TypeScript型'承诺<void | object> ' 不可分配给类型 'Promise<object> ' 在 Promise.then()<div id="text_translate"><p> 我目前正在尝试以 base64 格式实现缓存文档的服务。 我希望这个服务从 sessionStorage 中获取文档，如果 sessionStorage 中没有文档，则从 IRequestService 中获取它，然后将其保存到 sessionStorage。 我尝试了这种方法，但出现类型错误</p><pre>Type 'Promise&lt;void | GetDocumentResult&gt;' is not assignable to type 'Promise&lt;GetDocumentResult&gt;'.</pre><p> 代码如下所示：</p><pre> getDocument(requestId: string, documentId: DocumentID, requestService: IRequestService): Promise&lt;GetDocumentResult&gt; { if (this.storageKey in sessionStorage) { const documentsMap: Map&lt;DocumentID, GetDocumentResult&gt; = new Map(JSON.parse(sessionStorage.getItem(this.storageKey).)) if (documentsMap.get(documentId).== undefined) { return new Promise(resolve =&gt; resolve(documentsMap,get(documentId).)) } } return requestService.getDocument(requestId, documentId) .then(value =&gt; {this.setDocument(documentId, value)}) }</pre><p> 我期待 Promise&lt;GetDocumentResult&gt;.then 的返回类型是 Promise&lt;GetDocumentResult&gt;，但由于某种我不明白的原因，它是 Promise&lt;void | 获取文档结果&gt;</p><p> 有人知道为什么会这样吗？</p><p> 谢谢</p></div></object></void>

Question

I am currently trying to implement a service for caching document in base64 format. I want this service to fetch a document from sessionStorage, and if there is no document in sessionStorage, fetch it from IRequestService and then save it to sessionStorage. I tried this approach, but I am getting a type error

Type 'Promise<void | GetDocumentResult>' is not assignable to type 'Promise<GetDocumentResult>'.

The code looks like this:

getDocument(requestId: string, documentId: DocumentID, requestService: IRequestService): Promise<GetDocumentResult> {
    if (this.storageKey in sessionStorage) {
      const documentsMap: Map<DocumentID, GetDocumentResult> = new Map(JSON.parse(sessionStorage.getItem(this.storageKey)!))
      if (documentsMap.get(documentId) !== undefined) {
        return new Promise(resolve => resolve(documentsMap.get(documentId)!))
      }
    }

    return requestService.getDocument(requestId, documentId)
      .then(value => {this.setDocument(documentId, value)})
}

I was expecting return type of Promise< GetDocumentResult >.then to be Promise< GetDocumentResult >, but for some reason which I dont understand, it's Promise<void | GetDocumentResult>

Do someone knows why this is happening?

Thank you

Answer 1

Most likely you have an issue because you are not returning anything inside your .then()

here:

 .then(value => {this.setDocument(documentId, value)})

and that makes your returned type be Promise<void> instead of Promise<GetDocumentResult>

final versions should be like here:

getDocument(requestId: string, documentId: DocumentID, requestService: IRequestService): Promise<GetDocumentResult> {
    if (this.storageKey in sessionStorage) {
      const documentsMap: Map<DocumentID, GetDocumentResult> = new Map(JSON.parse(sessionStorage.getItem(this.storageKey)!))
      if (documentsMap.get(documentId) !== undefined) {
        return new Promise(resolve => resolve(documentsMap.get(documentId)!))
      }
    }

    return requestService.getDocument(requestId, documentId)
      .then(value => { return this.setDocument(documentId, value)})
}

Answer 2

As already explained, your then callback does return void not the result. You're looking for

getDocument(requestId: string, documentId: DocumentID, requestService: IRequestService): Promise<GetDocumentResult> {
  if (this.storageKey in sessionStorage) {
    const documentsMap: Map<DocumentID, GetDocumentResult> = new Map(JSON.parse(sessionStorage.getItem(this.storageKey)!));
    const document = documentsMap.get(documentId);
    if (document !== undefined) {
      return Promise.resolve(document);
    }
  }

  return requestService.getDocument(requestId, documentId).then(value => {
    this.setDocument(documentId, value);
    return value;
//  ^^^^^^^^^^^^
  });
}

Btw, I would recommend to extract the cache lookup and storage ( setDocument ?) into a separate service, and to have that service keep the documentsMap in memory instead of parsing JSON every time getDocument is called.