Question mark Typescript variable

In TypeScript, <name>?: <typename> a shorthand for <name>: <typename> | undefined.

This indicates to the type system that a symbol may contain a value of the indicated type or it may contain the value undefined (which is like null).

This is important when the (new in TypeScript 2) --strictNullChecks option is enabled. The documentation on Null- and undefined-aware types option is probably where you should start to understand why this is useful.