What defines a pure function in JavaScript?

A pure function in JavaScript is defined by its characteristics of returning the same output for the same input and not causing any side effects. This means that when you provide the same arguments to the function, it will consistently produce the same result, making it predictable and reliable. Additionally, a pure function does not modify any external state or variables, which ensures that it does not have unintended interactions with other parts of the program.

This predictability is crucial for functional programming paradigms, as it allows for easier testing, debugging, and reasoning about the behavior of the code. When functions do not depend on external states or cause changes to them, it minimizes complexity and enhances maintainability.

Given these criteria, the other options do not align with the definition of a pure function. A function being always asynchronous does not pertain to its purity. A function without parameters does not relate to the concept of purity, as it can still be pure even with parameters if it adheres to the required outputs and side effects. Likewise, a function that modifies external variables contradicts the essence of purity since changes to external state introduce side effects.