Property 'flatMap', 'flat' does not exist on type in TS

 项目TS 编译时，报以下错误

Property 'flatMap' does not exist on type 'Policy[]'. Do you need to change your target library? Try changing the 'lib' compiler option to 'es2019' or later.ts(2550)

错误提示很明了

解决方法：

To solve the "Property 'flatMap' or 'flat' does not exist on type" error, add the string "es2019" to the lib array in your tsconfig.json file and make sure you are running a recent version of TypeScript.

Open your tsconfig.json file and add es2019 to your lib array.

{
  "compilerOptions": {
    // ... other options
    "lib": [
      // ... other libs
      "es2019"
    ]
  }
}

　　

tsconfig.json 文件说明 参见官方文档

https://www.typescriptlang.org/tsconfig/