@broofa/jsondiff

Pragmatic and intuitive diff and patch functions for JSON data

Installation

npm install @broofa/jsondiff

Usage

Require it:

const jsondiff = require('@broofa/jsondiff');

// ... or ES6 module style:
// import jsondiff from '@broofa/jsondiff';

Start with some before and after state:

console.log(before);

⇒ {
⇒   name: 'my object',
⇒   description: "it's an object!",
⇒   details: { it: 'has', an: 'array', with: [ 'a', 'few', 'elements' ] }
⇒ }

console.log(after);

⇒ {
⇒   name: 'updated object',
⇒   title: "it's an object!",
⇒   details: {
⇒     it: 'has',
⇒     an: 'array',
⇒     with: [ 'a', 'few', 'more', 'elements', { than: 'before' } ]
⇒   }
⇒ }

Create a patch that descibes the difference between the two:

const patch = jsondiff.diff(before, after);
console.log(patch);

⇒ {
⇒   name: 'updated object',
⇒   description: '-',
⇒   details: { with: [ '+', '+', 'more', 'elements', { than: 'before' } ] },
⇒   title: "it's an object!"
⇒ }

(Note the special DROP and KEEP values ("-" and "+")! These are explained in Patch Objects, below.)

Apply patch to the before state to reproduce the after state:

const patched = jsondiff.patch(before, patch);
console.log(patched);

⇒ {
⇒   name: 'updated object',
⇒   details: {
⇒     it: 'has',
⇒     an: 'array',
⇒     with: [ 'a', 'few', 'more', 'elements', { than: 'before' } ]
⇒   },
⇒   title: "it's an object!"
⇒ }

Why yet-another diff module?

There are already several modules in this space - deep-diff, rfc6902, or fast-json-patch, to name a few. deep-diff is the most popular, however rfc6902 is (to my mind) the most compelling because it will interoperate with other libraries that support RFC6902 standard.

However ... the patch formats used by these modules tends to be cryptic and overly verbose - a list of the mutations needed to transform between the two states. In the case of deep-diff you end up with this patch:

console.log(deepPatch);

⇒ [
⇒   {
⇒     kind: 'E',
⇒     path: [ 'name' ],
⇒     lhs: 'my object',
⇒     rhs: 'updated object'
⇒   },
⇒   { kind: 'D', path: [ 'description' ], lhs: "it's an object!" },
⇒   {
⇒     kind: 'A',
⇒     path: [ 'details', 'with' ],
⇒     index: 4,
⇒     item: { kind: 'N', rhs: [ [Function: Object] ] }
⇒   },
⇒   {
⇒     kind: 'A',
⇒     path: [ 'details', 'with' ],
⇒     index: 3,
⇒     item: { kind: 'N', rhs: 'elements' }
⇒   },
⇒   {
⇒     kind: 'E',
⇒     path: [ 'details', 'with', 2 ],
⇒     lhs: 'elements',
⇒     rhs: 'more'
⇒   },
⇒   { kind: 'N', path: [ 'title' ], rhs: "it's an object!" }
⇒ ]

And for rfc6902:

console.log(rfcPatch);

⇒ [
⇒   { op: 'remove', path: '/description' },
⇒   { op: 'add', path: '/title', value: "it's an object!" },
⇒   { op: 'replace', path: '/name', value: 'updated object' },
⇒   { op: 'add', path: '/details/with/2', value: 'more' },
⇒   { op: 'add', path: '/details/with/-', value: { than: 'before' } }
⇒ ]

The advantage(?) of this module is that the patch structure mirrors the structure of the target data. As such, it terse, readable, and resilient.

That said, this module may not be for everyone. In particular, readers may find the DROP and KEEP values (described below) to be... "interesting".

API

jsondiff.diff(before, after)

Creates and returns a "patch object" that describes the differences between before and after. This object is suitable for use in patch().

jsondiff.patch(before, patch)

Applies a patch object to before and returns the result.

Note: Any result value that is deep-equal to it's before counterpart will reference the 'before' value directly, allowing === to be used as a test for deep equality.

jsondiff.value(val)

Normalize patch values. Currently this just converts DROP values to undefined, otherwise returns the value. This is useful in determining if a patch has a meaningful value. E.g.

const newPatch = {foo: jsondiff.DROP, bar: 123};

newPatch.foo; // ⇨ '-'
jsondiff.value(newPatch.foo); // ⇨ undefined
jsondiff.value(newPatch.bar); // ⇨ 123
jsondiff.value(newPatch.whups); // ⇨ undefined

Patch Objects

Patch objects are JSON objects with the same structure (schema) as the object they apply to. Applying a patch is (almost) as simple as doing a deep copy of the patch onto the target object. There are two special cases:

1. jsondiff.DROP ("-") values are "dropped" (deleted or set to undefined)
2. jsondiff.KEEP ("+") values are "kept" (resolve to the corresponding value in the target)

Note: DROP and KEEP are, admittedly, a hack. If these exact string values appear in data outside of patch objects, diff() and patch() may not function correctly. That said, this is not expected to be an issue in real-world conditions. (Both strings include a "private use" Unicode character that should make them fairly unique.)

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