Item 44: Prefer More Precise Variants of any to Plain any

要点

• When you use any, think about whether any JavaScript value is truly permissible.
• Prefer more precise forms of any such as any[] or {[id: string]: any} or () => any if they more accurately model your data.
• 当你使用 any 时，考虑是否真的允许任何 JavaScript 值。
• 如果更准确地描述数据，优先使用更精确的 any 形式，如 any[]、{[id: string]: any} 或 () => any。

正文

function getLengthBad(array: any) {
  // Don't do this!
  return array.length
}

function getLength(array: any[]) {
  // This is better
  return array.length
}

💻 playground

---

getLengthBad(/123/) // No error, returns undefined
getLength(/123/)
//        ~~~~~
// Argument of type 'RegExp' is not assignable to parameter of type 'any[]'.

getLengthBad(null) // No error, throws at runtime
getLength(null)
//        ~~~~
// Argument of type 'null' is not assignable to parameter of type 'any[]'.

💻 playground

---

function hasAKeyThatEndsWithZ(o: Record<string, any>) {
  for (const key in o) {
    if (key.endsWith('z')) {
      console.log(key, o[key])
      return true
    }
  }
  return false
}

💻 playground

---

function hasAKeyThatEndsWithZ(o: object) {
  for (const key in o) {
    if (key.endsWith('z')) {
      console.log(key, o[key])
      //               ~~~~~~ Element implicitly has an 'any' type
      //                      because type '{}' has no index signature
      return true
    }
  }
  return false
}

💻 playground

---

type Fn0 = () => any // any function callable with no params
type Fn1 = (arg: any) => any // With one param
type FnN = (...args: any[]) => any // With any number of params
// same as "Function" type

💻 playground

---

const numArgsBad = (...args: any) => args.length
//    ^? const numArgsBad: (...args: any) => any
const numArgsBetter = (...args: any[]) => args.length
//    ^? const numArgsBetter: (...args: any[]) => number

💻 playground