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Showing content from http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/length below:

Function: length - JavaScript | MDN

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The length data property of a Function instance indicates the number of parameters expected by the function.

function func1() {}

function func2(a, b) {}

console.log(func1.length);
// Expected output: 0

console.log(func2.length);
// Expected output: 2

A number.

A Function object's length property indicates how many arguments the function expects, i.e., the number of formal parameters:

• Only parameters before the first one with a default value are counted.
• A destructuring pattern counts as a single parameter.
• The rest parameter is excluded.

By contrast, arguments.length is local to a function and provides the number of arguments actually passed to the function.

The Function constructor is itself a Function object. Its length data property has a value of 1.

Due to historical reasons, Function.prototype is a callable itself. The length property of Function.prototype has a value of 0.

console.log(Function.length); // 1

console.log((() => {}).length); // 0
console.log(((a) => {}).length); // 1
console.log(((a, b) => {}).length); // 2 etc.

console.log(((...args) => {}).length);
// 0, rest parameter is not counted

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

console.log((({ a, b }, [c, d]) => {}).length);
// 2, destructuring patterns each count as
// a single parameter

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