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A leaner and meaner implementation of JSON-Patch. Small footprint. High performance.

Build Status

With JSON-Patch, you can:

  • apply patches
  • validate a sequence of patches
  • observe for changes (and generate patches when a change is detected)
  • compare two objects (to obtain the difference)

Why you should use JSON-Patch

JSON-Patch (RFC6902) is a standard format that allows you to update a JSON document by sending the changes rather than the whole document. JSON Patch plays well with the HTTP PATCH verb (method) and REST style programming.

Mark Nottingham has a nice blog about it.


1.22 KB minified and gzipped (3 KB minified)


add benchmark

screenshot 2015-10-22 20 35 22

replace benchmark

screenshot 2015-10-22 20 35 58

Tested on 22.10.2015. Compared libraries:

We aim the tests to be fair. Our library puts performance as the #1 priority, while other libraries can have different priorities. If you'd like to update the benchmarks or add a library, please edit the jsperf benchmarks linked above and open an issue to include new results.


  • Allows you to apply patches on object trees for incoming traffic.
  • Allows you to freely manipulate object trees and then generate patches for outgoing traffic.
  • Tested in IE 8-11, Firefox, Chrome, Safari and Node.js


Install the current version (and save it as a dependency):


$ npm install fast-json-patch --save


$ bower install fast-json-patch --save

download as ZIP

Adding to your project

In a web browser

Include json-patch.js if you want support for applying patches or include json-patch-duplex.js if you also want to generate patches.

In Node.js

Call require to get the instance:

var jsonpatch = require('fast-json-patch')


Applying patches:

var myobj = { firstName:"Albert", contactDetails: { phoneNumbers: [ ] } };
var patches = [
   {op:"replace", path:"/firstName", value:"Joachim" },
   {op:"add", path:"/lastName", value:"Wester" },
   {op:"add", path:"/contactDetails/phoneNumbers/0", value:{ number:"555-123" }  }
jsonpatch.apply( myobj, patches );
// myobj == { firstName:"Joachim", lastName:"Wester", contactDetails:{ phoneNumbers[ {number:"555-123"} ] } };

Generating patches:

var myobj = { firstName:"Joachim", lastName:"Wester", contactDetails: { phoneNumbers: [ { number:"555-123" }] } };
observer = jsonpatch.observe( myobj );
myobj.firstName = "Albert";
myobj.contactDetails.phoneNumbers[0].number = "123";
var patches = jsonpatch.generate(observer);
// patches  == [
//   { op:"replace", path="/firstName", value:"Albert"},
//   { op:"replace", path="/contactDetails/phoneNumbers/0/number", value:"123"},
//   { op:"add", path="/contactDetails/phoneNumbers/1", value:{number:"456"}}];

Comparing two object trees:

var objA = {user: {firstName: "Albert", lastName: "Einstein"}};
var objB = {user: {firstName: "Albert", lastName: "Collins"}};
var diff =, objB));
//diff == [{op: "replace", path: "/user/lastName", value: "Collins"}]

Validating a sequence of patches:

var obj = {user: {firstName: "Albert"}};
var patches = [{op: "replace", path: "/user/firstName", value: "Albert"}, {op: "replace", path: "/user/lastName", value: "Einstein"}];
var errors = jsonpatch.validate(patches, obj);
if (errors.length == 0) {
 //there are no errors!
else {
  for (var i=0; i < errors.length; i++) {
    if (!errors[i]) {
      console.log("Valid patch at index", i, patches[i]);
    else {
      console.error("Invalid patch at index", i, errors[i], patches[i]);


jsonpatch.apply (obj Object, patches Array, validate Boolean) : boolean

Available in json-patch.js and json-patch-duplex.js

Applies patches array on obj.

If the validate parameter is set to true, the patch is extensively validated before applying. An invalid patch results in throwing an error (see jsonpatch.validate for more information about the error object).

Returns an array of results - one item for each item in patches. The type of each item depends on type of operation applied

  • test - boolean result of the test
  • remove, replace and move - original object that has been removed
  • add (only when adding to an array) - index at which item has been inserted (useful when using - alias)

jsonpatch.observe (obj Object, callback Function (optional)) : observer Object

Available in json-patch-duplex.js

Sets up an deep observer on obj that listens for changes in object tree. When changes are detected, the optional callback is called with the generated patches array as the parameter.

Returns observer.

jsonpatch.generate (obj Object, observer Object) : patches Array

Available in json-patch-duplex.js

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.

If there are no pending changes in obj, returns an empty array (length 0).

jsonpatch.unobserve (obj Object, observer Object) : void

Available in json-patch-duplex.js

Destroys the observer set up on obj.

Any remaining changes are delivered synchronously (as in jsonpatch.generate). Note: this is different that ES6/7 Object.unobserve, which delivers remaining changes asynchronously. (obj1 Object, obj2 Object) : patches Array

Available in json-patch-duplex.js

Compares object trees obj1 and obj2 and returns the difference relative to obj1 as a patches array.

If there are no differences, returns an empty array (length 0).

jsonpatch.validate (patches Array, tree Object (optional)) : error JsonPatchError

Available in json-patch.js and json-patch-duplex.js

Validates a sequence of operations. If tree parameter is provided, the sequence is additionally validated against the object tree.

If there are no errors, returns undefined. If there is an errors, returns a JsonPatchError object with the following properties:

  • name String - short error code
  • message String - long human readable error message
  • index Number - index of the operation in the sequence
  • operation Object - reference to the operation
  • tree Object - reference to the tree

Possible errors:

Error name Error message
SEQUENCE_NOT_AN_ARRAY Patch sequence must be an array
OPERATION_NOT_AN_OBJECT Operation is not an object
OPERATION_OP_INVALID Operation op property is not one of operations defined in RFC-6902
OPERATION_PATH_INVALID Operation path property is not a valid string
OPERATION_FROM_REQUIRED Operation from property is not present (applicable in move and copy operations)
OPERATION_VALUE_REQUIRED Operation value property is not present, or undefined (applicable in add, replace and test operations)
OPERATION_VALUE_CANNOT_CONTAIN_UNDEFINED Operation value property object has at least one undefined value (applicable in add, replace and test operations)
OPERATION_PATH_CANNOT_ADD Cannot perform an add operation at the desired path
OPERATION_PATH_UNRESOLVABLE Cannot perform the operation at a path that does not exist
OPERATION_FROM_UNRESOLVABLE Cannot perform the operation from a path that does not exist
OPERATION_PATH_ILLEGAL_ARRAY_INDEX Expected an unsigned base-10 integer value, making the new referenced value the array element with the zero-based index
OPERATION_VALUE_OUT_OF_BOUNDS The specified index MUST NOT be greater than the number of elements in the array

undefineds (JS to JSON projection)

As undefined type does not exist in JSON, it's also not a valid value of JSON Patch operation. Therefore jsonpatch will not generate JSON Patches that sets anything to undefined.

Whenever a value is set to undefined in JS, JSON-Patch methods generate and compare will treat it similarly to how JavaScript method JSON.stringify (MDN) treats them:

If undefined (...) is encountered during conversion it is either omitted (when it is found in an object) or censored to null (when it is found in an array).

See the ECMAScript spec for details.




To see the list of recent changes, see Releases.




Lean and mean Javascript implementation of the JSON-Patch standard (RFC 6902). Update JSON documents using delta patches.



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