如何将泛型类型数组转换为 typescript 中的专用类型数组

Question

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

Answer 1

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.

Answer 2

 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); }); }); );