Javascript Function.length

The JavaScript Function length property returns the number of parameters expected by a function.

The syntax to access the length property is:


Here, func is a function.

Return value from Function.length

  • Returns the number of formal parameters of the given function.

Example: Using Function.length

function func() {}
console.log(func.length); // 0

function func1(a, b) {}
console.log(func1.length); // 2

function func2(...args) {}
console.log(func2.length); // 0 -> Rest parameters are not counted

function func3(a, b = 10, c) {}
// only parameters before the one with default value are counted
console.log(func3.length); // 1



Note: The length property excludes the rest parameters and only counts parameters until the first one with a default value. In this case, func3.length returns 1 and skips b(has default value) & c(comes after the default value).