JS: Function Length Property

By Xah Lee. Date: 2022-09-22. Last updated: 2026-02-06.

Every function has a own string property "length", including:

Arrow Function
user defined function using keyword function
Function 〔see JS: Function Constructor〕
Function.prototype 〔see JS: Function.prototype〕

Its value is total count of required parameters. (exclude parameters that have Argument Default Value, and exclude Rest Parameters)

// every function have a own property key named length

// arrow function
console.assert(Object.hasOwn((x) => x + 1, "length"));

// function defined by keyword function
console.assert(Object.hasOwn(function ff(x) {
 return x + 1;
}, "length"));

// also
console.assert(Object.hasOwn(Function, "length"));
console.assert(Object.hasOwn(Function.prototype, "length"));

// value of function length property is number of required arguments

console.assert((() => []).length === 0);
console.assert(((x) => [x]).length === 1);
console.assert(((x, y) => [x, y]).length === 2);

// param with default value is excluded
console.assert(((x, y = 4) => [x, y]).length === 1);

// rest param is excluded
console.assert(((x, ...y) => [x, y]).length === 1);