What is a function expression in JavaScript?

A function expression in JavaScript is a way to define a function that is typically assigned to a variable. This allows the function to be treated as a first-class citizen, meaning it can be passed around like any other value, assigned to other variables, or even returned from other functions.

Function expressions can be anonymous (without a name) or named (where the function has a name). When you assign a function to a variable, you can invoke the function using that variable name. This is a core concept in JavaScript, as it facilitates functional programming patterns, such as callbacks and closures.

The other choices relate to different concepts. The immediately invoked function expression (IIFE) describes a distinct pattern used to run a function as soon as it is defined, but it's not the general definition of a function expression. A function can exist without a return statement, but that does not qualify it as a function expression; it's simply a function that does not return a value. Parallel processing isn't related to function expressions; it pertains to performance optimizations that allow multiple processes to run concurrently, which is a separate topic in programming.