concat value of a resolved promise

Question

Task: I have a function playlist.expand() that returns a promise with data like [Item1, Item2, Item3]. I have a collection of these playlists and want to concat the data returned from all expand() functions and return it as a flat array. However I am stuck in the following code. I am hoping a few extra pair of eyes could help me spot the error:

Code:

breakdownPlaylist() {
    return this.playlists.reduce( (acc, playlist) => {
        acc.push( playlist.expand().then(result => result.collection));
        return acc;
    }, []);
}

$onInit() {
    console.log('test', this.breakdownPlaylist());
}

Issue: test is equal to [] at the time of execution. I can't understand why.