How to convert generic type array to specialized type array in typescript-CodePudding

I'm using firebase in my application. So, the snapshotChanges() method of firebase returning an Observable of type SnapshotAction array which I want to convert to specific type array after subscribing to an Observable. So, how can we do this in typescript?

Have a look at codebase below to understand it better,

// Below observable is returned by firebase snapshotChanges() method 
Observable<SnapshotAction<Product>[]>

// Now, after I subscribe to the above observable I get below type,
SnapshotAction<Product>[]

// Now I want to convert "SnapshotAction<Product>[]" to "Product[]" in typescript

CodePudding user response：

Snapshot action has a payload field which will give you a

DatabaseSnapshot<Product>

A DatabaseSnapshot has a val() method which will return a Product or null depending if the snapshot exists. So you can write a small function like this to extract the product:

function example(input: SnapshotAction<string>[]): (string | null)[] {
    return input.map(action => action.payload.val())
}

Note that this is return string | null because the snapshot might not exist. You could look at

action.payload.exists()

If you want to do additional checks or filtering.

CodePudding user response：

products: Product[] = [];

this.productService.getAll().pipe(
      switchMap((products: SnapshotAction<Product>[]) => {
      
        // Our intention starts here
        products.map(product => {
          let prod: Product = product.payload.val();
          prod.key = product.key;
          
          this.products.push(prod);
        });
        
      });
    );