Typescript 如果值未定义，设置属性的简短而清晰的语法

发布于05月10日

我只想在值未定义的情况下将属性设置为值，否则我不想设置属性.

我通常是这样处理这种情况的:

  const payload: UpdateDistributionCommandInput = {
    Id: cloudFrontId,
    DistributionConfig: {
      ...distribution.DistributionConfig,
      Aliases: {
        Items: modifiedAliasItems,
        Quantity: modifiedAliasItems.length
      }
    }
  }

  if (typeof distribution.ETag === "string") {
    payload.IfMatch = distribution.ETag;
  }

  const updateDistributionCommand = new UpdateDistributionCommand(payload)

我觉得有点冗长和凌乱.有没有更简短的语法来描述这个场景，或者用其他方式来描述这个场景？

推荐答案

如果payload.ifMatch被定义为ifMatch?: string，那么行为取决于exactOptionalPropertyTypes编译器选项.

考虑:

interface Payload {
  ifMatch?: string
}

With 100，这将起作用:

const payload: Payload = {
  ifMatch: undefined
}

我们也将:

const payload: Payload = {}
payload.ifMatch = undefined

With 100，你会发现错误:

const payload: Payload = {
  //  ^^^^^^^
  // Type '{ ifMatch: undefined; }' is not assignable to type 'Payload' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the types of the target's properties.
  //  Types of property 'ifMatch' are incompatible.
  //    Type 'undefined' is not assignable to type 'string'.(2375)
  ifMatch: undefined
}

payload.ifMatch = undefined
//^^^^^^^^^^^^^
// Type 'undefined' is not assignable to type 'string' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the type of the target.(2412)

Playground Link(在TS配置菜单中切换exactOptionalPropertyTypes)