diff --git a/README.md b/README.md
index 9c93221a..3701489a 100644
--- a/README.md
+++ b/README.md
@@ -154,6 +154,24 @@ var patch = jsonpatch.generate(observer);
 // ];
 ```
 
+Generating patches with test operations:
+
+```js
+var document = { firstName: "Joachim", lastName: "Wester", contactDetails: { phoneNumbers: [ { number:"555-123" }] } };
+var observer = jsonpatch.observe(document, undefined, true);
+document.firstName = "Albert";
+document.contactDetails.phoneNumbers[0].number = "123";
+document.contactDetails.phoneNumbers.push({ number:"456" });
+var patch = jsonpatch.generate(observer);
+// patch  == [
+//   { op: "test", path: "/firstName", value: "Joachim"},
+//   { op: "replace", path: "/firstName", value: "Albert"},
+//   { op: "test", path: "/contactDetails/phoneNumbers/0/number", value: "555-123" },
+//   { op: "replace", path: "/contactDetails/phoneNumbers/0/number", value: "123" },
+//   { op: "add", path: "/contactDetails/phoneNumbers/1", value: {number:"456"}}
+// ];
+```
+
 Comparing two object trees:
 
 ```js
@@ -163,6 +181,18 @@ var diff = jsonpatch.compare(documentA, documentB);
 //diff == [{op: "replace", path: "/user/lastName", value: "Collins"}]
 ```
 
+Comparing two object trees with test operations:
+
+```js
+var documentA = {user: {firstName: "Albert", lastName: "Einstein"}};
+var documentB = {user: {firstName: "Albert", lastName: "Collins"}};
+var diff = jsonpatch.compare(documentA, documentB, {inversible: true});
+//diff == [
+//   {op: "test", path: "/user/lastName", value: "Einstein"},
+//   {op: "replace", path: "/user/lastName", value: "Collins"}
+// ];
+```
+
 Validating a sequence of patches:
 
 ```js
@@ -262,17 +292,17 @@ Retrieves a value from a JSON document by a JSON pointer.
 
 Returns the value.
 
-#### `jsonpatch.observe(document: any, callback?: Function): Observer`
+#### `jsonpatch.observe(document: any, callback?: Function, inversible: boolean = false): Observer`
 
 Sets up an deep observer on `document` that listens for changes in object tree. When changes are detected, the optional
-callback is called with the generated patches array as the parameter.
+callback is called with the generated patches array as the parameter. If inversible is true, then observer will generate test operations.
 
 Returns `observer`.
 
-#### `jsonpatch.generate(document: any, observer: Observer): Operation[]`
+#### `jsonpatch.generate(document: any, observer: Observer, opts?: { inversible: boolean }): Operation[]`
 
 If there are pending changes in `obj`, returns them synchronously. If a `callback` was defined in `observe`
-method, it will be triggered synchronously as well.
+method, it will be triggered synchronously as well. If opts.inversible is undefined then fallback to observer.inversible.
 
 If there are no pending changes in `obj`, returns an empty array (length 0).
 
@@ -282,9 +312,9 @@ Destroys the observer set up on `document`.
 
 Any remaining changes are delivered synchronously (as in `jsonpatch.generate`). Note: this is different that ES6/7 `Object.unobserve`, which delivers remaining changes asynchronously.
 
-#### `jsonpatch.compare(document1: any, document2: any): Operation[]`
+#### `jsonpatch.compare(document1: any, document2: any, opts?: { inversible: boolean }): Operation[]`
 
-Compares object trees `document1` and `document2` and returns the difference relative to `document1` as a patches array.
+Compares object trees `document1` and `document2` and returns the difference relative to `document1` as a patches array. If opts.inversible is true, test operations will be generated.
 
 If there are no differences, returns an empty array (length 0).
 
diff --git a/dist/fast-json-patch.js b/dist/fast-json-patch.js
index 3c73b217..64971300 100644
--- a/dist/fast-json-patch.js
+++ b/dist/fast-json-patch.js
@@ -646,7 +646,7 @@ exports.validate = validate;
  */
 var helpers_1 = __webpack_require__(0);
 var core_1 = __webpack_require__(1);
-/* export all core functions */
+/* export all core functions and types */
 var core_2 = __webpack_require__(1);
 exports.applyOperation = core_2.applyOperation;
 exports.applyPatch = core_2.applyPatch;
@@ -694,7 +694,8 @@ exports.unobserve = unobserve;
 /**
  * Observes changes made to an object, which can then be retrieved using generate
  */
-function observe(obj, callback) {
+function observe(obj, callback, inversible) {
+    if (inversible === void 0) { inversible = false; }
     var patches = [];
     var observer;
     var mirror = getMirror(obj);
@@ -709,7 +710,7 @@ function observe(obj, callback) {
     if (observer) {
         return observer;
     }
-    observer = {};
+    observer = { inversible: inversible };
     mirror.value = helpers_1._deepClone(obj);
     if (callback) {
         observer.callback = callback;
@@ -766,9 +767,11 @@ exports.observe = observe;
 /**
  * Generate an array of patches from an observer
  */
-function generate(observer) {
+function generate(observer, opts) {
+    if (opts === void 0) { opts = {}; }
     var mirror = beforeDict.get(observer.object);
-    _generate(mirror.value, observer.object, observer.patches, "");
+    var inversible = typeof opts.inversible !== "undefined" ? opts.inversible : observer.inversible;
+    _generate(mirror.value, observer.object, observer.patches, "", { inversible: inversible });
     if (observer.patches.length) {
         core_1.applyPatch(mirror.value, observer.patches);
     }
@@ -783,7 +786,8 @@ function generate(observer) {
 }
 exports.generate = generate;
 // Dirty check if obj is different from mirror, generate patches and update mirror
-function _generate(mirror, obj, patches, path) {
+function _generate(mirror, obj, patches, path, opts) {
+    if (opts === void 0) { opts = { inversible: false }; }
     if (obj === mirror) {
         return;
     }
@@ -794,6 +798,7 @@ function _generate(mirror, obj, patches, path) {
     var oldKeys = helpers_1._objectKeys(mirror);
     var changed = false;
     var deleted = false;
+    var inversible = opts.inversible;
     //if ever "move" operation is implemented here, make sure this test runs OK: "should not generate the same patch twice (move)"
     for (var t = oldKeys.length - 1; t >= 0; t--) {
         var key = oldKeys[t];
@@ -801,20 +806,26 @@ function _generate(mirror, obj, patches, path) {
         if (helpers_1.hasOwnProperty(obj, key) && !(obj[key] === undefined && oldVal !== undefined && Array.isArray(obj) === false)) {
             var newVal = obj[key];
             if (typeof oldVal == "object" && oldVal != null && typeof newVal == "object" && newVal != null) {
-                _generate(oldVal, newVal, patches, path + "/" + helpers_1.escapePathComponent(key));
+                _generate(oldVal, newVal, patches, path + "/" + helpers_1.escapePathComponent(key), opts);
             }
             else {
                 if (oldVal !== newVal) {
                     changed = true;
+                    if (inversible)
+                        patches.push({ op: "test", path: path + "/" + helpers_1.escapePathComponent(key), value: helpers_1._deepClone(oldVal) });
                     patches.push({ op: "replace", path: path + "/" + helpers_1.escapePathComponent(key), value: helpers_1._deepClone(newVal) });
                 }
             }
         }
         else if (Array.isArray(mirror) === Array.isArray(obj)) {
+            if (inversible)
+                patches.push({ op: "test", path: path + "/" + helpers_1.escapePathComponent(key), value: helpers_1._deepClone(oldVal) });
             patches.push({ op: "remove", path: path + "/" + helpers_1.escapePathComponent(key) });
             deleted = true; // property has been deleted
         }
         else {
+            if (inversible)
+                patches.push({ op: "test", path: path, value: mirror });
             patches.push({ op: "replace", path: path, value: obj });
             changed = true;
         }
@@ -832,9 +843,9 @@ function _generate(mirror, obj, patches, path) {
 /**
  * Create an array of patches from the differences in two objects
  */
-function compare(tree1, tree2) {
+function compare(tree1, tree2, opts) {
     var patches = [];
-    _generate(tree1, tree2, patches, '');
+    _generate(tree1, tree2, patches, '', opts);
     return patches;
 }
 exports.compare = compare;
diff --git a/dist/fast-json-patch.min.js b/dist/fast-json-patch.min.js
index 61b99b1b..1568b3dc 100644
--- a/dist/fast-json-patch.min.js
+++ b/dist/fast-json-patch.min.js
@@ -1,2 +1,2 @@
 /*! fast-json-patch, version: 2.1.0 */
-var jsonpatch=function(a){function b(d){if(c[d])return c[d].exports;var e=c[d]={i:d,l:!1,exports:{}};return a[d].call(e.exports,e,e.exports,b),e.l=!0,e.exports}var c={};return b.m=a,b.c=c,b.i=function(a){return a},b.d=function(a,c,d){b.o(a,c)||Object.defineProperty(a,c,{configurable:!1,enumerable:!0,get:d})},b.n=function(a){var c=a&&a.__esModule?function(){return a['default']}:function(){return a};return b.d(c,'a',c),c},b.o=function(a,b){return Object.prototype.hasOwnProperty.call(a,b)},b.p='',b(b.s=2)}([function(a,b){function c(a,b){return j.call(a,b)}function d(a){if(Array.isArray(a)){for(var b=Array(a.length),d=0;d<b.length;d++)b[d]=''+d;return b}if(Object.keys)return Object.keys(a);var b=[];for(var e in a)c(a,e)&&b.push(e);return b}function e(a){return-1===a.indexOf('/')&&-1===a.indexOf('~')?a:a.replace(/~/g,'~0').replace(/\//g,'~1')}function f(a,b){var d;for(var g in a)if(c(a,g)){if(a[g]===b)return e(g)+'/';if('object'==typeof a[g]&&(d=f(a[g],b),''!=d))return e(g)+'/'+d}return''}function g(a){if(a===void 0)return!0;if(a)if(Array.isArray(a)){for(var b=0,c=a.length;b<c;b++)if(g(a[b]))return!0;}else if('object'==typeof a)for(var e=d(a),f=e.length,b=0;b<f;b++)if(g(a[e[b]]))return!0;return!1}function h(a,b){var c=[a];for(var d in b){var e='object'==typeof b[d]?JSON.stringify(b[d],null,2):b[d];'undefined'!=typeof e&&c.push(d+': '+e)}return c.join('\n')}var i=this&&this.__extends||function(a,c){function b(){this.constructor=a}for(var d in c)c.hasOwnProperty(d)&&(a[d]=c[d]);a.prototype=null===c?Object.create(c):(b.prototype=c.prototype,new b)},j=Object.prototype.hasOwnProperty;b.hasOwnProperty=c,b._objectKeys=d;b._deepClone=function(a){switch(typeof a){case'object':return JSON.parse(JSON.stringify(a));case'undefined':return null;default:return a;}},b.isInteger=function(a){for(var b,c=0,d=a.length;c<d;){if(b=a.charCodeAt(c),48<=b&&57>=b){c++;continue}return!1}return!0},b.escapePathComponent=e,b.unescapePathComponent=function(a){return a.replace(/~1/g,'/').replace(/~0/g,'~')},b._getPathRecursive=f,b.getPath=function(a,b){if(a===b)return'/';var c=f(a,b);if(''===c)throw new Error('Object not found in root');return'/'+c},b.hasUndefined=g;var k=function(a){function b(b,c,d,e,f){a.call(this,h(b,{name:c,index:d,operation:e,tree:f})),this.name=c,this.index=d,this.operation=e,this.tree=f,this.message=h(b,{name:c,index:d,operation:e,tree:f})}return i(b,a),b}(Error);b.PatchError=k},function(a,b,c){function d(a,b){if(''==b)return a;var c={op:'_get',path:b};return e(a,c),c.value}function e(a,c,e,f,h,i){if(void 0===e&&(e=!1),void 0===f&&(f=!0),void 0===h&&(h=!0),void 0===i&&(i=0),e&&('function'==typeof e?e(c,0,a,c.path):g(c,0)),''===c.path){var j={newDocument:a};if('add'===c.op)return j.newDocument=c.value,j;if('replace'===c.op)return j.newDocument=c.value,j.removed=a,j;if('move'===c.op||'copy'===c.op)return j.newDocument=d(a,c.from),'move'===c.op&&(j.removed=a),j;if('test'===c.op){if(j.test=k(a,c.value),!1===j.test)throw new b.JsonPatchError('Test operation failed','TEST_OPERATION_FAILED',i,c,a);return j.newDocument=a,j}if('remove'===c.op)return j.removed=a,j.newDocument=null,j;if('_get'===c.op)return c.value=a,j;if(e)throw new b.JsonPatchError('Operation `op` property is not one of operations defined in RFC-6902','OPERATION_OP_INVALID',i,c,a);else return j}else{f||(a=l._deepClone(a));var o,p,q,r=c.path||'',s=r.split('/'),u=a,v=1,t=s.length;for(q='function'==typeof e?e:g;;){if(p=s[v],h&&'__proto__'==p)throw new TypeError('JSON-Patch: modifying `__proto__` prop is banned for security reasons, if this was on purpose, please set `banPrototypeModifications` flag false and pass it to this function. More info in fast-json-patch README');if(e&&void 0==o&&(void 0===u[p]?o=s.slice(0,v).join('/'):v==t-1&&(o=c.path),void 0!==o&&q(c,0,a,o)),v++,Array.isArray(u)){if('-'===p)p=u.length;else if(e&&!l.isInteger(p))throw new b.JsonPatchError('Expected an unsigned base-10 integer value, making the new referenced value the array element with the zero-based index','OPERATION_PATH_ILLEGAL_ARRAY_INDEX',i,c,a);else l.isInteger(p)&&(p=~~p);if(v>=t){if(e&&'add'===c.op&&p>u.length)throw new b.JsonPatchError('The specified index MUST NOT be greater than the number of elements in the array','OPERATION_VALUE_OUT_OF_BOUNDS',i,c,a);var j=n[c.op].call(c,u,p,a);if(!1===j.test)throw new b.JsonPatchError('Test operation failed','TEST_OPERATION_FAILED',i,c,a);return j}}else if(p&&-1!=p.indexOf('~')&&(p=l.unescapePathComponent(p)),v>=t){var j=m[c.op].call(c,u,p,a);if(!1===j.test)throw new b.JsonPatchError('Test operation failed','TEST_OPERATION_FAILED',i,c,a);return j}u=u[p]}}}function f(a,c,d,f,g){if(void 0===f&&(f=!0),void 0===g&&(g=!0),d&&!Array.isArray(c))throw new b.JsonPatchError('Patch sequence must be an array','SEQUENCE_NOT_AN_ARRAY');f||(a=l._deepClone(a));for(var h=Array(c.length),j=0,i=c.length;j<i;j++)h[j]=e(a,c[j],d,!0,g,j),a=h[j].newDocument;return h.newDocument=a,h}function g(a,c,d,e){if('object'!=typeof a||null===a||Array.isArray(a))throw new b.JsonPatchError('Operation is not an object','OPERATION_NOT_AN_OBJECT',c,a,d);else if(!m[a.op])throw new b.JsonPatchError('Operation `op` property is not one of operations defined in RFC-6902','OPERATION_OP_INVALID',c,a,d);else if('string'!=typeof a.path)throw new b.JsonPatchError('Operation `path` property is not a string','OPERATION_PATH_INVALID',c,a,d);else if(0!==a.path.indexOf('/')&&0<a.path.length)throw new b.JsonPatchError('Operation `path` property must start with "/"','OPERATION_PATH_INVALID',c,a,d);else if(('move'===a.op||'copy'===a.op)&&'string'!=typeof a.from)throw new b.JsonPatchError('Operation `from` property is not 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window&&(window.removeEventListener?(window.removeEventListener('mouseup',l),window.removeEventListener('keyup',l),window.removeEventListener('mousedown',l),window.removeEventListener('keydown',l)):(document.documentElement.detachEvent('onmouseup',l),document.documentElement.detachEvent('onkeyup',l),document.documentElement.detachEvent('onmousedown',l),document.documentElement.detachEvent('onkeydown',l)))},h.observers.set(b,new o(b,c)),c},b.generate=g,b.compare=function(a,b){var c=[];return h(a,b,c,''),c}},function(a,b,c){function d(a){return null===a||a===void 0}function e(a){return a&&'object'==typeof a&&'number'==typeof a.length&&('function'!=typeof a.copy||'function'!=typeof a.slice?!1:0<a.length&&'number'!=typeof a[0]?!1:!0)}function f(c,f,l){var m,i;if(d(c)||d(f))return!1;if(c.prototype!==f.prototype)return!1;if(j(c))return!!j(f)&&(c=g.call(c),f=g.call(f),k(c,f,l));if(e(c)){if(!e(f))return!1;if(c.length!==f.length)return!1;for(m=0;m<c.length;m++)if(c[m]!==f[m])return!1;return!0}try{var n=h(c),o=h(f)}catch(a){return!1}if(n.length!=o.length)return!1;for(n.sort(),o.sort(),m=n.length-1;0<=m;m--)if(n[m]!=o[m])return!1;for(m=n.length-1;0<=m;m--)if(i=n[m],!k(c[i],f[i],l))return!1;return typeof c==typeof f}var g=Array.prototype.slice,h=c(5),j=c(4),k=a.exports=function(a,b,c){return c||(c={}),a===b||(a instanceof Date&&b instanceof Date?a.getTime()===b.getTime():a&&b&&('object'==typeof a||'object'==typeof b)?f(a,b,c):c.strict?a===b:a==b)}},function(a,b){function c(a){return'[object Arguments]'==Object.prototype.toString.call(a)}function d(a){return a&&'object'==typeof a&&'number'==typeof a.length&&Object.prototype.hasOwnProperty.call(a,'callee')&&!Object.prototype.propertyIsEnumerable.call(a,'callee')||!1}var e='[object Arguments]'==function(){return Object.prototype.toString.call(arguments)}();b=a.exports=e?c:d,b.supported=c;b.unsupported=d},function(a,b){function c(a){var b=[];for(var c in a)b.push(c);return b}b=a.exports='function'==typeof Object.keys?Object.keys:c,b.shim=c}]);
\ No newline at end of file
+var jsonpatch=function(a){function b(d){if(c[d])return c[d].exports;var e=c[d]={i:d,l:!1,exports:{}};return a[d].call(e.exports,e,e.exports,b),e.l=!0,e.exports}var c={};return b.m=a,b.c=c,b.i=function(a){return a},b.d=function(a,c,d){b.o(a,c)||Object.defineProperty(a,c,{configurable:!1,enumerable:!0,get:d})},b.n=function(a){var c=a&&a.__esModule?function(){return a['default']}:function(){return a};return b.d(c,'a',c),c},b.o=function(a,b){return Object.prototype.hasOwnProperty.call(a,b)},b.p='',b(b.s=2)}([function(a,b){function c(a,b){return j.call(a,b)}function d(a){if(Array.isArray(a)){for(var b=Array(a.length),d=0;d<b.length;d++)b[d]=''+d;return b}if(Object.keys)return Object.keys(a);var b=[];for(var e in a)c(a,e)&&b.push(e);return b}function e(a){return-1===a.indexOf('/')&&-1===a.indexOf('~')?a:a.replace(/~/g,'~0').replace(/\//g,'~1')}function f(a,b){var d;for(var g in a)if(c(a,g)){if(a[g]===b)return e(g)+'/';if('object'==typeof a[g]&&(d=f(a[g],b),''!=d))return e(g)+'/'+d}return''}function g(a){if(a===void 0)return!0;if(a)if(Array.isArray(a)){for(var b=0,c=a.length;b<c;b++)if(g(a[b]))return!0;}else if('object'==typeof a)for(var e=d(a),f=e.length,b=0;b<f;b++)if(g(a[e[b]]))return!0;return!1}function h(a,b){var c=[a];for(var d in b){var e='object'==typeof b[d]?JSON.stringify(b[d],null,2):b[d];'undefined'!=typeof e&&c.push(d+': '+e)}return c.join('\n')}var i=this&&this.__extends||function(a,c){function b(){this.constructor=a}for(var d in c)c.hasOwnProperty(d)&&(a[d]=c[d]);a.prototype=null===c?Object.create(c):(b.prototype=c.prototype,new b)},j=Object.prototype.hasOwnProperty;b.hasOwnProperty=c,b._objectKeys=d;b._deepClone=function(a){switch(typeof a){case'object':return JSON.parse(JSON.stringify(a));case'undefined':return null;default:return a;}},b.isInteger=function(a){for(var b,c=0,d=a.length;c<d;){if(b=a.charCodeAt(c),48<=b&&57>=b){c++;continue}return!1}return!0},b.escapePathComponent=e,b.unescapePathComponent=function(a){return a.replace(/~1/g,'/').replace(/~0/g,'~')},b._getPathRecursive=f,b.getPath=function(a,b){if(a===b)return'/';var c=f(a,b);if(''===c)throw new Error('Object not found in root');return'/'+c},b.hasUndefined=g;var k=function(a){function b(b,c,d,e,f){a.call(this,h(b,{name:c,index:d,operation:e,tree:f})),this.name=c,this.index=d,this.operation=e,this.tree=f,this.message=h(b,{name:c,index:d,operation:e,tree:f})}return i(b,a),b}(Error);b.PatchError=k},function(a,b,c){function d(a,b){if(''==b)return a;var c={op:'_get',path:b};return e(a,c),c.value}function e(a,c,e,f,h,i){if(void 0===e&&(e=!1),void 0===f&&(f=!0),void 0===h&&(h=!0),void 0===i&&(i=0),e&&('function'==typeof e?e(c,0,a,c.path):g(c,0)),''===c.path){var j={newDocument:a};if('add'===c.op)return j.newDocument=c.value,j;if('replace'===c.op)return j.newDocument=c.value,j.removed=a,j;if('move'===c.op||'copy'===c.op)return j.newDocument=d(a,c.from),'move'===c.op&&(j.removed=a),j;if('test'===c.op){if(j.test=k(a,c.value),!1===j.test)throw new b.JsonPatchError('Test operation failed','TEST_OPERATION_FAILED',i,c,a);return j.newDocument=a,j}if('remove'===c.op)return j.removed=a,j.newDocument=null,j;if('_get'===c.op)return c.value=a,j;if(e)throw new b.JsonPatchError('Operation `op` property is not one of operations defined in RFC-6902','OPERATION_OP_INVALID',i,c,a);else return j}else{f||(a=l._deepClone(a));var o,p,q,r=c.path||'',s=r.split('/'),u=a,v=1,t=s.length;for(q='function'==typeof e?e:g;;){if(p=s[v],h&&'__proto__'==p)throw new TypeError('JSON-Patch: modifying `__proto__` prop is banned for security reasons, if this was on purpose, please set `banPrototypeModifications` flag false and pass it to this function. More info in fast-json-patch README');if(e&&void 0==o&&(void 0===u[p]?o=s.slice(0,v).join('/'):v==t-1&&(o=c.path),void 0!==o&&q(c,0,a,o)),v++,Array.isArray(u)){if('-'===p)p=u.length;else if(e&&!l.isInteger(p))throw new b.JsonPatchError('Expected an unsigned base-10 integer value, making the new referenced value the array element with the zero-based index','OPERATION_PATH_ILLEGAL_ARRAY_INDEX',i,c,a);else l.isInteger(p)&&(p=~~p);if(v>=t){if(e&&'add'===c.op&&p>u.length)throw new b.JsonPatchError('The specified index MUST NOT be greater than the number of elements in the array','OPERATION_VALUE_OUT_OF_BOUNDS',i,c,a);var j=n[c.op].call(c,u,p,a);if(!1===j.test)throw new b.JsonPatchError('Test operation failed','TEST_OPERATION_FAILED',i,c,a);return j}}else if(p&&-1!=p.indexOf('~')&&(p=l.unescapePathComponent(p)),v>=t){var j=m[c.op].call(c,u,p,a);if(!1===j.test)throw new b.JsonPatchError('Test operation failed','TEST_OPERATION_FAILED',i,c,a);return j}u=u[p]}}}function f(a,c,d,f,g){if(void 0===f&&(f=!0),void 0===g&&(g=!0),d&&!Array.isArray(c))throw new b.JsonPatchError('Patch sequence must be an array','SEQUENCE_NOT_AN_ARRAY');f||(a=l._deepClone(a));for(var h=Array(c.length),j=0,i=c.length;j<i;j++)h[j]=e(a,c[j],d,!0,g,j),a=h[j].newDocument;return h.newDocument=a,h}function g(a,c,d,e){if('object'!=typeof a||null===a||Array.isArray(a))throw new b.JsonPatchError('Operation is not an object','OPERATION_NOT_AN_OBJECT',c,a,d);else if(!m[a.op])throw new b.JsonPatchError('Operation `op` property is not one of operations defined in RFC-6902','OPERATION_OP_INVALID',c,a,d);else if('string'!=typeof a.path)throw new b.JsonPatchError('Operation `path` property is not a string','OPERATION_PATH_INVALID',c,a,d);else if(0!==a.path.indexOf('/')&&0<a.path.length)throw new b.JsonPatchError('Operation `path` property must start with "/"','OPERATION_PATH_INVALID',c,a,d);else if(('move'===a.op||'copy'===a.op)&&'string'!=typeof a.from)throw new b.JsonPatchError('Operation `from` property is not present (applicable in `move` and `copy` operations)','OPERATION_FROM_REQUIRED',c,a,d);else if(('add'===a.op||'replace'===a.op||'test'===a.op)&&a.value===void 0)throw new b.JsonPatchError('Operation `value` property is not present (applicable in `add`, `replace` and `test` operations)','OPERATION_VALUE_REQUIRED',c,a,d);else if(('add'===a.op||'replace'===a.op||'test'===a.op)&&l.hasUndefined(a.value))throw new b.JsonPatchError('Operation `value` property is not present (applicable in `add`, `replace` and `test` operations)','OPERATION_VALUE_CANNOT_CONTAIN_UNDEFINED',c,a,d);else if(d)if('add'==a.op){var f=a.path.split('/').length,g=e.split('/').length;if(f!==g+1&&f!==g)throw new b.JsonPatchError('Cannot perform an `add` operation at the desired path','OPERATION_PATH_CANNOT_ADD',c,a,d)}else if('replace'===a.op||'remove'===a.op||'_get'===a.op){if(a.path!==e)throw new b.JsonPatchError('Cannot perform the operation at a path that does not exist','OPERATION_PATH_UNRESOLVABLE',c,a,d);}else if('move'===a.op||'copy'===a.op){var i={op:'_get',path:a.from,value:void 0},j=h([i],d);if(j&&'OPERATION_PATH_UNRESOLVABLE'===j.name)throw new b.JsonPatchError('Cannot perform the operation from a path that does not exist','OPERATION_FROM_UNRESOLVABLE',c,a,d)}}function h(a,c,d){try{if(!Array.isArray(a))throw new b.JsonPatchError('Patch sequence must be an array','SEQUENCE_NOT_AN_ARRAY');if(c)f(l._deepClone(c),l._deepClone(a),d||!0);else{d=d||g;for(var e=0;e<a.length;e++)d(a[e],e,c,void 0)}}catch(a){if(a instanceof b.JsonPatchError)return a;throw a}}var i={strict:!0},j=c(3),k=function(c,a){return j(c,a,i)},l=c(0);b.JsonPatchError=l.PatchError,b.deepClone=l._deepClone;var m={add:function(a,b,c){return a[b]=this.value,{newDocument:c}},remove:function(a,b,c){var d=a[b];return delete a[b],{newDocument:c,removed:d}},replace:function(a,b,c){var d=a[b];return a[b]=this.value,{newDocument:c,removed:d}},move:function(a,b,c){var f=d(c,this.path);f&&(f=l._deepClone(f));var g=e(c,{op:'remove',path:this.from}).removed;return e(c,{op:'add',path:this.path,value:g}),{newDocument:c,removed:f}},copy:function(a,b,c){var f=d(c,this.from);return e(c,{op:'add',path:this.path,value:l._deepClone(f)}),{newDocument:c}},test:function(a,b,c){return{newDocument:c,test:k(a[b],this.value)}},_get:function(a,b,c){return this.value=a[b],{newDocument:c}}},n={add:function(a,b,c){return l.isInteger(b)?a.splice(b,0,this.value):a[b]=this.value,{newDocument:c,index:b}},remove:function(a,b,c){var d=a.splice(b,1);return{newDocument:c,removed:d[0]}},replace:function(a,b,c){var d=a[b];return a[b]=this.value,{newDocument:c,removed:d}},move:m.move,copy:m.copy,test:m.test,_get:m._get};b.getValueByPointer=d,b.applyOperation=e,b.applyPatch=f,b.applyReducer=function(a,c,d){var f=e(a,c);if(!1===f.test)throw new b.JsonPatchError('Test operation failed','TEST_OPERATION_FAILED',d,c,a);return f.newDocument},b.validator=g,b.validate=h},function(a,b,c){function d(a){return m.get(a)}function e(a,b){return a.observers.get(b)}function f(a,b){a.observers.delete(b.callback)}function g(a,b){void 0===b&&(b={});var c=m.get(a.object),d='undefined'==typeof b.inversible?a.inversible:b.inversible;h(c.value,a.object,a.patches,'',{inversible:d}),a.patches.length&&j.applyPatch(c.value,a.patches);var e=a.patches;return 0<e.length&&(a.patches=[],a.callback&&a.callback(e)),e}function h(a,b,c,d,e){if(void 0===e&&(e={inversible:!1}),b!==a){'function'==typeof b.toJSON&&(b=b.toJSON());for(var f=i._objectKeys(b),g=i._objectKeys(a),j=!1,k=!1,l=e.inversible,m=g.length-1;0<=m;m--){var n=g[m],o=a[n];if(i.hasOwnProperty(b,n)&&(void 0!==b[n]||void 0===o||!1!==Array.isArray(b))){var p=b[n];'object'==typeof o&&null!=o&&'object'==typeof p&&null!=p?h(o,p,c,d+'/'+i.escapePathComponent(n),e):o!==p&&(j=!0,l&&c.push({op:'test',path:d+'/'+i.escapePathComponent(n),value:i._deepClone(o)}),c.push({op:'replace',path:d+'/'+i.escapePathComponent(n),value:i._deepClone(p)}))}else Array.isArray(a)===Array.isArray(b)?(l&&c.push({op:'test',path:d+'/'+i.escapePathComponent(n),value:i._deepClone(o)}),c.push({op:'remove',path:d+'/'+i.escapePathComponent(n)}),k=!0):(l&&c.push({op:'test',path:d,value:a}),c.push({op:'replace',path:d,value:b}),j=!0)}if(k||f.length!=g.length)for(var n,m=0;m<f.length;m++)n=f[m],i.hasOwnProperty(a,n)||void 0===b[n]||c.push({op:'add',path:d+'/'+i.escapePathComponent(n),value:i._deepClone(b[n])})}}var i=c(0),j=c(1),k=c(1);b.applyOperation=k.applyOperation,b.applyPatch=k.applyPatch,b.applyReducer=k.applyReducer,b.getValueByPointer=k.getValueByPointer,b.validate=k.validate,b.validator=k.validator;var l=c(0);b.JsonPatchError=l.PatchError,b.deepClone=l._deepClone,b.escapePathComponent=l.escapePathComponent,b.unescapePathComponent=l.unescapePathComponent;var m=new WeakMap,n=function(){return function(a){this.observers=new Map,this.obj=a}}(),o=function(){return function(a,b){this.callback=a,this.observer=b}}();b.unobserve=function(a,b){b.unobserve()},b.observe=function(a,b,c){void 0===c&&(c=!1);var h,j=d(a);if(!j)j=new n(a),m.set(a,j);else{var k=e(j,b);h=k&&k.observer}if(h)return h;if(h={inversible:c},j.value=i._deepClone(a),b){h.callback=b,h.next=null;var l=function(){g(h)},p=function(){clearTimeout(h.next),h.next=setTimeout(l)};'undefined'!=typeof window&&(window.addEventListener?(window.addEventListener('mouseup',p),window.addEventListener('keyup',p),window.addEventListener('mousedown',p),window.addEventListener('keydown',p),window.addEventListener('change',p)):(document.documentElement.attachEvent('onmouseup',p),document.documentElement.attachEvent('onkeyup',p),document.documentElement.attachEvent('onmousedown',p),document.documentElement.attachEvent('onkeydown',p),document.documentElement.attachEvent('onchange',p)))}return h.patches=[],h.object=a,h.unobserve=function(){g(h),clearTimeout(h.next),f(j,h),'undefined'!=typeof window&&(window.removeEventListener?(window.removeEventListener('mouseup',p),window.removeEventListener('keyup',p),window.removeEventListener('mousedown',p),window.removeEventListener('keydown',p)):(document.documentElement.detachEvent('onmouseup',p),document.documentElement.detachEvent('onkeyup',p),document.documentElement.detachEvent('onmousedown',p),document.documentElement.detachEvent('onkeydown',p)))},j.observers.set(b,new o(b,h)),h},b.generate=g,b.compare=function(a,b,c){var d=[];return h(a,b,d,'',c),d}},function(a,b,c){function d(a){return null===a||a===void 0}function e(a){return a&&'object'==typeof a&&'number'==typeof a.length&&('function'!=typeof a.copy||'function'!=typeof a.slice?!1:0<a.length&&'number'!=typeof a[0]?!1:!0)}function f(c,f,l){var m,i;if(d(c)||d(f))return!1;if(c.prototype!==f.prototype)return!1;if(j(c))return!!j(f)&&(c=g.call(c),f=g.call(f),k(c,f,l));if(e(c)){if(!e(f))return!1;if(c.length!==f.length)return!1;for(m=0;m<c.length;m++)if(c[m]!==f[m])return!1;return!0}try{var n=h(c),o=h(f)}catch(a){return!1}if(n.length!=o.length)return!1;for(n.sort(),o.sort(),m=n.length-1;0<=m;m--)if(n[m]!=o[m])return!1;for(m=n.length-1;0<=m;m--)if(i=n[m],!k(c[i],f[i],l))return!1;return typeof c==typeof f}var g=Array.prototype.slice,h=c(5),j=c(4),k=a.exports=function(a,b,c){return c||(c={}),a===b||(a instanceof Date&&b instanceof Date?a.getTime()===b.getTime():a&&b&&('object'==typeof a||'object'==typeof b)?f(a,b,c):c.strict?a===b:a==b)}},function(a,b){function c(a){return'[object Arguments]'==Object.prototype.toString.call(a)}function d(a){return a&&'object'==typeof a&&'number'==typeof a.length&&Object.prototype.hasOwnProperty.call(a,'callee')&&!Object.prototype.propertyIsEnumerable.call(a,'callee')||!1}var e='[object Arguments]'==function(){return Object.prototype.toString.call(arguments)}();b=a.exports=e?c:d,b.supported=c;b.unsupported=d},function(a,b){function c(a){var b=[];for(var c in a)b.push(c);return b}b=a.exports='function'==typeof Object.keys?Object.keys:c,b.shim=c}]);
\ No newline at end of file
diff --git a/src/duplex.ts b/src/duplex.ts
index faefea24..16a52267 100644
--- a/src/duplex.ts
+++ b/src/duplex.ts
@@ -6,8 +6,8 @@
 import { _deepClone, _objectKeys, escapePathComponent, hasOwnProperty } from './helpers';
 import { applyPatch, Operation } from './core';
 
-/* export all core functions */
-export { applyOperation, applyPatch, applyReducer, getValueByPointer, Operation, validate, validator, OperationResult } from './core';
+/* export all core functions and types */
+export { applyOperation, applyPatch, applyReducer, getValueByPointer, Operation, AddOperation, RemoveOperation, ReplaceOperation, MoveOperation, CopyOperation,  TestOperation, GetOperation, validate, validator, OperationResult } from './core';
 
 /* export some helpers */
 export { PatchError as JsonPatchError, _deepClone as deepClone, escapePathComponent, unescapePathComponent } from './helpers';
@@ -17,6 +17,7 @@ export interface Observer<T> {
   patches: Operation[];
   unobserve: () => void;
   callback: (patches: Operation[]) => void;
+  inversible: boolean;
 }
 
 var beforeDict = new WeakMap();
@@ -63,7 +64,7 @@ export function unobserve<T>(root: T, observer: Observer<T>) {
 /**
  * Observes changes made to an object, which can then be retrieved using generate
  */
-export function observe<T>(obj: Object|Array<T>, callback?: (patches: Operation[]) => void): Observer<T> {
+export function observe<T>(obj: Object|Array<T>, callback?: (patches: Operation[]) => void, inversible: boolean = false): Observer<T> {
   var patches = [];
   var observer;
   var mirror = getMirror(obj);
@@ -80,7 +81,7 @@ export function observe<T>(obj: Object|Array<T>, callback?: (patches: Operation[
     return observer;
   }
 
-  observer = {};
+  observer = { inversible };
 
   mirror.value = _deepClone(obj);
 
@@ -144,9 +145,11 @@ export function observe<T>(obj: Object|Array<T>, callback?: (patches: Operation[
 /**
  * Generate an array of patches from an observer
  */
-export function generate<T>(observer: Observer<Object>): Operation[] {
+export function generate<T>(observer: Observer<Object>, opts: { inversible?: boolean } = {}): Operation[] {
   var mirror = beforeDict.get(observer.object);
-  _generate(mirror.value, observer.object, observer.patches, "");
+  var inversible = typeof opts.inversible !== "undefined" ? opts.inversible : observer.inversible;
+
+  _generate(mirror.value, observer.object, observer.patches, "", { inversible });
   if (observer.patches.length) {
     applyPatch(mirror.value, observer.patches);
   }
@@ -161,7 +164,7 @@ export function generate<T>(observer: Observer<Object>): Operation[] {
 }
 
 // Dirty check if obj is different from mirror, generate patches and update mirror
-function _generate(mirror, obj, patches, path) {
+function _generate(mirror, obj, patches, path, opts = { inversible: false }) {
   if (obj === mirror) {
     return;
   }
@@ -174,6 +177,7 @@ function _generate(mirror, obj, patches, path) {
   var oldKeys = _objectKeys(mirror);
   var changed = false;
   var deleted = false;
+  var { inversible } = opts;
 
   //if ever "move" operation is implemented here, make sure this test runs OK: "should not generate the same patch twice (move)"
 
@@ -185,19 +189,22 @@ function _generate(mirror, obj, patches, path) {
       var newVal = obj[key];
 
       if (typeof oldVal == "object" && oldVal != null && typeof newVal == "object" && newVal != null) {
-        _generate(oldVal, newVal, patches, path + "/" + escapePathComponent(key));
+        _generate(oldVal, newVal, patches, path + "/" + escapePathComponent(key), opts);
       }
       else {
         if (oldVal !== newVal) {
           changed = true;
+          if (inversible) patches.push({ op: "test", path: path + "/" + escapePathComponent(key), value: _deepClone(oldVal) });
           patches.push({ op: "replace", path: path + "/" + escapePathComponent(key), value: _deepClone(newVal) });
         }
       }
     }
     else if(Array.isArray(mirror) === Array.isArray(obj)) {
+      if (inversible) patches.push({ op: "test", path: path + "/" + escapePathComponent(key), value: _deepClone(oldVal) })
       patches.push({ op: "remove", path: path + "/" + escapePathComponent(key) });
       deleted = true; // property has been deleted
     } else {
+      if (inversible) patches.push({ op: "test", path, value: mirror });
       patches.push({ op: "replace", path, value: obj });
       changed = true;
     }
@@ -217,8 +224,8 @@ function _generate(mirror, obj, patches, path) {
 /**
  * Create an array of patches from the differences in two objects
  */
-export function compare(tree1: Object | Array<any>, tree2: Object | Array<any>): Operation[] {
+export function compare(tree1: Object | Array<any>, tree2: Object | Array<any>, opts?: { inversible: boolean }): Operation[] {
   var patches = [];
-  _generate(tree1, tree2, patches, '');
+  _generate(tree1, tree2, patches, '', opts);
   return patches;
 }
diff --git a/test/spec/duplexBenchmark.js b/test/spec/duplexBenchmark.js
index d619f66e..ef278804 100644
--- a/test/spec/duplexBenchmark.js
+++ b/test/spec/duplexBenchmark.js
@@ -145,6 +145,136 @@ suite.add('compare operation same but deep objects', {
   }
 });
 
+// Benchmark generating test operations
+suite.add('generate operation, with inversible set to true', {
+  setup: function() {
+    var obj = {
+      firstName: 'Albert',
+      lastName: 'Einstein',
+      phoneNumbers: [
+        {
+          number: '12345'
+        },
+        {
+          number: '45353'
+        }
+      ]
+    };
+    var observer = jsonpatch.observe(obj, undefined, true);
+  },
+  fn: function() {
+    obj.firstName = 'Joachim';
+    obj.lastName = 'Wester';
+    obj.phoneNumbers[0].number = '123';
+    obj.phoneNumbers[1].number = '456';
+
+    var patches = jsonpatch.generate(observer);
+  }
+});
+suite.add('generate operation and re-apply, with inversible set to true', {
+  setup: function() {
+    var obj = {
+      firstName: 'Albert',
+      lastName: 'Einstein',
+      phoneNumbers: [
+        {
+          number: '12345'
+        },
+        {
+          number: '45353'
+        }
+      ]
+    };
+    var observer = jsonpatch.observe(obj, undefined, true);
+  },
+  fn: function() {
+    obj.firstName = 'Joachim';
+    obj.lastName = 'Wester';
+    obj.phoneNumbers[0].number = '123';
+    obj.phoneNumbers[1].number = '456';
+
+    var patches = jsonpatch.generate(observer);
+    obj2 = {
+      firstName: 'Albert',
+      lastName: 'Einstein',
+      phoneNumbers: [
+        {
+          number: '12345'
+        },
+        {
+          number: '45353'
+        }
+      ]
+    };
+
+    jsonpatch.applyPatch(obj2, patches);
+  }
+});
+suite.add('compare operation, with inversible set to true', {
+  setup: function() {
+    var obj = {
+      firstName: 'Albert',
+      lastName: 'Einstein',
+      phoneNumbers: [
+        {
+          number: '12345'
+        },
+        {
+          number: '45353'
+        }
+      ]
+    };
+    var obj2 = {
+      firstName: 'Joachim',
+      lastName: 'Wester',
+      mobileNumbers: [
+        {
+          number: '12345'
+        },
+        {
+          number: '45353'
+        }
+      ]
+    };
+  },
+  fn: function() {
+    var patches = jsonpatch.compare(obj, obj2, { inversible: true });
+  }
+});
+
+suite.add('compare operation same but deep objects, with inversible set to true', {
+  setup: function() {
+    var depth = 10;
+
+    function shallowObj() {
+      return {
+        shallow: {
+          firstName: 'Tomek',
+          lastName: 'Wytrebowicz',
+          mobileNumbers: [
+            {
+              number: '12345'
+            },
+            {
+              number: '45353'
+            }
+          ]
+        }
+      };
+    }
+    var obj = shallowObj();
+    var node = obj;
+    while (depth-- > 0) {
+      node.nested = shallowObj();
+      node = node.nested;
+    }
+    var obj2 = obj;
+  },
+  fn: function() {
+    var patches = jsonpatch.compare(obj, obj2, { inversible: true });
+  }
+});
+
 // if we are in the browser with benchmark < 2.1.2
 if (typeof benchmarkReporter !== 'undefined') {
   benchmarkReporter(suite);
diff --git a/test/spec/duplexSpec.js b/test/spec/duplexSpec.js
index ce65f1fd..70155e34 100644
--- a/test/spec/duplexSpec.js
+++ b/test/spec/duplexSpec.js
@@ -198,10 +198,15 @@ describe('duplex', function() {
       person1.firstName = 'Alexander';
       person2.firstName = 'Lucas';
 
-      var patch1 = jsonpatch.generate(observer1);
-      var patch2 = jsonpatch.generate(observer2);
+      var patch1 = jsonpatch.generate(observer1, { inversible: true });
+      var patch2 = jsonpatch.generate(observer2, { inversible: true });
 
       expect(patch1).toReallyEqual([
+        {
+          op: "test",
+          path: "/firstName",
+          value: "Alexandra"
+        },
         {
           op: 'replace',
           path: '/firstName',
@@ -209,6 +214,11 @@ describe('duplex', function() {
         }
       ]);
       expect(patch2).toReallyEqual([
+        {
+          op: "test",
+          path: "/firstName",
+          value: "Lisa"
+        },
         {
           op: 'replace',
           path: '/firstName',
@@ -231,11 +241,16 @@ describe('duplex', function() {
         ]
       };
 
-      var observer = jsonpatch.observe(obj);
+      var observer = jsonpatch.observe(obj, undefined, true);
       obj.firstName = 'Marcin';
 
       var patches = jsonpatch.generate(observer);
       expect(patches).toReallyEqual([
+        {
+          op: "test",
+          path: "/firstName",
+          value: "Albert"
+        },
         {
           op: 'replace',
           path: '/firstName',
@@ -246,6 +261,11 @@ describe('duplex', function() {
       obj.lastName = 'Warp';
       patches = jsonpatch.generate(observer); //first patch should NOT be reported again here
       expect(patches).toReallyEqual([
+        {
+          op: "test",
+          path: "/lastName",
+          value: "Einstein"
+        },
         {
           op: 'replace',
           path: '/lastName',
@@ -281,11 +301,16 @@ describe('duplex', function() {
         ]
       };
 
-      var observer = jsonpatch.observe(obj);
+      var observer = jsonpatch.observe(obj, undefined, true);
       obj.phoneNumbers[0].number = '123';
 
       var patches = jsonpatch.generate(observer);
       expect(patches).toReallyEqual([
+        {
+          op: "test",
+          path: "/phoneNumbers/0/number",
+          value: "12345"
+        },
         {
           op: 'replace',
           path: '/phoneNumbers/0/number',
@@ -296,6 +321,11 @@ describe('duplex', function() {
       obj.phoneNumbers[1].number = '456';
       patches = jsonpatch.generate(observer); //first patch should NOT be reported again here
       expect(patches).toReallyEqual([
+        {
+          op: "test",
+          path: "/phoneNumbers/1/number",
+          value: "45353"
+        },
         {
           op: 'replace',
           path: '/phoneNumbers/1/number',