typescript - TypeScript-为什么在编译时不知道设置为已定义对象的静态属性要定义？

Question

我正在尝试访问类的静态属性。尝试编译时出现错误Object is possibly 'undefined'。为什么设置为已定义对象的静态属性在编译时未知？
我在自己的文件中定义了三个类:
DirectoryContents.ts

export class DirectoryContents
{
    public readonly directories: ReadonlyArray<string>;

    public readonly files: ReadonlyArray<string>;

    public constructor(directories: ReadonlyArray<string>, files: ReadonlyArray<string>)
    {
        this.directories = directories;
        this.files = files;
    }
}

RootDirStructure.ts

01| import {DirectoryContents} from "./DirectoryContents";
02| import ReadOnlyDict = NodeJS.ReadOnlyDict;
03|
04| export class RootDirStructure
05| {
06|     private static readonly REQUIRED_NPM_DIRECTORIES: ReadonlyArray<string> =
07|         ["node_modules"];
08|
09|     private static readonly REQUIRED_NPM_FILES: ReadonlyArray<string> =
10|         ["package.json", "package-lock.json"];
11|
12|     // RootDirStructure.required.npm set to defined object
13|     public static readonly required: ReadOnlyDict<DirectoryContents> =
14|         {
15|             npm: new DirectoryContents(RootDirStructure.REQUIRED_NPM_DIRECTORIES, RootDirStructure.REQUIRED_NPM_FILES)
16|         }
17| }

索引

01| import {RootDirStructure} from "./RootDirStructure";
02|
03| class Index
04| {
05|     // The following reference to RootDirStructure.required.npm causes error
06|     const requiredNPMDirectories: ReadonlyArray<string> = RootDirStructure.required.npm.directories;
07| }

在RootDirStructure.ts的第15行，其静态属性被设置为已定义的对象。但是，当在Index.ts的第6行上引用该对象时，它将创建Object is possibly 'undefined'错误。
由于它是一个静态属性，其值应在编译时知道，为什么设置为已定义对象的静态属性在编译时不被定义？

Answer 1

我可以通过重写RootDirStructure.ts来修复它。
我将静态属性RootDirStructure.required设置为静态内部类，而不是类型为ReadOnlyDict<DirectoryContents>的属性，所以现在RootDirStructure.ts看起来像:

import {DirectoryContents} from "./DirectoryContents";

export class RootDirStructure
{
    private static readonly REQUIRED_NPM_DIRECTORIES: ReadonlyArray<string> =
        ["node_modules"];
    private static readonly REQUIRED_NPM_FILES: ReadonlyArray<string> =
        ["package.json", "package-lock.json"];
    // RootDirStructure.required.npm set to defined object
    public static readonly required = class
    {
        public static readonly npm: DirectoryContents = new DirectoryContents(RootDirStructure.REQUIRED_NPM_DIRECTORIES, RootDirStructure.REQUIRED_NPM_FILES);
    }
}

我仍然很感兴趣，想知道为什么以前我得到了错误。我更喜欢原始语法。