为什么这些三元表达式会产生不同的结果？

Question

我的第一个reduce返回一个表达式：a + (condition)? b : c，我的第二个reduce返回等价的（???）表达式：(condition)? a+b : a+c为什么它们的行为不同？

function addressLengthWrong(address) {
  let keys = ['street', 'city', 'state', 'zip']
  return keys.reduce((acc, key) => acc + (address[key]) ? address[key].length : 0, 0)
}

let address = {
  street: 'My Street',
  city: '',
  state: '',
  zip: ''
}
console.log(`wrong address len ${addressLengthWrong(address)}`)

address.zip = '01234'
console.log(`wrong address len ${addressLengthWrong(address)}`)

运行替代方案......

function addressLengthCorrect(address) {
  let keys = ['street', 'city', 'state', 'zip']
  return keys.reduce((acc, key) => (address[key]) ? acc + address[key].length : acc, 0)

}

address = {
  street: 'My Street',
  city: '',
  state: '',
  zip: ''
}
console.log(`correct address len ${addressLengthCorrect(address)}`)

address.zip = '01234'
console.log(`correct address len ${addressLengthCorrect(address)}`)

我有一种兴奋的感觉，我即将学习新的东西。这里发生了什么？

Answer 1

你的括号顺序是错误的。试试这个：

function addressLengthWrong(address) {
  let keys = ['street', 'city', 'state', 'zip']
  return keys.reduce((acc, key) => acc + (address[key] ? address[key].length : 0), 0)
}

你应该用括号包装整个三元表达式，否则将包括acc。

另外，引用Andreas的回答：

... + ... precedence: 13, ... ? ... : ... precedence: 4; the higher value 'wins'

Answer 2

我认为这是因为第一次操作中的三元操作不正确，并且没有达到初始值。而不是添加（+），在累加器上使用+ =。

let a = 1, b = 2
let c = 0

c + a === 1 ? a : b //  fails, c = 0

c += a === 1 ? a : b //  success, c = 1