subfunction

A subfunction is a function defined within another function or scope to perform a small, well-scoped task. The concept is most often associated with programming languages that support nested or inner functions, such as Python, JavaScript, and Lisp. A subfunction can typically access variables from its enclosing scope, forming a closure, and is usually not accessible from outside the outer function unless it is explicitly exposed or returned.

In typical use, subfunctions help break complex logic into simpler parts, improve readability, and reduce duplication by localizing helper code. They may reference non-local variables from the outer function, enabling short-term state to be carried without passing everything through function arguments. Because of this encapsulation, subfunctions also help prevent name clashes by keeping helper identifiers local to the enclosing function.

In languages that do not support nested definitions, a closely related concept is a private helper function or subroutine defined at a broader scope. These serve a similar purpose of modularizing implementation details, but they are not defined within the local scope of another function.

In mathematics and formal analysis, the term subfunction is less standardized. It can be used informally to refer to a function defined on a subset of a domain or as a component function in a decomposition of a larger function, though this usage varies and is not a formal designation.

Example (Python): def outer(x): def inner(y): return y*y; return inner(x) + 1. This illustrates a subfunction defined inside another function, illustrating scope and closure behavior.