Today I found out about another super cool Javascript feature that I'll never use in my life, and I'm here to share it with you! Introducing Function.prototype.length.

// there are 2 expected arguments
function foo(bar, baz) {
  // ...
}

foo.length; // so 2 is outputted

It's as simple as that! The length property exposes the amount of arguments expected by the function in question.

Specifications

Rest parameters are not included in the final count!

function foo(bar, ...baz) {
  // ...
}

foo.length; // 1 - rest parameters are not counted

Also, only parameters before a parameter with a default value are counted.

function foo(bar, baz = true, foobar) {
  // ...
}

foo.length; // 1 - only parameters before one with a default value

What's the difference between arguments.length and Function#length?

As I described above, Function#length will show how many parameters are expected in a function. However, arguments.length (when used inside the function) will show how many were actually passed, regardless of whether they were expected.

function foo(bar, baz) {
  return arguments.length;
}

foo.length; // 2 - expects `bar` and `baz`
foo(1, 2, 3, 4, 5); // 5 - five arguments were actually passed

Use Cases

You tell me! I have no idea 🤣

I hope you learned a bit about the Function#length property! If you have any questions, corrections, or addons, I would love to hear them. Peace ✌