gistfile1.js

/*
 We have two classes: resultRenderer and dbQuery
 resultRenderer will ask dbQuery to perform a database query, and asyncronously receives the results
*/

function resultRenderer(query) { this._query = query }

resultRenderer.prototype = {
	queryResultsAndRender: function () {
		this._query.runQueryAsync(this.renderResults.bind(this));
	},
	
	renderResults: function (results) {
		// now render them!
	}
}

function dbQuery() {}
dbQuery.prototype = {
	runQueryAsync: function (callback) {
		// query the database, and callback once we have results:
		callback(queryResults);
	}
}

// to test resultRenderer using a mocked dbQuery, we write something like:

query = new dbQuery;
spyOn(dbQuery, 'runQueryAsync');

renderer = new resultRenderer(query);
renderer.queryResultsAndRender();

// expect that our async query method was called, and supplied with an appropriate callback
expect(dbQuery.runQueryAsync).toHaveBeenCalledWith(/* What goes here? */);