To select elements with a specific class name, you should use the querySelector() method. This method allows you to select elements using CSS-like selectors, including class names. For example, document.querySelector('.classname') selects all elements with the specified class name.

The data type of NaN in JavaScript is "Not a Number" (NaN). NaN is a special value representing an unrepresentable or undefined value in the context of numbers. It is not a regular number, undefined, or a string. Understanding the data type of NaN is important for handling unexpected mathematical operations in JavaScript.

Using a switch statement is often more performant when dealing with numerous conditions because it uses direct lookup tables, making it faster and more efficient than a series of if-else statements, which involve sequential comparisons.

If the condition in a while loop is always true, an infinite loop will occur. The code block will keep executing repeatedly, and the loop will never exit. This can lead to the program becoming unresponsive, and it's essential to ensure that the condition in a while loop eventually becomes false to prevent infinite loops.

The let keyword is used to declare a variable with block scope in JavaScript. Variables declared with let are limited to the block or scope in which they are defined, which is typically within a pair of curly braces {}.

Promises in JavaScript allow you to handle asynchronous operations more readably and reliably. Promises provide a structured way to work with asynchronous code, making it easier to manage tasks like fetching data from an API, performing operations, and updating the UI when the data is ready.

The keyword finally is used to specify a block of code that will be executed regardless of whether an exception is thrown or not in a try...catch block. It ensures that cleanup code is always executed.

If you omit the break statement in a switch case, JavaScript will execute the code for that case and continue to execute the code for the following case(s) until it encounters a break statement or reaches the end of the switch statement. This behavior is known as "fall-through."

The concept that allows JavaScript objects to inherit properties and behavior from an object of another class is known as "inheritance." Inheritance is a fundamental aspect of object-oriented programming and helps in code reusability.

To avoid iterating over prototype properties with a for...in loop, you should use the Object.keys method. This method returns an array of an object's own enumerable property names, allowing you to iterate over only the object's properties without including those from its prototype chain.

The method getElementById selects an element by its ID attribute. It returns the element with the specified ID attribute, providing a direct and efficient way to access an element in the DOM by its unique identifier.

A recursive function expression can be named, allowing it to refer to itself by name. This is useful in cases where a function needs to call itself during its execution, typically seen in solving recursive problems or implementing recursive algorithms.