ES6 结构赋值+默认参数 的funtion为什么报错？({a=1}) => console.log(a)

Question

const fn1  = ({a=1}) => console.log(a)
fn1()

错误信息：

const fn1  = ({a=1}) => console.log(a)
               ^

TypeError: Cannot read property 'a' of undefined

请问这是为什么呢？ 怎么改写比较合适呢？

Answer 1

const fn1 = ({a=1} = {}) => console.log(a)
fn1()

const fn1 = ({a=1}) => console.log(a)
fn1({})

Answer 2

解构赋值默认值结合使用