Enforcing the type of the indexed members of a Typescript object

Question

I would like to store a mapping of string -> string in a Typescript object and enforce that all of the keys map to strings. For example:

var stuff = {};
stuff["a"] = "foo";   
stuff["b"] = "bar";   
stuff["c"] = false;   

Is there a way for me to enforce that the values must be strings (or whatever type..)?