diff --git a/js/src/observe.js b/js/src/observe.js
new file mode 100644
index 0000000..0125fd4
--- /dev/null
+++ b/js/src/observe.js
@@ -0,0 +1,143 @@
+/*
+ * μlogger
+ *
+ * Copyright(C) 2019 Bartek Fabiszewski (www.fabiszewski.net)
+ *
+ * This is free software; you can redistribute it and/or modify it under
+ * the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+/* eslint-disable no-underscore-dangle */
+export default class uObserve {
+
+  /**
+   * Observe object's property or all properties if not specified.
+   * On change call observer function.
+   * observe(obj, prop, observer) observes given property prop;
+   * observe(obj, observer) observes all properties of object obj.
+   * @param {Object} obj
+   * @param {(string|function)} p1
+   * @param {function=} p2
+   */
+  static observe(obj, p1, p2) {
+    if (typeof p2 === 'function') {
+      this.observeProperty(obj, p1, p2);
+    } else if (typeof p1 === 'function') {
+        if (Array.isArray(obj)) {
+          this.observeArray(obj, p1);
+        } else {
+          this.observeRecursive(obj, p1);
+        }
+    } else {
+      throw new Error('Invalid arguments');
+    }
+  }
+
+  /**
+   * Observe object's proporty. On change call observer
+   * @param {Object} obj
+   * @param {?string} property
+   * @param {function} observer
+   */
+  static observeProperty(obj, property, observer) {
+    this.addObserver(obj, observer, property);
+    if (!obj.hasOwnProperty('_values')) {
+      Object.defineProperty(obj, '_values', { enumerable: false, configurable: false, value: [] });
+    }
+    obj._values[property] = obj[property];
+    Object.defineProperty(obj, property, {
+      get: () => obj._values[property],
+      set: (newValue) => {
+        if (obj._values[property] !== newValue) {
+          obj._values[property] = newValue;
+          uObserve.notify(obj._observers[property], newValue);
+        }
+        if (Array.isArray(obj[property])) {
+          this.observeArray(obj[property], observer);
+        }
+      }
+    });
+    if (Array.isArray(obj[property])) {
+      this.observeArray(obj[property], observer);
+    }
+  }
+
+  /**
+   * Recursively add observer to all properties
+   * @param {Object} obj
+   * @param {function} observer
+   */
+  static observeRecursive(obj, observer) {
+    for (const prop in obj) {
+      if (obj.hasOwnProperty(prop)) {
+        uObserve.observeProperty(obj, prop, observer);
+      }
+    }
+  }
+
+  /**
+   * Observe array
+   * @param {Object} arr
+   * @param {function} observer
+   */
+  static observeArray(arr, observer) {
+    this.addObserver(arr, observer);
+    [ 'pop', 'push', 'reverse', 'shift', 'sort', 'splice', 'unshift' ].forEach(
+      (operation) => {
+        const descriptor = Object.getOwnPropertyDescriptor(Array.prototype, operation);
+        descriptor.value = function () {
+          const result = Array.prototype[operation].apply(arr, arguments);
+          uObserve.notify(arr._observers, arr);
+          return result;
+        };
+        Object.defineProperty(arr, operation, descriptor);
+      });
+  }
+
+  /**
+   * Store observer in object
+   * @param {Object} obj Object
+   * @param {function} observer Observer
+   * @param {string=} property Optional property
+   */
+  static addObserver(obj, observer, property) {
+    if (!obj.hasOwnProperty('_observers')) {
+      Object.defineProperty(obj, '_observers', {
+        enumerable: false,
+        configurable: false,
+        value: (arguments.length === 3) ? [] : new Set()
+      });
+    }
+    if (arguments.length === 3) {
+      if (!obj._observers[property]) {
+        obj._observers[property] = new Set();
+      }
+      obj._observers[property].add(observer);
+    } else {
+      obj._observers.add(observer);
+    }
+  }
+
+  /**
+   * Notify observers
+   * @param {Set<function>} observers
+   * @param {*} value
+   */
+  static notify(observers, value) {
+    for (const observer of observers) {
+      (async () => {
+        await observer(value);
+      })();
+    }
+  }
+}
diff --git a/js/test/observe.test.js b/js/test/observe.test.js
new file mode 100644
index 0000000..247b24a
--- /dev/null
+++ b/js/test/observe.test.js
@@ -0,0 +1,169 @@
+/*
+ * μlogger
+ *
+ * Copyright(C) 2019 Bartek Fabiszewski (www.fabiszewski.net)
+ *
+ * This is free software; you can redistribute it and/or modify it under
+ * the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+import uObserve from '../src/observe.js';
+
+describe('Observe tests', () => {
+  let object;
+  let result = false;
+  let resultValue;
+
+  beforeEach(() => {
+    object = { observed: 1, nonObserved: 1 };
+    result = false;
+  });
+
+  describe('when object is observed', () => {
+
+    it('should throw error if observer is missing', () => {
+      expect(() => { uObserve.observe(object, 'observed'); }).toThrow(new Error('Invalid arguments'));
+    });
+
+    it('should notify observers when observed property is modified', () => {
+      // given
+      uObserve.observe(object, 'observed', (value) => {
+        result = true;
+        resultValue = value;
+      });
+      // when
+      expect(result).toBe(false);
+      object.observed = 2;
+      // then
+      expect(result).toBe(true);
+      expect(resultValue).toBe(2);
+    });
+
+    it('should not notify observers when non-observed property is modified', () => {
+      // given
+      uObserve.observe(object, 'observed', () => {
+        result = true;
+      });
+      // when
+      expect(result).toBe(false);
+      object.nonObserved = 2;
+      // then
+      expect(result).toBe(false);
+    });
+
+    it('should not notify observers when modified value is same', () => {
+      // given
+      uObserve.observe(object, 'observed', () => {
+        result = true;
+      });
+      // when
+      expect(result).toBe(false);
+      object.observed = 1;
+      // then
+      expect(result).toBe(false);
+    });
+
+    it('should notify observers when any property is modified', () => {
+      // given
+      uObserve.observe(object, (value) => {
+        result = true;
+        resultValue = value;
+      });
+      // when
+      expect(result).toBe(false);
+      object.observed = 2;
+      // then
+      expect(result).toBe(true);
+      expect(resultValue).toBe(2);
+
+      // given
+      result = false;
+      resultValue = null;
+
+      // when
+      expect(result).toBe(false);
+      object.nonObserved = 2;
+      // then
+      expect(result).toBe(true);
+      expect(resultValue).toBe(2);
+    });
+
+    it('should notify observers when observed array property is modified', () => {
+      // given
+      const array = [ 1, 2 ];
+      object = { array: array };
+      uObserve.observe(object, 'array', (value) => {
+        result = true;
+        resultValue = value;
+      });
+      // when
+      expect(result).toBe(false);
+      array.push(3);
+      // then
+      expect(result).toBe(true);
+      expect(resultValue).toEqual(array);
+    });
+
+    it('should notify observers when observed array object is modified', () => {
+      // given
+      const array = [ 1, 2 ];
+      uObserve.observe(array, (value) => {
+        result = true;
+        resultValue = value;
+      });
+      // when
+      expect(result).toBe(false);
+      array.push(3);
+      // then
+      expect(result).toBe(true);
+      expect(resultValue).toEqual(array);
+    });
+
+    it('should retain observers after array is reassigned', () => {
+      // given
+      const array = [ 1, 2 ];
+      const newArray = [ 3, 4 ];
+      object = { array: array };
+      uObserve.observe(object, 'array', (value) => {
+        result = true;
+        resultValue = value;
+      });
+      // when
+      object.array = newArray;
+      result = false;
+
+      expect(result).toBe(false);
+      object.array.push(5);
+      // then
+      expect(result).toBe(true);
+      expect(resultValue).toEqual(newArray);
+    });
+  });
+
+  describe('when notify is called directly', () => {
+    it('should call observers with given value', () => {
+      // given
+      const observers = new Set();
+      let result2 = false;
+      observers.add((value) => { result = value; });
+      observers.add((value) => { result2 = value; });
+      // when
+      expect(result).toBe(false);
+      expect(result2).toBe(false);
+      uObserve.notify(observers, true);
+      // then
+      expect(result).toBe(true);
+      expect(result2).toBe(true);
+    });
+  });
+});