The obscure `Function#length` property!

#todayilearned #javascript #programming #tutorial

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 ✌

Top comments (9)

Jon Randy 🎖️ • Oct 15 '21 • Edited

Not that obscure - I was using it in my project just last week!

I had two different functions to run for attaching methods to objects - one for methods that took arguments, another for methods that didn't. The Function length property was very useful to create a convenience function to automatically pick the correct method adder.

It is also used in the commonly used function that will curry the passed function:

function curry(func) {

  return function curried(...args) {
    if (args.length >= func.length) {
      return func.apply(this, args);
    } else {
      return function(...args2) {
        return curried.apply(this, args.concat(args2));
      }
    }
  };

}

Lioness100 • Oct 15 '21

Oooh, genius! (And the project that you linked is super cool too!)

Jakub T. Jankiewicz • Oct 17 '21

This is very useful and not that obscure, you can use it for curry function. Or any meta/functional programming.

Here is another example of curry function with ES5:

Aditya • Oct 15 '21

Wow, I also have no idea where to use this, maybe you can use to find out the no. of agrs taken by a builtin function, who knows, or if you are lazy or crazy then you can use it.
:)