How to get an array of user IDs in WordPress?

I'm having difficulty getting an array of my WordPress user's IDs. I'm using WP_User_Query. Here's what I've tried so far:

$array = new WP_User_Query( array( 'fields' => 'ID' ) );

var_dump( $array ); is giving me an object of type WP_User_Query. I was expecting an array of user IDs.

How can I get an array of user IDs in WordPress?

Ref: http://ift.tt/19sDZTo