映射数组时如何解决“隐式具有“任何”类型的问题？

Question

我正在尝试像这样映射数组：

const formatted = results.map(item => item.payload)

但是当我将鼠标悬停在“项目”上时，它隐含地具有“任何”类型。我用打字稿知道了，我应该为这个函数定义参数，所以我这样尝试：

interface IItem {
   alert_id: null
   event_type: string
   id: string
   payload: string
   timestamp: string
}

const formatted = results.map(item: IItem => item.payload)

一个项目基本上由字符串和一些返回为空的项目组成：

{
   alert_id: null
   event_type: "test"
   id: "sd09f82-f8asf"
   payload: "{'test': 'accepted'}"
   timestamp: "2019-12-10"
}

我得到了所有这些错误

    return new TSError(diagnosticText, diagnosticCodes)
           ^
TSError: ⨯ Unable to compile TypeScript:
src/application.ts:31:11 - error TS6196: 'IItem' is declared but never used.

31 interface IItem {
             ~~~~~
src/application.ts:65:54 - error TS7006: Parameter 'IItem' implicitly has an 'any' type.

65     const formatted = results.map(item: IItem => item.payload)
                                                        ~~~~~
src/application.ts:65:63 - error TS2532: Object is possibly 'undefined'.

65     const formatted = results.map(item: IItem => item.payload)
                                                                 ~~~~~
src/application.ts:65:69 - error TS2339: Property 'payload' does not exist on type 'Event'.

65     const formatted = results.map(item: IItem => item.payload)
                                                                       ~~~~~~~
src/application.ts:65:52 - error TS1005: ',' expected.

65     const formatted = results.map(item: IItem => item.payload)

Answer 1

将要映射的参数括在括号中

const formatted = results.map((item: IItem) => item.payload).