diff --git a/src/services/MarkerService.ts b/src/services/MarkerService.ts
index 6c788d31..1bde6b3b 100644
--- a/src/services/MarkerService.ts
+++ b/src/services/MarkerService.ts
@@ -41,7 +41,7 @@ export class MarkerService {
       val.unmark();
       val.mark(term || this.prevTerm);
     });
-    this.prevTerm = term || this.prevTerm || '';
+    this.prevTerm = term || this.prevTerm;
   }
 
   unmark() {
diff --git a/src/services/__tests__/MarkerService.test.ts b/src/services/__tests__/MarkerService.test.ts
new file mode 100644
index 00000000..95b8168f
--- /dev/null
+++ b/src/services/__tests__/MarkerService.test.ts
@@ -0,0 +1,103 @@
+const markSpy = jest.fn();
+const unmarkSpy = jest.fn();
+
+class FakeMark {
+  mark = markSpy;
+  unmark = unmarkSpy;
+}
+
+jest.mock('mark.js', () => FakeMark);
+
+import { MarkerService } from '../MarkerService';
+
+describe('Marker service', () => {
+  let marker: MarkerService;
+  const element = document.createElement('span');
+
+  beforeEach(() => {
+    marker = new MarkerService();
+    markSpy.mockClear();
+    unmarkSpy.mockClear();
+  });
+
+  test('add element to Map', () => {
+    marker.add(element);
+    expect(marker.map.size).toBeGreaterThan(0);
+  });
+
+  test('delete element from Map', () => {
+    marker.add(element);
+    marker.delete(element);
+
+    expect(marker.map.size).toEqual(0);
+  });
+
+  test('addOnly: should unmark and remove old elements', () => {
+    const e1 = document.createElement('span');
+    const e2 = document.createElement('span');
+    const e3 = document.createElement('span');
+
+    marker.add(e1);
+    marker.add(e2);
+
+    marker.addOnly([element, e2, e3]);
+
+    expect(unmarkSpy).toHaveBeenCalledTimes(1);
+    expect(marker.map.size).toEqual(3);
+  });
+
+  test('unmark: should unmark all elements', () => {
+    const e1 = document.createElement('span');
+    const e2 = document.createElement('span');
+    marker.add(e1);
+    marker.add(e2);
+    marker.add(element);
+
+    marker.unmark();
+
+    expect(unmarkSpy).toHaveBeenCalledTimes(3);
+    expect(markSpy).not.toHaveBeenCalled();
+  });
+  test('clearAll: should unmark and remove all elements', () => {
+    const e1 = document.createElement('span');
+    const e2 = document.createElement('span');
+    marker.add(e1);
+    marker.add(e2);
+    marker.add(element);
+
+    marker.clearAll();
+
+    expect(unmarkSpy).toHaveBeenCalledTimes(3);
+    expect(markSpy).not.toHaveBeenCalled();
+    expect(marker.map.size).toEqual(0);
+  });
+
+  test('mark: should unmark and mark again each element', () => {
+    const e1 = document.createElement('span');
+    const e2 = document.createElement('span');
+    marker.add(e1);
+    marker.add(e2);
+    marker.add(element);
+
+    marker.mark('test');
+
+    expect(unmarkSpy).toHaveBeenCalledTimes(3);
+    expect(markSpy).toHaveBeenCalledTimes(3);
+    expect(markSpy).toHaveBeenCalledWith('test');
+    expect(marker.map.size).toEqual(3);
+  });
+
+  test('mark: should do nothing if no term provided', () => {
+    marker.add(element);
+    marker.mark();
+
+    expect(markSpy).not.toHaveBeenCalled();
+  });
+  test('mark: should save previous marked term and use it if no term is provided', () => {
+    marker.add(element);
+    marker.mark('test');
+    marker.mark();
+
+    expect(markSpy).toHaveBeenLastCalledWith('test');
+  });
+});