Question

我的第一个reduce返回以下形式的表达式： a + (condition)? b : c，第二个reduce会返回等价（???）表达式：(condition)? a+b : a+c为什么它们的行为有所不同？

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)}`)

我对即将学习新东西感到很兴奋。这是怎么回事？

Answer 1

您的括号顺序错误。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)
}

您应该用括号将整个三元表达式括起来，否则将包含acc。

还要引用安德里亚斯的答案：

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

Answer 2

我认为这是因为三元运算在开始时是不正确的，并且没有获得初始值。代替加号（+），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

为什么这些三元表达式给出不同的结果？

2 个答案: