Function arity

The arity of a function is the number of arguments that the function takes. A unary function is a function that only takes a single argument:

function isNull<T>(a: T|null) {
    return (a === null);
}

Unary functions are very important in functional programming because they facilitate utilization of the function composition pattern.

A binary function is a function that takes two arguments:

function add(a: number, b: number) {
    return a + b;
}

Functions with two or more arguments are also important because some of the most common FP patterns and techniques (for example, partial application and currying) have been designed to transform functions that allow multiple arguments into unary functions.

There are also functions with three (ternary functions) or more arguments. However, functions that accept a variable number of arguments, known as variadic functions, are particularly interesting in functional programming, as demonstrated in the following code snippet:

function addMany(...numbers: number[]) {
    numbers.reduce((p, c) => p + c, 0);
}