Item 37: Limit the Use of Optional Properties

要点

• Optional properties can prevent the type checker from finding bugs and can lead to repeated and possibly inconsistent code for filling in default values.
• Think twice before adding an optional property to an interface. Consider whether you could make it required instead.
• Consider creating distinct types for un-normalized input data and normalized data for use in your code.
• Avoid a combinatorial explosion of options.
• 可选属性可能会阻止类型检查器发现错误，并可能导致重复且可能不一致的代码来填充默认值。
• 在向接口添加可选属性之前，三思而后行。考虑是否可以将其改为必填属性。
• 考虑为未标准化的输入数据和标准化的数据创建不同的类型，以便在代码中使用。
• 避免选项的组合爆炸。

正文

interface FormattedValue {
  value: number
  units: string
}
function formatValue(value: FormattedValue) {
  /* ... */
}

💻 playground

---

interface Hike {
  miles: number
  hours: number
}
function formatHike({ miles, hours }: Hike) {
  const distanceDisplay = formatValue({ value: miles, units: 'miles' })
  const paceDisplay = formatValue({ value: miles / hours, units: 'mph' })
  return `${distanceDisplay} at ${paceDisplay}`
}

💻 playground

---

type UnitSystem = 'metric' | 'imperial'
interface FormattedValue {
  value: number
  units: string
  /** default is imperial */
  unitSystem?: UnitSystem
}

💻 playground

---

interface AppConfig {
  darkMode: boolean
  // ... other settings ...
  /** default is imperial */
  unitSystem?: UnitSystem
}

💻 playground

---

function formatHike({ miles, hours }: Hike, config: AppConfig) {
  const { unitSystem } = config
  const distanceDisplay = formatValue({
    value: miles,
    units: 'miles',
    unitSystem,
  })
  const paceDisplay = formatValue({
    value: miles / hours,
    units: 'mph', // forgot unitSystem, oops!
  })
  return `${distanceDisplay} at ${paceDisplay}`
}

💻 playground

---

declare let config: AppConfig
const unitSystem = config.unitSystem ?? 'imperial'

💻 playground

---

const unitSystem = config.unitSystem ?? 'metric'

💻 playground

---

interface InputAppConfig {
  darkMode: boolean
  // ... other settings ...
  /** default is imperial */
  unitSystem?: UnitSystem
}
interface AppConfig extends InputAppConfig {
  unitSystem: UnitSystem // required
}

💻 playground

---

function normalizeAppConfig(inputConfig: InputAppConfig): AppConfig {
  return {
    ...inputConfig,
    unitSystem: inputConfig.unitSystem ?? 'imperial',
  }
}

💻 playground