diff --git a/.travis.yml b/.travis.yml
index 4b8f518a..7c35fe92 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,4 +5,4 @@ before_script:
   - npm install
   - npm run serve &
 script:
-  - npm run test-sauce
+  - npm run test
diff --git a/README.md b/README.md
index 9c93221a..e1e9f262 100644
--- a/README.md
+++ b/README.md
@@ -154,6 +154,24 @@ var patch = jsonpatch.generate(observer);
 // ];
 ```
 
+Generating patches with test operations for values in the first object:
+
+```js
+var document = { firstName: "Joachim", lastName: "Wester", contactDetails: { phoneNumbers: [ { number:"555-123" }] } };
+var observer = jsonpatch.observe(document);
+document.firstName = "Albert";
+document.contactDetails.phoneNumbers[0].number = "123";
+document.contactDetails.phoneNumbers.push({ number:"456" });
+var patch = jsonpatch.generate(observer, true);
+// 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 for values in the first object:
+
+```js
+var documentA = {user: {firstName: "Albert", lastName: "Einstein"}};
+var documentB = {user: {firstName: "Albert", lastName: "Collins"}};
+var diff = jsonpatch.compare(documentA, documentB, true);
+//diff == [
+//   {op: "test", path: "/user/lastName", value: "Einstein"},
+//   {op: "replace", path: "/user/lastName", value: "Collins"}
+// ];
+```
+
 Validating a sequence of patches:
 
 ```js
@@ -269,10 +299,10 @@ callback is called with the generated patches array as the parameter.
 
 Returns `observer`.
 
-#### `jsonpatch.generate(document: any, observer: Observer): Operation[]`
+#### `jsonpatch.generate(document: any, observer: Observer, invertible = false): 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 `invertible` is true, then each change will be preceded by a test operation of the value before the change.
 
 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, invertible = false): 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 `invertible` is true, then each change will be preceded by a test operation of the value in `document1`.
 
 If there are no differences, returns an empty array (length 0).
 
@@ -346,7 +376,7 @@ Functions `applyPatch`, `applyOperation`, and `validate` accept a `validate`/ `v
 If you pass a validator, it will be called with four parameters for each operation, `function(operation, index, tree, existingPath)` and it is expected to throw `JsonPatchError` when your conditions are not met.
 
 - `operation` The operation it self.
-- `index` `operation`'s index in the patch array (if application).  
+- `index` `operation`'s index in the patch array (if application).
 - `tree` The object that is supposed to be patched.
 - `existingPath` the path `operation` points to.
 
diff --git a/dist/fast-json-patch.js b/dist/fast-json-patch.js
index 54ee455d..4048e979 100644
--- a/dist/fast-json-patch.js
+++ b/dist/fast-json-patch.js
@@ -674,7 +674,7 @@ Object.defineProperty(exports, "__esModule", { value: true });
  */
 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;
@@ -722,7 +722,8 @@ 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);
@@ -737,7 +738,7 @@ function observe(obj, callback) {
     if (observer) {
         return observer;
     }
-    observer = {};
+    observer = { inversible: inversible };
     mirror.value = helpers_1._deepClone(obj);
     if (callback) {
         observer.callback = callback;
@@ -794,9 +795,11 @@ 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);
     }
@@ -811,7 +814,8 @@ 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;
     }
@@ -822,6 +826,7 @@ 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];
@@ -829,20 +834,26 @@ 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;
         }
@@ -860,9 +871,9 @@ 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 a3eb6c16..46eef41b 100644
--- a/dist/fast-json-patch.min.js
+++ b/dist/fast-json-patch.min.js
@@ -5,10 +5,10 @@ var jsonpatch=function(e){var t={};function n(r){if(t[r])return t[r].exports;var
  * (c) 2017 Joachim Wester
  * MIT license
  */
-var n,r=this&&this.__extends||(n=function(e,t){return(n=Object.setPrototypeOf||{__proto__:[]}instanceof Array&&function(e,t){e.__proto__=t}||function(e,t){for(var n in t)t.hasOwnProperty(n)&&(e[n]=t[n])})(e,t)},function(e,t){function r(){this.constructor=e}n(e,t),e.prototype=null===t?Object.create(t):(r.prototype=t.prototype,new r)});Object.defineProperty(t,"__esModule",{value:!0});var o=Object.prototype.hasOwnProperty;function a(e,t){return o.call(e,t)}function i(e){if(Array.isArray(e)){for(var t=new Array(e.length),n=0;n<t.length;n++)t[n]=""+n;return t}if(Object.keys)return Object.keys(e);t=[];for(var r in e)a(e,r)&&t.push(r);return t}function u(e){return-1===e.indexOf("/")&&-1===e.indexOf("~")?e:e.replace(/~/g,"~0").replace(/\//g,"~1")}function p(e,t){var n;for(var r in e)if(a(e,r)){if(e[r]===t)return u(r)+"/";if("object"==typeof e[r]&&""!=(n=p(e[r],t)))return u(r)+"/"+n}return""}function c(e,t){var n=[e];for(var r in t){var o="object"==typeof t[r]?JSON.stringify(t[r],null,2):t[r];void 0!==o&&n.push(r+": "+o)}return n.join("\n")}t.hasOwnProperty=a,t._objectKeys=i,t._deepClone=function(e){switch(typeof e){case"object":return JSON.parse(JSON.stringify(e));case"undefined":return null;default:return e}},t.isInteger=function(e){for(var t,n=0,r=e.length;n<r;){if(!((t=e.charCodeAt(n))>=48&&t<=57))return!1;n++}return!0},t.escapePathComponent=u,t.unescapePathComponent=function(e){return e.replace(/~1/g,"/").replace(/~0/g,"~")},t._getPathRecursive=p,t.getPath=function(e,t){if(e===t)return"/";var n=p(e,t);if(""===n)throw new Error("Object not found in root");return"/"+n},t.hasUndefined=function e(t){if(void 0===t)return!0;if(t)if(Array.isArray(t)){for(var n=0,r=t.length;n<r;n++)if(e(t[n]))return!0}else if("object"==typeof t){var o=i(t),a=o.length;for(n=0;n<a;n++)if(e(t[o[n]]))return!0}return!1};var s=function(e){function t(t,n,r,o,a){var i=this.constructor,u=e.call(this,c(t,{name:n,index:r,operation:o,tree:a}))||this;return u.name=n,u.index=r,u.operation=o,u.tree=a,Object.setPrototypeOf(u,i.prototype),u.message=c(t,{name:n,index:r,operation:o,tree:a}),u}return r(t,e),t}(Error);t.PatchError=s},function(e,t,n){Object.defineProperty(t,"__esModule",{value:!0});var r=n(3),o=n(0);t.JsonPatchError=o.PatchError,t.deepClone=o._deepClone;var a={add:function(e,t,n){return e[t]=this.value,{newDocument:n}},remove:function(e,t,n){var r=e[t];return delete e[t],{newDocument:n,removed:r}},replace:function(e,t,n){var r=e[t];return e[t]=this.value,{newDocument:n,removed:r}},move:function(e,t,n){var r=u(n,this.path);r&&(r=o._deepClone(r));var a=p(n,{op:"remove",path:this.from}).removed;return p(n,{op:"add",path:this.path,value:a}),{newDocument:n,removed:r}},copy:function(e,t,n){var r=u(n,this.from);return p(n,{op:"add",path:this.path,value:o._deepClone(r)}),{newDocument:n}},test:function(e,t,n){return{newDocument:n,test:r(e[t],this.value)}},_get:function(e,t,n){return this.value=e[t],{newDocument:n}}},i={add:function(e,t,n){return o.isInteger(t)?e.splice(t,0,this.value):e[t]=this.value,{newDocument:n,index:t}},remove:function(e,t,n){return{newDocument:n,removed:e.splice(t,1)[0]}},replace:function(e,t,n){var r=e[t];return e[t]=this.value,{newDocument:n,removed:r}},move:a.move,copy:a.copy,test:a.test,_get:a._get};function u(e,t){if(""==t)return e;var n={op:"_get",path:t};return p(e,n),n.value}function p(e,n,p,c,f,l){if(void 0===p&&(p=!1),void 0===c&&(c=!0),void 0===f&&(f=!0),void 0===l&&(l=0),p&&("function"==typeof p?p(n,0,e,n.path):s(n,0)),""===n.path){var d={newDocument:e};if("add"===n.op)return d.newDocument=n.value,d;if("replace"===n.op)return d.newDocument=n.value,d.removed=e,d;if("move"===n.op||"copy"===n.op)return d.newDocument=u(e,n.from),"move"===n.op&&(d.removed=e),d;if("test"===n.op){if(d.test=r(e,n.value),!1===d.test)throw new t.JsonPatchError("Test operation failed","TEST_OPERATION_FAILED",l,n,e);return d.newDocument=e,d}if("remove"===n.op)return d.removed=e,d.newDocument=null,d;if("_get"===n.op)return n.value=e,d;if(p)throw new t.JsonPatchError("Operation `op` property is not one of operations defined in RFC-6902","OPERATION_OP_INVALID",l,n,e);return d}c||(e=o._deepClone(e));var h=(n.path||"").split("/"),v=e,m=1,y=h.length,w=void 0,E=void 0,_=void 0;for(_="function"==typeof p?p:s;;){if(E=h[m],f&&"__proto__"==E)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(p&&void 0===w&&(void 0===v[E]?w=h.slice(0,m).join("/"):m==y-1&&(w=n.path),void 0!==w&&_(n,0,e,w)),m++,Array.isArray(v)){if("-"===E)E=v.length;else{if(p&&!o.isInteger(E))throw new t.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",l,n,e);o.isInteger(E)&&(E=~~E)}if(m>=y){if(p&&"add"===n.op&&E>v.length)throw new t.JsonPatchError("The specified index MUST NOT be greater than the number of elements in the array","OPERATION_VALUE_OUT_OF_BOUNDS",l,n,e);if(!1===(d=i[n.op].call(n,v,E,e)).test)throw new t.JsonPatchError("Test operation failed","TEST_OPERATION_FAILED",l,n,e);return d}}else if(E&&-1!=E.indexOf("~")&&(E=o.unescapePathComponent(E)),m>=y){if(!1===(d=a[n.op].call(n,v,E,e)).test)throw new t.JsonPatchError("Test operation failed","TEST_OPERATION_FAILED",l,n,e);return d}v=v[E]}}function c(e,n,r,a,i){if(void 0===a&&(a=!0),void 0===i&&(i=!0),r&&!Array.isArray(n))throw new t.JsonPatchError("Patch sequence must be an array","SEQUENCE_NOT_AN_ARRAY");a||(e=o._deepClone(e));for(var u=new Array(n.length),c=0,s=n.length;c<s;c++)u[c]=p(e,n[c],r,!0,i,c),e=u[c].newDocument;return u.newDocument=e,u}function s(e,n,r,i){if("object"!=typeof e||null===e||Array.isArray(e))throw new t.JsonPatchError("Operation is not an object","OPERATION_NOT_AN_OBJECT",n,e,r);if(!a[e.op])throw new t.JsonPatchError("Operation `op` property is not one of operations defined in RFC-6902","OPERATION_OP_INVALID",n,e,r);if("string"!=typeof e.path)throw new t.JsonPatchError("Operation `path` property is not a string","OPERATION_PATH_INVALID",n,e,r);if(0!==e.path.indexOf("/")&&e.path.length>0)throw new t.JsonPatchError('Operation `path` property must start with "/"',"OPERATION_PATH_INVALID",n,e,r);if(("move"===e.op||"copy"===e.op)&&"string"!=typeof e.from)throw new t.JsonPatchError("Operation `from` property is not present (applicable in `move` and `copy` operations)","OPERATION_FROM_REQUIRED",n,e,r);if(("add"===e.op||"replace"===e.op||"test"===e.op)&&void 0===e.value)throw new t.JsonPatchError("Operation `value` property is not present (applicable in `add`, `replace` and `test` operations)","OPERATION_VALUE_REQUIRED",n,e,r);if(("add"===e.op||"replace"===e.op||"test"===e.op)&&o.hasUndefined(e.value))throw new t.JsonPatchError("Operation `value` property is not present (applicable in `add`, `replace` and `test` operations)","OPERATION_VALUE_CANNOT_CONTAIN_UNDEFINED",n,e,r);if(r)if("add"==e.op){var u=e.path.split("/").length,p=i.split("/").length;if(u!==p+1&&u!==p)throw new t.JsonPatchError("Cannot perform an `add` operation at the desired path","OPERATION_PATH_CANNOT_ADD",n,e,r)}else if("replace"===e.op||"remove"===e.op||"_get"===e.op){if(e.path!==i)throw new t.JsonPatchError("Cannot perform the operation at a path that does not exist","OPERATION_PATH_UNRESOLVABLE",n,e,r)}else if("move"===e.op||"copy"===e.op){var c=f([{op:"_get",path:e.from,value:void 0}],r);if(c&&"OPERATION_PATH_UNRESOLVABLE"===c.name)throw new t.JsonPatchError("Cannot perform the operation from a path that does not exist","OPERATION_FROM_UNRESOLVABLE",n,e,r)}}function f(e,n,r){try{if(!Array.isArray(e))throw new t.JsonPatchError("Patch sequence must be an array","SEQUENCE_NOT_AN_ARRAY");if(n)c(o._deepClone(n),o._deepClone(e),r||!0);else{r=r||s;for(var a=0;a<e.length;a++)r(e[a],a,n,void 0)}}catch(e){if(e instanceof t.JsonPatchError)return e;throw e}}t.getValueByPointer=u,t.applyOperation=p,t.applyPatch=c,t.applyReducer=function(e,n,r){var o=p(e,n);if(!1===o.test)throw new t.JsonPatchError("Test operation failed","TEST_OPERATION_FAILED",r,n,e);return o.newDocument},t.validator=s,t.validate=f},function(e,t,n){Object.defineProperty(t,"__esModule",{value:!0});
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 /*!
  * https://github.com/Starcounter-Jack/JSON-Patch
  * (c) 2017 Joachim Wester
  * MIT license
  */
-var r=n(0),o=n(1),a=n(1);t.applyOperation=a.applyOperation,t.applyPatch=a.applyPatch,t.applyReducer=a.applyReducer,t.getValueByPointer=a.getValueByPointer,t.validate=a.validate,t.validator=a.validator;var i=n(0);t.JsonPatchError=i.PatchError,t.deepClone=i._deepClone,t.escapePathComponent=i.escapePathComponent,t.unescapePathComponent=i.unescapePathComponent;var u=new WeakMap,p=function(){return function(e){this.observers=new Map,this.obj=e}}(),c=function(){return function(e,t){this.callback=e,this.observer=t}}();function s(e){var t=u.get(e.object);f(t.value,e.object,e.patches,""),e.patches.length&&o.applyPatch(t.value,e.patches);var n=e.patches;return n.length>0&&(e.patches=[],e.callback&&e.callback(n)),n}function f(e,t,n,o){if(t!==e){"function"==typeof t.toJSON&&(t=t.toJSON());for(var a=r._objectKeys(t),i=r._objectKeys(e),u=!1,p=i.length-1;p>=0;p--){var c=e[l=i[p]];if(!r.hasOwnProperty(t,l)||void 0===t[l]&&void 0!==c&&!1===Array.isArray(t))Array.isArray(e)===Array.isArray(t)?(n.push({op:"remove",path:o+"/"+r.escapePathComponent(l)}),u=!0):(n.push({op:"replace",path:o,value:t}),!0);else{var s=t[l];"object"==typeof c&&null!=c&&"object"==typeof s&&null!=s?f(c,s,n,o+"/"+r.escapePathComponent(l)):c!==s&&(!0,n.push({op:"replace",path:o+"/"+r.escapePathComponent(l),value:r._deepClone(s)}))}}if(u||a.length!=i.length)for(p=0;p<a.length;p++){var l=a[p];r.hasOwnProperty(e,l)||void 0===t[l]||n.push({op:"add",path:o+"/"+r.escapePathComponent(l),value:r._deepClone(t[l])})}}}t.unobserve=function(e,t){t.unobserve()},t.observe=function(e,t){var n,o=function(e){return u.get(e)}(e);if(o){var a=function(e,t){return e.observers.get(t)}(o,t);n=a&&a.observer}else o=new p(e),u.set(e,o);if(n)return n;if(n={},o.value=r._deepClone(e),t){n.callback=t,n.next=null;var i=function(){s(n)},f=function(){clearTimeout(n.next),n.next=setTimeout(i)};"undefined"!=typeof window&&(window.addEventListener?(window.addEventListener("mouseup",f),window.addEventListener("keyup",f),window.addEventListener("mousedown",f),window.addEventListener("keydown",f),window.addEventListener("change",f)):(document.documentElement.attachEvent("onmouseup",f),document.documentElement.attachEvent("onkeyup",f),document.documentElement.attachEvent("onmousedown",f),document.documentElement.attachEvent("onkeydown",f),document.documentElement.attachEvent("onchange",f)))}return n.patches=[],n.object=e,n.unobserve=function(){s(n),clearTimeout(n.next),function(e,t){e.observers.delete(t.callback)}(o,n),"undefined"!=typeof window&&(window.removeEventListener?(window.removeEventListener("mouseup",f),window.removeEventListener("keyup",f),window.removeEventListener("mousedown",f),window.removeEventListener("keydown",f)):(document.documentElement.detachEvent("onmouseup",f),document.documentElement.detachEvent("onkeyup",f),document.documentElement.detachEvent("onmousedown",f),document.documentElement.detachEvent("onkeydown",f)))},o.observers.set(t,new c(t,n)),n},t.generate=s,t.compare=function(e,t){var n=[];return f(e,t,n,""),n}},function(e,t,n){"use strict";var r=Array.isArray,o=Object.keys,a=Object.prototype.hasOwnProperty;e.exports=function e(t,n){if(t===n)return!0;if(t&&n&&"object"==typeof t&&"object"==typeof n){var i,u,p,c=r(t),s=r(n);if(c&&s){if((u=t.length)!=n.length)return!1;for(i=u;0!=i--;)if(!e(t[i],n[i]))return!1;return!0}if(c!=s)return!1;var f=t instanceof Date,l=n instanceof Date;if(f!=l)return!1;if(f&&l)return t.getTime()==n.getTime();var d=t instanceof RegExp,h=n instanceof RegExp;if(d!=h)return!1;if(d&&h)return t.toString()==n.toString();var v=o(t);if((u=v.length)!==o(n).length)return!1;for(i=u;0!=i--;)if(!a.call(n,v[i]))return!1;for(i=u;0!=i--;)if(!e(t[p=v[i]],n[p]))return!1;return!0}return t!=t&&n!=n}}]);
\ No newline at end of file
+var r=n(0),o=n(1),a=n(1);t.applyOperation=a.applyOperation,t.applyPatch=a.applyPatch,t.applyReducer=a.applyReducer,t.getValueByPointer=a.getValueByPointer,t.validate=a.validate,t.validator=a.validator;var i=n(0);t.JsonPatchError=i.PatchError,t.deepClone=i._deepClone,t.escapePathComponent=i.escapePathComponent,t.unescapePathComponent=i.unescapePathComponent;var p=new WeakMap,u=function(){return function(e){this.observers=new Map,this.obj=e}}(),c=function(){return function(e,t){this.callback=e,this.observer=t}}();function s(e,t){void 0===t&&(t={});var n=p.get(e.object),r=void 0!==t.inversible?t.inversible:e.inversible;f(n.value,e.object,e.patches,"",{inversible:r}),e.patches.length&&o.applyPatch(n.value,e.patches);var a=e.patches;return a.length>0&&(e.patches=[],e.callback&&e.callback(a)),a}function f(e,t,n,o,a){if(void 0===a&&(a={inversible:!1}),t!==e){"function"==typeof t.toJSON&&(t=t.toJSON());for(var i=r._objectKeys(t),p=r._objectKeys(e),u=!1,c=a.inversible,s=p.length-1;s>=0;s--){var l=e[h=p[s]];if(!r.hasOwnProperty(t,h)||void 0===t[h]&&void 0!==l&&!1===Array.isArray(t))Array.isArray(e)===Array.isArray(t)?(c&&n.push({op:"test",path:o+"/"+r.escapePathComponent(h),value:r._deepClone(l)}),n.push({op:"remove",path:o+"/"+r.escapePathComponent(h)}),u=!0):(c&&n.push({op:"test",path:o,value:e}),n.push({op:"replace",path:o,value:t}),!0);else{var d=t[h];"object"==typeof l&&null!=l&&"object"==typeof d&&null!=d?f(l,d,n,o+"/"+r.escapePathComponent(h),a):l!==d&&(!0,c&&n.push({op:"test",path:o+"/"+r.escapePathComponent(h),value:r._deepClone(l)}),n.push({op:"replace",path:o+"/"+r.escapePathComponent(h),value:r._deepClone(d)}))}}if(u||i.length!=p.length)for(s=0;s<i.length;s++){var h=i[s];r.hasOwnProperty(e,h)||void 0===t[h]||n.push({op:"add",path:o+"/"+r.escapePathComponent(h),value:r._deepClone(t[h])})}}}t.unobserve=function(e,t){t.unobserve()},t.observe=function(e,t,n){void 0===n&&(n=!1);var o,a=function(e){return p.get(e)}(e);if(a){var i=function(e,t){return e.observers.get(t)}(a,t);o=i&&i.observer}else a=new u(e),p.set(e,a);if(o)return o;if(o={inversible:n},a.value=r._deepClone(e),t){o.callback=t,o.next=null;var f=function(){s(o)},l=function(){clearTimeout(o.next),o.next=setTimeout(f)};"undefined"!=typeof window&&(window.addEventListener?(window.addEventListener("mouseup",l),window.addEventListener("keyup",l),window.addEventListener("mousedown",l),window.addEventListener("keydown",l),window.addEventListener("change",l)):(document.documentElement.attachEvent("onmouseup",l),document.documentElement.attachEvent("onkeyup",l),document.documentElement.attachEvent("onmousedown",l),document.documentElement.attachEvent("onkeydown",l),document.documentElement.attachEvent("onchange",l)))}return o.patches=[],o.object=e,o.unobserve=function(){s(o),clearTimeout(o.next),function(e,t){e.observers.delete(t.callback)}(a,o),"undefined"!=typeof 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)))},a.observers.set(t,new c(t,o)),o},t.generate=s,t.compare=function(e,t,n){var r=[];return f(e,t,r,"",n),r}},function(e,t,n){"use strict";var r=Array.isArray,o=Object.keys,a=Object.prototype.hasOwnProperty;e.exports=function e(t,n){if(t===n)return!0;if(t&&n&&"object"==typeof t&&"object"==typeof n){var i,p,u,c=r(t),s=r(n);if(c&&s){if((p=t.length)!=n.length)return!1;for(i=p;0!=i--;)if(!e(t[i],n[i]))return!1;return!0}if(c!=s)return!1;var f=t instanceof Date,l=n instanceof Date;if(f!=l)return!1;if(f&&l)return t.getTime()==n.getTime();var d=t instanceof RegExp,h=n instanceof RegExp;if(d!=h)return!1;if(d&&h)return t.toString()==n.toString();var v=o(t);if((p=v.length)!==o(n).length)return!1;for(i=p;0!=i--;)if(!a.call(n,v[i]))return!1;for(i=p;0!=i--;)if(!e(t[u=v[i]],n[u]))return!1;return!0}return t!=t&&n!=n}}]);
\ No newline at end of file
diff --git a/src/duplex.ts b/src/duplex.ts
index faefea24..150859cd 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';
@@ -144,9 +144,10 @@ 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>, invertible = false): Operation[] {
   var mirror = beforeDict.get(observer.object);
-  _generate(mirror.value, observer.object, observer.patches, "");
+
+  _generate(mirror.value, observer.object, observer.patches, "", invertible);
   if (observer.patches.length) {
     applyPatch(mirror.value, observer.patches);
   }
@@ -161,7 +162,7 @@ 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, invertible) {
   if (obj === mirror) {
     return;
   }
@@ -185,19 +186,28 @@ 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), invertible);
       }
       else {
         if (oldVal !== newVal) {
           changed = true;
+          if (invertible) { 
+            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 (invertible) {
+        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 (invertible) {
+        patches.push({ op: "test", path, value: mirror });
+      }
       patches.push({ op: "replace", path, value: obj });
       changed = true;
     }
@@ -217,8 +227,8 @@ 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>, invertible = false): Operation[] {
   var patches = [];
-  _generate(tree1, tree2, patches, '');
+  _generate(tree1, tree2, patches, '', invertible);
   return patches;
 }
diff --git a/test/spec/duplexBenchmark.js b/test/spec/duplexBenchmark.js
index 4a25e31d..7e583793 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, invertible = true', {
+  setup: function() {
+    var obj = {
+      firstName: 'Albert',
+      lastName: 'Einstein',
+      phoneNumbers: [
+        {
+          number: '12345'
+        },
+        {
+          number: '45353'
+        }
+      ]
+    };
+    var observer = jsonpatch.observe(obj);
+  },
+  fn: function() {
+    obj.firstName = 'Joachim';
+    obj.lastName = 'Wester';
+    obj.phoneNumbers[0].number = '123';
+    obj.phoneNumbers[1].number = '456';
+
+    var patches = jsonpatch.generate(observer, true);
+  }
+});
+suite.add('generate operation and re-apply, invertible = true', {
+  setup: function() {
+    var obj = {
+      firstName: 'Albert',
+      lastName: 'Einstein',
+      phoneNumbers: [
+        {
+          number: '12345'
+        },
+        {
+          number: '45353'
+        }
+      ]
+    };
+    var observer = jsonpatch.observe(obj);
+  },
+  fn: function() {
+    obj.firstName = 'Joachim';
+    obj.lastName = 'Wester';
+    obj.phoneNumbers[0].number = '123';
+    obj.phoneNumbers[1].number = '456';
+
+    var patches = jsonpatch.generate(observer, true);
+    obj2 = {
+      firstName: 'Albert',
+      lastName: 'Einstein',
+      phoneNumbers: [
+        {
+          number: '12345'
+        },
+        {
+          number: '45353'
+        }
+      ]
+    };
+
+    jsonpatch.applyPatch(obj2, patches);
+  }
+});
+suite.add('compare operation, invertible = 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, true);
+  }
+});
+
+suite.add('compare operation same but deep objects, invertible = 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, 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 19f52b1f..223d3c3a 100644
--- a/test/spec/duplexSpec.js
+++ b/test/spec/duplexSpec.js
@@ -74,6 +74,16 @@ var customMatchers = {
   }
 };
 
+function variantIt(name, variants, fn) {
+  variants.forEach(variant => {
+    it(`${name} | variant: ${variant[0]}`, fn(...variant.slice(1)));
+  });
+}
+
+function insertIf(condition, ...elements) {
+  return condition ? elements : [];
+}
+
 describe('duplex', function() {
   beforeEach(function() {
     jasmine.addMatchers(customMatchers);
@@ -183,139 +193,184 @@ describe('duplex', function() {
       expect(obj2).toReallyEqual(obj);
     });
 
-    it('should generate replace (2 observers)', function() {
-      var person1 = {
-        firstName: 'Alexandra',
-        lastName: 'Galbreath'
-      };
-      var person2 = {
-        firstName: 'Lisa',
-        lastName: 'Mendoza'
-      };
-
-      var observer1 = jsonpatch.observe(person1);
-      var observer2 = jsonpatch.observe(person2);
+    variantIt('should generate replace (2 observers)', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        var person1 = {
+          firstName: 'Alexandra',
+          lastName: 'Galbreath'
+        };
+        var person2 = {
+          firstName: 'Lisa',
+          lastName: 'Mendoza'
+        };
 
-      person1.firstName = 'Alexander';
-      person2.firstName = 'Lucas';
+        var observer1 = jsonpatch.observe(person1);
+        var observer2 = jsonpatch.observe(person2);
 
-      var patch1 = jsonpatch.generate(observer1);
-      var patch2 = jsonpatch.generate(observer2);
+        person1.firstName = 'Alexander';
+        person2.firstName = 'Lucas';
 
-      expect(patch1).toReallyEqual([
-        {
-          op: 'replace',
-          path: '/firstName',
-          value: 'Alexander'
-        }
-      ]);
-      expect(patch2).toReallyEqual([
-        {
-          op: 'replace',
-          path: '/firstName',
-          value: 'Lucas'
-        }
-      ]);
-    });
+        var patch1 = jsonpatch.generate(observer1, testInvertible);
+        var patch2 = jsonpatch.generate(observer2, testInvertible);
 
-    it('should generate replace (double change, shallow object)', function() {
-      obj = {
-        firstName: 'Albert',
-        lastName: 'Einstein',
-        phoneNumbers: [
+        expect(patch1).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: "test",
+            path: "/firstName",
+            value: "Alexandra"
+          }),
           {
-            number: '12345'
-          },
+            op: 'replace',
+            path: '/firstName',
+            value: 'Alexander'
+          }
+        ]);
+        expect(patch2).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: "test",
+            path: "/firstName",
+            value: "Lisa"
+          }),
           {
-            number: '45353'
+            op: 'replace',
+            path: '/firstName',
+            value: 'Lucas'
           }
-        ]
-      };
-
-      var observer = jsonpatch.observe(obj);
-      obj.firstName = 'Marcin';
+        ]);
+      }
+    });
 
-      var patches = jsonpatch.generate(observer);
-      expect(patches).toReallyEqual([
-        {
-          op: 'replace',
-          path: '/firstName',
-          value: 'Marcin'
-        }
-      ]);
+    variantIt('should generate test and replace (double change, shallow object)', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        obj = {
+          firstName: 'Albert',
+          lastName: 'Einstein',
+          phoneNumbers: [
+            {
+              number: '12345'
+            },
+            {
+              number: '45353'
+            }
+          ]
+        };
 
-      obj.lastName = 'Warp';
-      patches = jsonpatch.generate(observer); //first patch should NOT be reported again here
-      expect(patches).toReallyEqual([
-        {
-          op: 'replace',
-          path: '/lastName',
-          value: 'Warp'
-        }
-      ]);
+        var observer = jsonpatch.observe(obj);
+        obj.firstName = 'Marcin';
 
-      expect(obj).toReallyEqual({
-        firstName: 'Marcin',
-        lastName: 'Warp',
-        phoneNumbers: [
-          {
-            number: '12345'
-          },
+        var patches = jsonpatch.generate(observer, testInvertible);
+        expect(patches).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: "test",
+            path: "/firstName",
+            value: "Albert"
+          }),
           {
-            number: '45353'
+            op: 'replace',
+            path: '/firstName',
+            value: 'Marcin'
           }
-        ]
-      }); //objects should be still the same
-    });
+        ]);
 
-    it('should generate replace (double change, deep object)', function() {
-      obj = {
-        firstName: 'Albert',
-        lastName: 'Einstein',
-        phoneNumbers: [
-          {
-            number: '12345'
-          },
+        obj.lastName = 'Warp';
+        patches = jsonpatch.generate(observer, testInvertible); //first patch should NOT be reported again here
+        expect(patches).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: "test",
+            path: "/lastName",
+            value: "Einstein"
+          }),
           {
-            number: '45353'
+            op: 'replace',
+            path: '/lastName',
+            value: 'Warp'
           }
-        ]
-      };
+        ]);
 
-      var observer = jsonpatch.observe(obj);
-      obj.phoneNumbers[0].number = '123';
+        expect(obj).toReallyEqual({
+          firstName: 'Marcin',
+          lastName: 'Warp',
+          phoneNumbers: [
+            {
+              number: '12345'
+            },
+            {
+              number: '45353'
+            }
+          ]
+        }); //objects should be still the same
+      }
+      });
 
-      var patches = jsonpatch.generate(observer);
-      expect(patches).toReallyEqual([
-        {
-          op: 'replace',
-          path: '/phoneNumbers/0/number',
-          value: '123'
-        }
-      ]);
+    variantIt('should generate test and replace (double change, shallow object)', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        obj = {
+          firstName: 'Albert',
+          lastName: 'Einstein',
+          phoneNumbers: [
+            {
+              number: '12345'
+            },
+            {
+              number: '45353'
+            }
+          ]
+        };
 
-      obj.phoneNumbers[1].number = '456';
-      patches = jsonpatch.generate(observer); //first patch should NOT be reported again here
-      expect(patches).toReallyEqual([
-        {
-          op: 'replace',
-          path: '/phoneNumbers/1/number',
-          value: '456'
-        }
-      ]);
+        var observer = jsonpatch.observe(obj);
+        obj.phoneNumbers[0].number = '123';
 
-      expect(obj).toReallyEqual({
-        firstName: 'Albert',
-        lastName: 'Einstein',
-        phoneNumbers: [
+        var patches = jsonpatch.generate(observer, testInvertible);
+        expect(patches).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: "test",
+            path: "/phoneNumbers/0/number",
+            value: "12345"
+          }),
           {
-            number: '123'
-          },
+            op: 'replace',
+            path: '/phoneNumbers/0/number',
+            value: '123'
+          }
+        ]);
+
+        obj.phoneNumbers[1].number = '456';
+        patches = jsonpatch.generate(observer, testInvertible); //first patch should NOT be reported again here
+        expect(patches).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: "test",
+            path: "/phoneNumbers/1/number",
+            value: "45353"
+          }),
           {
-            number: '456'
+            op: 'replace',
+            path: '/phoneNumbers/1/number',
+            value: '456'
           }
-        ]
-      }); //objects should be still the same
+        ]);
+
+        expect(obj).toReallyEqual({
+          firstName: 'Albert',
+          lastName: 'Einstein',
+          phoneNumbers: [
+            {
+              number: '123'
+            },
+            {
+              number: '456'
+            }
+          ]
+        }); //objects should be still the same
+      }
     });
 
     it('should generate replace (changes in new array cell, primitive values)', function() {
@@ -475,34 +530,49 @@ describe('duplex', function() {
       expect(obj2).toEqualInJson(obj);
     });
 
-    it('should generate remove (array indexes should be sorted descending)', function() {
-      obj = {
-        items: ['a', 'b', 'c']
-      };
-      var observer = jsonpatch.observe(obj);
+    variantIt('should generate test and replace (double change, shallow object)', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        obj = {
+          items: ['a', 'b', 'c']
+        };
+        var observer = jsonpatch.observe(obj);
 
-      obj.items.pop();
-      obj.items.pop();
+        obj.items.pop();
+        obj.items.pop();
 
-      patches = jsonpatch.generate(observer);
+        patches = jsonpatch.generate(observer, testInvertible);
 
-      //array indexes must be sorted descending, otherwise there is an index collision in apply
-      expect(patches).toReallyEqual([
-        {
-          op: 'remove',
-          path: '/items/2'
-        },
-        {
-          op: 'remove',
-          path: '/items/1'
-        }
-      ]);
+        //array indexes must be sorted descending, otherwise there is an index collision in apply
+        expect(patches).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: 'test',
+            path: '/items/2',
+            value: 'c'
+          }),
+          {
+            op: 'remove',
+            path: '/items/2'
+          },
+          ...insertIf(testInvertible, {
+            op: 'test',
+            path: '/items/1',
+            value: 'b'
+          }),
+          {
+            op: 'remove',
+            path: '/items/1'
+          },
+        ]);
 
-      obj2 = {
-        items: ['a', 'b', 'c']
-      };
-      jsonpatch.applyPatch(obj2, patches);
-      expect(obj).toEqualInJson(obj2);
+        obj2 = {
+          items: ['a', 'b', 'c']
+        };
+        jsonpatch.applyPatch(obj2, patches);
+        expect(obj).toEqualInJson(obj2);
+      }
     });
 
     it('should not generate the same patch twice (replace)', function() {
@@ -1428,172 +1498,262 @@ describe('duplex', function() {
     });
   });
   describe('compare', function() {
-    it('Replacing a root array with an object should be handled well', function() {
-
-      const obj = {};
-      var patches = jsonpatch.compare(['jack'], obj);
-      expect(patches).toEqual([
-        {
-          op: 'replace',
-          path: '',
-          value: obj
-        }
-      ]);
-
+    variantIt('Replacing a root array with an object should be handled well', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+
+        const objA = ['jack'];
+        const objB = {};
+        var patches = jsonpatch.compare(objA, objB, testInvertible);
+        expect(patches).toEqual([
+          ...insertIf(testInvertible, {
+            op: 'test',
+            path: '',
+            value: objA
+          }),
+          {
+            op: 'replace',
+            path: '',
+            value: objB
+          }
+        ]);
+      }
     });
-    it('Replacing an array with an object should be handled well', function() {
-
-      const obj = {};
-      var patches = jsonpatch.compare({arr: ['jack']}, {arr: obj});
-      expect(patches).toEqual([
-        {
-          op: 'replace',
-          path: '/arr',
-          value: obj
-        }
-      ]);
-            
+    variantIt('Replacing an array with an object should be handled well', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        const arr = ['jack'];
+        const obj = {};
+        var patches = jsonpatch.compare({ arr: arr }, { arr: obj }, testInvertible);
+        expect(patches).toEqual([
+          ...insertIf(testInvertible, {
+            op: 'test',
+            path: '/arr',
+            value: arr
+          }),
+          {
+            op: 'replace',
+            path: '/arr',
+            value: obj
+          }
+        ]);
+      }
     });
-    it('Replacing an array that nested in an object with an object nested in an an object should be handled well', function() {
-
-      const obj = {};
-      var patches = jsonpatch.compare({arr: {deeperArray: ['jack']}}, {arr: {deeperArray: obj}});
-
-      expect(patches).toEqual([
-        {
-          op: 'replace',
-          path: '/arr/deeperArray',
-          value: obj
-        }
-      ]);
-            
+    variantIt('Replacing an array that nested in an object with an object nested in an an object should be handled well', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+
+        const arr = ['jack'];
+        const obj = {};
+        var patches = jsonpatch.compare({ arr: { deeperArray: arr } }, { arr: { deeperArray: obj } }, testInvertible);
+
+        expect(patches).toEqual([
+          ...insertIf(testInvertible, {
+            op: 'test',
+            path: '/arr/deeperArray',
+            value: arr
+          }),
+          {
+            op: 'replace',
+            path: '/arr/deeperArray',
+            value: obj
+          }
+        ]);
+      }
     });
-    it('should return an add for a property that does not exist in the first obj', function() {
-      var objA = {
-        user: {
-          firstName: 'Albert'
-        }
-      };
-      var objB = {
-        user: {
-          firstName: 'Albert',
-          lastName: 'Einstein'
-        }
-      };
+    variantIt('should return an add for a property that does not exist in the first obj, without a test operation', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        var objA = {
+          user: {
+            firstName: 'Albert'
+          }
+        };
+        var objB = {
+          user: {
+            firstName: 'Albert',
+            lastName: 'Einstein'
+          }
+        };
 
-      expect(jsonpatch.compare(objA, objB)).toReallyEqual([
-        {
-          op: 'add',
-          path: '/user/lastName',
-          value: 'Einstein'
-        }
-      ]);
+        expect(jsonpatch.compare(objA, objB, testInvertible)).toReallyEqual([
+          // no test operation, because undefined values are not serialized to JSON
+          {
+            op: 'add',
+            path: '/user/lastName',
+            value: 'Einstein'
+          }
+        ]);
+      }
     });
 
-    it('should return a remove for a property that does not exist in the second obj', function() {
-      var objA = {
-        user: {
-          firstName: 'Albert',
-          lastName: 'Einstein'
-        }
-      };
-      var objB = {
-        user: {
-          firstName: 'Albert'
-        }
-      };
+    variantIt('should return test and remove for a property that does not exist in the second obj', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        var objA = {
+          user: {
+            firstName: 'Albert',
+            lastName: 'Einstein'
+          }
+        };
+        var objB = {
+          user: {
+            firstName: 'Albert'
+          }
+        };
 
-      expect(jsonpatch.compare(objA, objB)).toReallyEqual([
-        {
-          op: 'remove',
-          path: '/user/lastName'
-        }
-      ]);
+        expect(jsonpatch.compare(objA, objB, testInvertible)).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: 'test',
+            path: '/user/lastName',
+            value: 'Einstein'
+          }),
+          {
+            op: 'remove',
+            path: '/user/lastName'
+          }
+        ]);
+      }
     });
 
-    it('should return a replace for a property that exists in both', function() {
-      var objA = {
-        user: {
-          firstName: 'Albert',
-          lastName: 'Einstein'
-        }
-      };
-      var objB = {
-        user: {
-          firstName: 'Albert',
-          lastName: 'Collins'
-        }
-      };
+    variantIt('should return test and replace for a property that exists in both', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        var objA = {
+          user: {
+            firstName: 'Albert',
+            lastName: 'Einstein'
+          }
+        };
+        var objB = {
+          user: {
+            firstName: 'Albert',
+            lastName: 'Collins'
+          }
+        };
 
-      expect(jsonpatch.compare(objA, objB)).toReallyEqual([
-        {
-          op: 'replace',
-          path: '/user/lastName',
-          value: 'Collins'
-        }
-      ]);
+        expect(jsonpatch.compare(objA, objB, testInvertible)).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: 'test',
+            path: '/user/lastName',
+            value: 'Einstein'
+          }),
+          {
+            op: 'replace',
+            path: '/user/lastName',
+            value: 'Collins'
+          }
+        ]);
+      }
     });
 
-    it('should replace null with object', function() {
-      var objA = {
-        user: null
-      };
-      var objB = {
-        user: {}
-      };
+    variantIt('should replace null with object', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        var objA = {
+          user: null
+        };
+        var objB = {
+          user: {}
+        };
 
-      expect(jsonpatch.compare(objA, objB)).toReallyEqual([
-        {
-          op: 'replace',
-          path: '/user',
-          value: {}
-        }
-      ]);
-    });
+        expect(jsonpatch.compare(objA, objB, testInvertible)).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: 'test',
+            path: '/user',
+            value: null
+          }),
+          {
+            op: 'replace',
+            path: '/user',
+            value: {}
+          }
+        ]);
+      }
+      });
 
-    it('should replace object with null', function() {
-      var objA = {
-        user: {}
-      };
-      var objB = {
-        user: null
-      };
+    variantIt('should replace object with null', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        var objA = {
+          user: {}
+        };
+        var objB = {
+          user: null
+        };
 
-      expect(jsonpatch.compare(objA, objB)).toReallyEqual([
-        {
-          op: 'replace',
-          path: '/user',
-          value: null
-        }
-      ]);
+        expect(jsonpatch.compare(objA, objB, testInvertible)).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: 'test',
+            path: '/user',
+            value: {}
+          }),
+          {
+            op: 'replace',
+            path: '/user',
+            value: null
+          }
+        ]);
+      }
     });
 
-    it('should not remove undefined', function() {
-      var objA = {
-        user: undefined
-      };
-      var objB = {
-        user: undefined
-      };
+    variantIt('should not remove undefined', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        var objA = {
+          user: undefined
+        };
+        var objB = {
+          user: undefined
+        };
 
-      expect(jsonpatch.compare(objA, objB)).toReallyEqual([]);
+        expect(jsonpatch.compare(objA, objB, testInvertible)).toReallyEqual([]);
+      }
     });
 
-    it('should replace 0 with empty string', function() {
-      var objA = {
-        user: 0
-      };
-      var objB = {
-        user: ''
-      };
+    variantIt('should replace 0 with empty string', [
+      ['invertible = FALSE', false],
+      ['invertible = TRUE', true]
+    ], function (testInvertible) {
+      return function () {
+        var objA = {
+          user: 0
+        };
+        var objB = {
+          user: ''
+        };
 
-      expect(jsonpatch.compare(objA, objB)).toReallyEqual([
-        {
-          op: 'replace',
-          path: '/user',
-          value: ''
-        }
-      ]);
+        expect(jsonpatch.compare(objA, objB, testInvertible)).toReallyEqual([
+          ...insertIf(testInvertible, {
+            op: 'test',
+            path: '/user',
+            value: 0
+          }),
+          {
+            op: 'replace',
+            path: '/user',
+            value: ''
+          }
+        ]);
+      };
     });
   });