Typescript casting object's property

问题

I'm working with indexeddb and typescript. My issue is that TS doesn't seem to be able handle the event.target.result property. Case in point:

request.onsuccess = (event) => {
    namespace.db = event.target.result; //the property 'results' does not 
                                        //exist on the value of type 'EventTarget'
    var a = event.target;
    var b = <IDBOpenDBRequest>a;
    var c = b.result; // <-- magically there's a results property here

    version = parseInt(namespace.db.version);
    console.log("version: " + version);
    deferred.resolve();
}

So my question is: Is there an easier way to cast the target property to <IDBOpenDBRequest> other then the a, b method above?

回答1:

If you are looking for a oneliner you can cast it by adding some extra parenthesis like so:

indexedDB.open("test", 1).onsuccess = (ev) => {
    var result: IDBDatabase = (<IDBOpenDBRequest>ev.target).result;
}

Also notice the : IDBDatabase because result is typed as any in the Typescript definition file. It isn't needed but using it as an "any" type would mean no typechecking by the compiler.

Now you can use the result like you want with the methods available as defined here: http://www.w3.org/TR/IndexedDB/#database-interface

来源：https://stackoverflow.com/questions/27490535/typescript-casting-objects-property