Closure Compiler - Bad Type Annotation. Unknown type (with externs)

Question

Starting with the basics. I'm using the JavaScript Closure Compiler and got the error Bad Type Annotation. Unknown type VRPose Bad Type Annotation. Unknown type VRPose I was trying to have a function return a {VRPose} . ie

/**
 * @returns {VRPose}
 */
VRDisplay.prototype.getPose = function() {};

I did the obvious thing and tried to define the VRPose like:

function VRPose() {};

Unfortunately, this didn't work; I have the same error. How should I go about defining a type/class in Closure?

Answer 1

At first, I tried

goog.forwardDeclare('VRPose');

This actually worked fine. And maybe this was correct behavior for defining externs. But it seemed to be to sort of be skirting the problem.

I looked around for a bit more and found that I was missing the @constructor tag. So here's what I have now:

/** @constructor */
function VRPose() {};

I'm not really sure what the difference between these two is, so if anyone would like to answer the question and include a better explanation, I'd be happy to accept!