What does ?: mean in Typescript?

I found the following in a TypeScript declaration file for Angular:

interface IDirective{ compile?: (templateElement: IAugmentedJQuery, 

What does the ?: after compile do?

2 Answers

See: Walkthrough: Interfaces | TypeScript :: Describing Simple Types.

Basically, ? marks the member as being optional in the interface.

(EDIT: As noted in comments, this is not restricted to interfaces.)

In this case, the ?: is not a single operator, but rather two operators:

• ? (optional),

• : (specify type).

In other languages/cases, ?: would be the Elvis Operator.