The number of arguments a function expects/supports.

In JavaScript, the arity of a function can be determined using its length property.

As an example, the following function has an arity of 3.

const getFullName = (firstName, middleName, lastName) => {
  return `${firstName} ${middleName} ${lastName}`;

getFullName.length returns 3 accordingly.

This way, you can implement differing behaviour depending on the type of e.g. callback function that was passed. For example, express uses function arity to distinguish regular middleware functions from middleware functions for error handling: these are defined in the same way as other middleware functions, except that error handling functions have four arguments instead of three. The following example visualizes this logic.

const getMiddlewareType = (callbackFunction) => {
  if (callbackFunction.length === 4) {
    return 'Error handler';
  return 'Regular middleware';
// Returns "Error handler"
getMiddlewareType((err, req, res, next) => {
  /* NOOP */
// Returns "Regular middleware"
getMiddlewareType((req, res, next) => {
  /* NOOP */