Angular 2 return data from arrow function

Question

I have the following code

getEnquiryWithId(id: number) {
    let enquiryStore = this.db.transaction('enquiries', 'readonly').objectStore('enquiries');
    let index = enquiryStore.index('enquiries');
    let request = index.get(id);
    return request.onsuccess = (event) => {
      return event.target.result;
    }
  }

which i'm trying to use in a pipe

transform(value: number): string {
    let request = this.dbStore.getEnquiryWithId(value);
    let result = request();
  }

I get an error saying Cannot invoke an expression whose type lacks a call signature .

Note: that I cannot rely RxJs subjects here as my transform function needs to return something, as its a pipe. How do I make this work?

Answer 1

The title misleading. You don't want data to be returned from an arrow function (which is trivially possible), but you want your arrow function to be synchronously executed - which is not possible as it's a callback that will be called by the JS engine.

As others mentioned you can return a Promise in your function that resolves as soon as the the inner query succeeds:

getEnquiryWithId(id: number) {
  let enquiryStore = this.db.transaction('enquiries', 'readonly').objectStore('enquiries');
  let index = enquiryStore.index('enquiries');
  let request = index.get(id);
  return new Promise((resolve, reject) => {
    request.onsuccess = (event) => {
      resolve(event.target.result);
    };
    request.onerror = (event) => {
      reject(request.errorCode);
    };
  }
}

If you want to display the result of a promise you could simply use AsyncPipe . It's not necessary to write an own transform function.