Function: length

The length data property of a Function instance indicates the number of parameters expected by the function.

A Function object’s length property indicates how many arguments the function expects, i.e., the number of formal parameters:

Only parameters before the first one with a default value are counted. A destructuring pattern counts as a single parameter. The rest parameter is excluded.

By contrast, arguments.length is local to a function and provides the number of arguments actually passed to the function.

The Function constructor is itself a Function object. Its length data property has a value of 1.