功能：length

¥Value

一个号码。

¥A number.

¥Description

Function 对象的 length 属性指示函数需要多少个参数，即形式参数的数量。该数字不包括 rest parameter，并且仅包括第一个具有默认值的参数之前的参数。相比之下，arguments.length 是函数的本地函数，提供实际传递给函数的参数数量。

¥A Function object's length property indicates how many arguments the function expects, i.e. the number of formal parameters. This number excludes the rest parameter and only includes parameters before the first one with a default value. By contrast, arguments.length is local to a function and provides the number of arguments actually passed to the function.

Function 构造函数本身就是一个 Function 对象。其 length 数据属性的值为 1。

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

由于历史原因，Function.prototype 本身就是一个可调用的。Function.prototype 的 length 属性的值为 0。

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

¥Examples

¥Using function length

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

¥See also

• Function