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X2 Framework for Node.js | JSON Patches

This module is an implementation of JSON Patch (RFC 6902) for use with the record objects as defined by the X2 Framework's x2node-records module. Given patch specification JSON, the module builds a patch object that can be applied to records of the specified type.

The module also supports Merge Patch specification (RFC 7396).

See module's API Reference Documentation.


A patch is represented by a RecordPatch class object and is parsed from the JSON using module's build() function. For example:

const records = require('x2node-records');
const patches = require('x2node-patches');

const recordTypes = records.buildLibrary({
    recordTypes: {
        'Order': {

// build the patch
const patch =, 'Order', [
    { "op": "test", "path": "/a/b/c", "value": "foo" },
    { "op": "remove", "path": "/a/b/c" },
    { "op": "add", "path": "/a/b/c", "value": [ "foo", "bar" ] },
    { "op": "replace", "path": "/a/b/c", "value": 42 },
    { "op": "move", "from": "/a/b/c", "path": "/a/b/d" },
    { "op": "copy", "from": "/a/b/d", "path": "/a/b/e" }

// apply patch to a record
const order = ...

The build() function takes the following arguments:

  • recordTypes - A RecordTypesLibrary instance.

  • recordTypeName - Name of the record type, against records of which the patch is going to be applied.

  • patch - The JSON patch specification according to the RFC 6902 specification.

If the record type is invalid, an X2UsageError is thrown. If anything is wrong with the provided patch specification, the function throws an X2SyntaxError. Otherwise, it returns a RecordPatch instance, which exposes the following properties and methods:

  • involvedPropPaths - A Set of paths (in dot notation) of all record properties involved (read, erased and updated) in the patch.

  • updatedPropPaths - A Set of paths (in dot notaion) of those properties that may be updated by the patch, including paths of all parent properties of updated nested object properties. It will exclude properties that are only involved in "test" operations or as "from" properties of "copy" operations. Note that whether the patch actually changes the property value will depend on the current value in the supplied record. Naturally, if the value is the same, it won't change even though the property is still listed in the updatedPropPaths.

  • apply(record, [handlers]) - Applies the patch to the specified record. If the record is not good for the patch (e.g. some properties are missing that are expected to be present by the patch specification), the method throws an X2DataError. Otherwise, it makes the necessary modifications to the provided record object and returns either true if all good, or false if a "test" patch operation fails. Note, that the method does not provide transactionality, so in case of an error or a failed "test" operation the provided record object may be left partially modified.

Optionally, the apply() method can be provided with a handlers object that implements RecordPatchHandlers interface. The interface methods on the object, if present, are invoked during the patch application to notify it about the changes that the patch is making to the record as it goes through the patch operations. The methods are:

  • onInsert(op, ptr, newValue, oldValue) - Called when a value is added to an array or map property as a result of an "add", "move" or "copy" patch operation. The op argument is the operation, which can be "add", "move" or "copy". The ptr is a RecordElementPointer from the x2node-pointers module pointing at the array or map element, and the newValue is the value being inserted. The value may be null for a simple (non nested object) value array or map element, but never undefined. The oldValue is the previous value at the pointer location as would be returned by the pointer's getValue() method.

  • onRemove(op, ptr, oldValue) - Called when a value is removed from an array or map property as a result of a "remove" or "move" patch operation. The op argument is the operation, which can be "remove" or "move". The ptr is a RecordElementPointer pointing at the array or map element. The oldValue is the previous value at the pointer location.

  • onSet(op, ptr, newValue, oldValue) - Called when a value is set to a property (or an array or a map element is replaced) as a result of an "add", "remove" (the newValue is null), "replace", "move" or "copy" patch operation. The op argument is the operation, which can be "add", "remove", "replace", "move" or "copy". The ptr is a RecordElementPointer, and the value is the value being set. The value may be null but never undefined. The oldValue is the previous value at the pointer location.

  • onTest(ptr, value, passed) - Called when a property value is tested as a result of a "test" patch operation. The ptr is a RecordElementPointer pointing at the property, value is the value, against which it is tested and passed is true if the test was successful.

The methods are called only if present on the provided handlers object and only if the record is actually modified as a result of the operation (except the onTest(), which does not modify the record and is called always, if present).

Merge Patch

Alternatively, instead of JSON Patch the patch may be specified using Merge Patch format:

const patch = patches.buildMerge(recordTypes, 'Order', {
    quantity: 10,
    status: 'ADJUSTED'

The only difference is that buildMerge() function is used instead of the regular build(). The resulting patch object follows the same specification as described above.

Diffing Records

Another option is to build JSON patch specification by analyzing differences between two records. The module provides fromDiff() function for that:

const patchSpec = patches.fromDiff(recordTypes, 'Order', orignalOrder, updatedOrder);

const patch =, 'Order', patchSpec);

A few notes about fromDiff() function:

  • The top record id property is allowed to be missing in the provided updated record (if present, must be the same).
  • View, calculated and record meta-info properties are ignored.
  • Unrecognized properties in the updated record are not allowed.
  • The resulting patch specification may still be invalid. For example, fromDiff() does not check if properties are modifiable or optional. Attempt to build a patch from the resulting specification will reveal the error.
  • The array properties are assumed to be sorted using the same criteria in both the original and the updated records.
  • Elements of nested object arrays must have id property. All elements in the original record must have an id value. Elements in the updated record that do not have the id or have an id that is not found in the original array are assumed to be new and are inserted.


RFC 6902 JSON Patch implementation for X2 Framework.








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