The length data property of a Function instance indicates the number of parameters expected by the function.
Value
A number.
Description
A Function object's length property indicates how many arguments the function expects, i.e. the number of formal parameters. This number excludes the rest parameter and only includes parameters before the first one with a default value. 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.
Due to historical reasons, Function.prototype is a callable itself. The length property of Function.prototype has a value of 0.
Examples
Using function length
console.log(Function.length); // 1
console.log((() => {}).length); // 0
console.log(((a) => {}).length); // 1
console.log(((a, b) => {}).length); // 2 etc.
console.log(((...args) => {}).length);
// 0, rest parameter is not counted
console.log(((a, b = 1, c) => {}).length);
// 1, only parameters before the first one with
// a default value are counted