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The obscure `Function#length` property!

 2 years ago
source link: https://dev.to/lioness100/the-obscure-functionlength-property-32il
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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
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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
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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
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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
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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 ✌


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