How to define this property in a typescript definition file

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Javascript

Typescript

I am attempting to include when.js (a public Promise library on github that can be found here) as a property of Cesium (a public 3-D modelling framework described here) in a Typescript definition file I have for the Cesium library. This is because I am trying to convert some Javascript files from a project that utilizes Cesium to Typescript, and the Cesium.js code that is referenced has the below code snippet included. I am at a loss at how to declare when.js in the d.ts file, however. It doesn't appear to be a class, and I don't think it's an interface. How should I go about this property - what kind of object is it?

(function(define) { 'use strict';
define('ThirdParty/when',[],function () {
	var reduceArray, slice, undef;

	//
	// Public API
	//

	when.defer     = defer;     // Create a deferred
	/* more when.otherProperties set here */

    /**
	 * Register an observer for a promise or immediate value.
	 *
	 * @param {*} promiseOrValue
	 * @param {function?} [onFulfilled]
	 * @param {function?} [onRejected]
	 * @param {function?} [onProgress]
	 * @returns {Promise}
	 */
	function when(promiseOrValue, onFulfilled, onRejected, onProgress) {
		// Get a trusted promise for the input promiseOrValue, and then
		// register promise handlers
		return resolve(promiseOrValue).then(onFulfilled, onRejected, onProgress);
	}
    /* more functions defined here, including a 'defer' function */

What I have tried:

Currently I can get code referencing this to work if I declare a class called "when" in the Cesium.d.ts file I have. The only problem is that in this case "when" is a constructor, but as is clearly defined in the code snippet, the function "when" returns a Promise.