’click’` is correct.

The HTML5 History API (specifically `history.pushState()` and `history.replaceState()`) allows web applications to add and modify history entries, and change the URL displayed in the browser’s address bar, without triggering a page reload. This is essential for creating a navigable single-page application where the back/forward buttons work as users expect.

Web Workers allow you to run JavaScript in background threads, separate from the main execution thread. This means long-running scripts can perform tasks without interfering with the user interface’s responsiveness. While `async/await` and Promises help manage asynchronous operations, they still run on the main thread. `setTimeout` simply defers execution but does not run it concurrently on a separate thread.

A common cause of `getElementById()` returning null is that the script executes before the HTML element with that ID has been parsed and added to the DOM. Placing scripts in the without waiting for the DOM to be fully loaded (e.g., using `window.onload` or placing scripts at the end of the ) is a classic source of this error. The `getElementById()` method itself is very widely and consistently supported.0