javascript - Typescript 类构造函数生成的代码顺序

Question

我正在编写一个服务，但在运行它时出现以下错误。有问题的部分是:

import { Configuration } from "./Configuration";
import { BooksApplication } from "./application/BookApplication";
import { PersistenceBookRepository } from "./application/PersistenceBookRepository";
import { DatabaseAccessor } from "./database/DatabaseAccessor";

export class Context {
  public readonly configuration: Configuration;

  public constructor(configuration: Configuration) {
    this.configuration = configuration;
  }

  private readonly databaseAccessor = new DatabaseAccessor(
    this.configuration.databaseConfiguration
  );

  private readonly bookRepository = new PersistenceBookRepository(
    this.databaseAccessor
  );

  private readonly bookService = new BooksApplication(this.bookRepository);
}

错误:

TypeError: Cannot read property 'databaseConfiguration' of undefined
    at new Context (/sandbox/src/Context.ts:14:24)
    at Object.<anonymous> (/sandbox/src/index.ts:6:17)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:742:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:743:3)
error Command failed with exit code 1.

我检查了 Typescript 生成的代码，它看起来像这样:

// ...
var Context = (function () {
    function Context(configuration) {
        this.databaseAccessor = new DatabaseAccessor_1.DatabaseAccessor(this.configuration.databaseConfiguration);
        this.bookRepository = new PersistenceBookRepository_1.PersistenceBookRepository(this.databaseAccessor);
        this.bookService = new BooksApplication_1.BooksApplication(this.bookRepository);
        this.configuration = configuration;
    }
    return Context;
}());
// ...

如您所见，this.configuration = configuration; 位于末尾，因此 this.databaseAccessor = new DatabaseAccessor_1.DatabaseAccessor(this.configuration.databaseConfiguration); 失败。

我做错了什么或者 Typescript 中有错误吗？

我尝试更改类(class)成员的顺序，但结果相同。

我有一个项目在这里重现了这个问题:https://codesandbox.io/embed/644k5kwr4r

谢谢。

Answer 1

根据spec proposal，这实际上是正确的启动顺序。 1:

When field initializers are evaluated and fields are added to instances:

• Base class: At the beginning of the constructor execution, even before parameter destructuring.

• Derived class: Right after super() returns. (The flexibility in how super() can be called has led many implementations to make a separate invisible initialize() method for this case.)

TLDR:类字段首先执行，在构造函数主体之前。

不过，您可以将初始化移至构造函数中。

<小时/>

由于 TS 是 JS 的超集，因此它也必须符合 ES 规范。