Add tests for listenImage function, fix private variables initialization

test/spec/ol/image.test.js

@@ -0,0 +1,46 @@
+import {listenImage} from '../../../src/ol/Image.js';
+
+
+describe('HTML Image loading', function() {
+  let handleLoad, handleError, img;
+
+  beforeEach(function() {
+    handleLoad = sinon.spy();
+    handleError = sinon.spy();
+    img = new Image();
+  });
+
+  it('handles load event', function(done) {
+    img.src = 'spec/ol/data/dot.png';
+    listenImage(img, handleLoad, handleError);
+
+    setTimeout(function() {
+      expect(handleLoad).to.be.called();
+      expect(handleError).not.to.be.called();
+      done();
+    }, 200);
+  });
+
+  it('handles error event', function(done) {
+    img.src = 'invalid.jpeg';
+    listenImage(img, handleLoad, handleError);
+
+    setTimeout(function() {
+      expect(handleLoad).not.to.be.called();
+      expect(handleError).to.be.called();
+      done();
+    }, 200);
+  });
+
+  it('handles cancelation', function(done) {
+    img.src = 'spec/ol/data/dot.png';
+    listenImage(img, handleLoad, handleError)();
+
+    setTimeout(function() {
+      expect(handleLoad).not.to.be.called();
+      expect(handleError).not.to.be.called();
+      done();
+    }, 200);
+  });
+
+});