Q:

{!GETRECORDIDS($ObjectType.TestObject__c) in Lightning Component

I have GETRECORDIDS in OnClick Javascript button and im trying to convert the OnClick Javascript button to Lightning Component. How do i do this ?

A:

Somewhat counter-intuitively, the technique I've seen most recommended is to create a Visualforce page based on a StandardSetController. Lightning Experience supports the passing of the selected record Ids to the page in this case.
Google for examples of this that fit with what you want to do with the Id values. A document that I found helpful is Converting JavaScript Buttons for Lightning Experience from a company called Bayforce.

