diff --git a/js/src/lang.js b/js/src/lang.js
index f5e899d..f1b0e4f 100644
--- a/js/src/lang.js
+++ b/js/src/lang.js
@@ -17,6 +17,8 @@
  * along with this program; if not, see <http://www.gnu.org/licenses/>.
  */
 
+import uUtils from './utils.js';
+
 export default class uLang {
   constructor() {
     this.strings = {};
@@ -35,10 +37,18 @@ export default class uLang {
     }
   }
 
-  _(name) {
+  /**
+   * @param {string} name
+   * @param {...(string|number)=} params Optional parameters
+   * @return {string}
+   */
+  _(name, ...params) {
     if (typeof this.strings[name] === 'undefined') {
       throw new Error('Unknown localized string');
     }
+    if (params.length) {
+      return uUtils.sprintf(this.strings[name], ...params);
+    }
     return this.strings[name];
   }
 
diff --git a/js/src/mapapi/api_gmaps.js b/js/src/mapapi/api_gmaps.js
index 0190dcf..6bc7b11 100644
--- a/js/src/mapapi/api_gmaps.js
+++ b/js/src/mapapi/api_gmaps.js
@@ -74,7 +74,7 @@ export default class GoogleMapsApi {
       };
       window.gm_authFailure = () => {
         GoogleMapsApi.authError = true;
-        let message = uUtils.sprintf($._('apifailure'), 'Google Maps');
+        let message = $._('apifailure', 'Google Maps');
         message += '<br><br>' + $._('gmauthfailure');
         message += '<br><br>' + $._('gmapilink');
         if (GoogleMapsApi.gmInitialized) {
diff --git a/js/src/mapviewmodel.js b/js/src/mapviewmodel.js
index a8cd613..1f293e6 100644
--- a/js/src/mapviewmodel.js
+++ b/js/src/mapviewmodel.js
@@ -90,7 +90,7 @@ export default class MapViewModel extends ViewModel {
     this.api.init()
       .then(() => this.onReady())
       .catch((e) => {
-        let txt = uUtils.sprintf($._('apifailure'), apiName);
+        let txt = $._('apifailure', apiName);
         if (e && e.message) {
           txt += ` (${e.message})`;
         }
@@ -181,7 +181,7 @@ export default class MapViewModel extends ViewModel {
         ${(pos.altitude !== null) ? `<img class="icon" alt="${$._('altitude')}" title="${$._('altitude')}" src="images/altitude_dark.svg">${$.getLocaleAltitude(pos.altitude, true)}<br>` : ''}
         ${(pos.accuracy !== null) ? `<img class="icon" alt="${$._('accuracy')}" title="${$._('accuracy')}" src="images/accuracy_dark.svg">${$.getLocaleAccuracy(pos.accuracy, true)}${provider}<br>` : ''}
         </div>${stats}</div>
-        <div id="pfooter">${uUtils.sprintf($._('pointof'), id + 1, count)}</div>
+        <div id="pfooter">${$._('pointof', id + 1, count)}</div>
         </div>`;
   }
 
diff --git a/js/src/trackviewmodel.js b/js/src/trackviewmodel.js
index b8d15ff..5c57dbd 100644
--- a/js/src/trackviewmodel.js
+++ b/js/src/trackviewmodel.js
@@ -158,7 +158,7 @@ export default class TrackViewModel extends ViewModel {
     const form = this.importEl.parentElement;
     const sizeMax = form.elements['MAX_FILE_SIZE'].value;
     if (this.importEl.files && this.importEl.files.length === 1 && this.importEl.files[0].size > sizeMax) {
-      uUtils.error(uUtils.sprintf($._('isizefailure'), sizeMax));
+      uUtils.error($._('isizefailure', sizeMax));
       return;
     }
     if (!auth.isAuthenticated) {
@@ -169,7 +169,7 @@ export default class TrackViewModel extends ViewModel {
       .then((trackList) => {
         if (trackList.length) {
           if (trackList.length > 1) {
-            alert(uUtils.sprintf($._('imultiple'), trackList.length));
+            alert($._('imultiple', trackList.length));
           }
           this.model.trackList = trackList.concat(this.model.trackList);
           this.model.currentTrackId = trackList[0].listValue;
diff --git a/js/src/utils.js b/js/src/utils.js
index 218ab58..cc03fa1 100644
--- a/js/src/utils.js
+++ b/js/src/utils.js
@@ -41,16 +41,23 @@ export default class uUtils {
    * @param {...(string|number)=} params Optional parameters
    * @returns {string}
    */
-  static sprintf(fmt, params) { // eslint-disable-line no-unused-vars
-    const args = Array.prototype.slice.call(arguments);
-    const format = args.shift();
+  static sprintf(fmt, ...params) {
     let i = 0;
-    return format.replace(/%%|%s|%d/g, (match) => {
+    const ret = fmt.replace(/%%|%s|%d/g, (match) => {
       if (match === '%%') {
         return '%';
+      } else if (match === '%d' && isNaN(params[i])) {
+        throw new Error(`Wrong format specifier ${match} for ${params[i]} argument`);
       }
-      return (typeof args[i] !== 'undefined') ? args[i++] : match;
+      if (typeof params[i] === 'undefined') {
+        throw new Error(`Missing argument for format specifier ${match}`);
+      }
+      return params[i++];
     });
+    if (i < params.length) {
+      throw new Error(`Unused argument for format specifier ${fmt}`);
+    }
+    return ret;
   }
 
   /**
diff --git a/js/test/lang.test.js b/js/test/lang.test.js
index d68fd12..7d1838a 100644
--- a/js/test/lang.test.js
+++ b/js/test/lang.test.js
@@ -39,6 +39,7 @@ describe('Lang tests', () => {
     };
     mockStrings = {
       string1: 'łańcuch1',
+      placeholders: '%s : %d',
       units: 'jp',
       unitd: 'jo',
       unitdm: 'jo / 1000',
@@ -66,6 +67,15 @@ describe('Lang tests', () => {
     expect(lang._('string1')).toBe(mockStrings.string1);
   });
 
+  it('should return localized string with replaced placeholders', () => {
+    // when
+    lang.init(mockConfig, mockStrings);
+    const p1 = 'str';
+    const p2 = 4;
+    // then
+    expect(lang._('placeholders', p1, p2)).toBe(`${p1} : ${p2}`);
+  });
+
   it('should throw error on unknown string', () => {
     // when
     lang.init(mockConfig, mockStrings);
diff --git a/js/test/mapviewmodel.test.js b/js/test/mapviewmodel.test.js
index 09f85c7..6782af6 100644
--- a/js/test/mapviewmodel.test.js
+++ b/js/test/mapviewmodel.test.js
@@ -257,7 +257,6 @@ describe('MapViewModel tests', () => {
   it('should get popup html content', () => {
     // given
     const id = 0;
-    spyOn(uUtils, 'sprintf');
     state.currentTrack = TrackFactory.getTrack(2);
     // when
     const html = vm.getPopupHtml(id);
@@ -265,8 +264,8 @@ describe('MapViewModel tests', () => {
     // then
     expect(element).toBeInstanceOf(HTMLDivElement);
     expect(element.id).toBe('popup');
-    expect(uUtils.sprintf.calls.mostRecent().args[1]).toBe(id + 1);
-    expect(uUtils.sprintf.calls.mostRecent().args[2]).toBe(state.currentTrack.length);
+    expect(lang._.calls.mostRecent().args[1]).toBe(id + 1);
+    expect(lang._.calls.mostRecent().args[2]).toBe(state.currentTrack.length);
   });
 
   it('should get popup with stats when track does not contain only latest positions', () => {
diff --git a/js/test/trackviewmodel.test.js b/js/test/trackviewmodel.test.js
index 3ffcb86..7ff932a 100644
--- a/js/test/trackviewmodel.test.js
+++ b/js/test/trackviewmodel.test.js
@@ -451,7 +451,7 @@ describe('TrackViewModel tests', () => {
       return Promise.resolve(imported);
     });
     spyOn(uPositionSet, 'fetch').and.returnValue(Promise.resolve(positions));
-    spyOn(uUtils, 'sprintf');
+    spyOn(window, 'alert');
     const options = '<option selected value="1">track1</option><option value="2">track2</option>';
     trackEl.insertAdjacentHTML('afterbegin', options);
     const optLength = trackEl.options.length;
@@ -476,7 +476,7 @@ describe('TrackViewModel tests', () => {
       expect(state.currentTrack).toBe(imported[0]);
       expect(vm.model.currentTrackId).toBe(imported[0].listValue);
       expect(state.currentTrack.length).toBe(positions.length);
-      expect(uUtils.sprintf.calls.mostRecent().args[1]).toBe(imported.length);
+      expect(window.alert).toHaveBeenCalledTimes(1);
       expect(trackEl.options.length).toBe(optLength + imported.length);
       expect(vm.model.trackList.length).toBe(optLength + imported.length);
       expect(vm.model.inputFile).toBe('');
@@ -493,7 +493,6 @@ describe('TrackViewModel tests', () => {
     ];
     spyOn(uTrack, 'import').and.returnValue(Promise.resolve(imported));
     spyOn(uPositionSet, 'fetch').and.returnValue(Promise.resolve(positions));
-    spyOn(uUtils, 'sprintf');
     spyOn(uUtils, 'error');
     const options = '<option selected value="1">track1</option><option value="2">track2</option>';
     trackEl.insertAdjacentHTML('afterbegin', options);
@@ -517,7 +516,7 @@ describe('TrackViewModel tests', () => {
       expect(uTrack.import).not.toHaveBeenCalled();
       expect(state.currentTrack).toBe(track1);
       expect(vm.model.currentTrackId).toBe(track1.listValue);
-      expect(uUtils.sprintf.calls.mostRecent().args[1]).toBe(MAX_FILE_SIZE.toString());
+      expect(lang._.calls.mostRecent().args[1]).toBe(MAX_FILE_SIZE.toString());
       expect(trackEl.options.length).toBe(optLength);
       expect(vm.model.trackList.length).toBe(optLength);
       done();