JSON Diff and Patch

Jiff is an implementation of JSON Patch RFC6902, plus a Diff implementation that generates compliant patches.

It also provides advanced and experimental APIs based on patch algebra, such as patch inverses ("reverse" patches), commutation (patch reordering), and even rebasing (moving patches from one history to another).

Get it

npm install --save jiff

bower install --save jiff

Example

var a = [
    { name: 'a' },
    { name: 'b' },
    { name: 'c' },
]

var b = a.slice();
b.splice(1, 1);
b.push({ name: 'd' });

// Generate diff (ie JSON Patch) from a to b
var patch = jiff.diff(a, b);

// [{"op":"add","path":"/3","value":{"name":"d"}},{"op":"remove","path":"/1"}]
console.log(JSON.stringify(patch));

var patched = jiff.patch(patch, a);

// [{"name":"a"},{"name":"c"},{"name":"d"}]
console.log(JSON.stringify(patched));

API

patch

var b = jiff.patch(patch, a);

Given an rfc6902 JSON Patch, apply it to a and return a new patched JSON object/array/value. Patching is atomic, and is performed on a clone of a. Thus, if patching fails mid-patch, a will still be in a consistent state.

Throws InvalidPatchOperationError and TestFailedError.

patchInPlace

a = jiff.patchInPlace(patch, a);

Given an rfc6902 JSON Patch, apply it directly to a, mutating a.

Note that this is an opt-in violation of the patching algorithm outlined in rfc6902. It may provide some performance benefits as it avoids creating a new clone of a before patching.

However, if patching fails mid-patch, a will be left in an inconsistent state.

Throws InvalidPatchOperationError and TestFailedError.

diff

var patch = jiff.diff(a, b [, hashFunction]);

Computes and returns a JSON Patch from a to b: a and b must be valid JSON objects/arrays/values of the same type. If patch is applied to a, it will yield b.

If provided, the optional hashFunction will be used to recognize when two objects are the same. If not provided, JSON.stringify will be used.

While jiff's patch algorithm handles all the JSON Patch operations required by rfc6902, the diff algorithm currently does not generate move, or copy operations, only add, remove, and replace.

inverse

var patchInverse = jiff.inverse(patch);

Compute an inverse patch. Applying the inverse of a patch will undo the effect of the original.

Due to the current JSON Patch format defined in rfc6902, not all patches can be inverted. To be invertible, a patch must have the following characteristics:

1. Each remove and replace operation must be preceded by a test operation that verifies the value at the path being removed/replaced.
2. The patch must not contain any copy operations. Read this discussion to understand why copy operations are not (yet) invertible. You can achieve the same effect by using add instead of copy, albeit potentially at the cost of data size.

clone

var b = jiff.clone(a);

Creates a deep copy of a, which must be a valid JSON object/array/value.

Experimental APIs

jiff/lib/rebase

var rebase = require('jiff/lib/rebase');
var patchRebased = rebase(patchHistory, patch);

Yes, this is git rebase for JSON Patch.

Given a patchHistory (Array of patches), and a single patch rooted at the same starting document context, rebase patch onto patchHistory, so that it may be applied after patchHistory.

Rebasing is dependent on commutation, and so is also highly experimental. If the rebase cannot be performed, it will throw a TypeError.

jiff/lib/commute

var commute = require('jiff/lib/commute');
var [p2c, p1c] = commute(p1, p2);

Given two patches p1 and p2, which are intended to be applied in the order p1 then p2, transform them so that they can be safely applied in the order p2c and then p1c.

Commutation is currently highly experimental. It works for patch operations whose path refers to a common array ancestor by transforming array indices. Operations that share a common object ancestor are simply swapped for now, which is likely not the right thing in most cases!

Commutation does attempt to detect operations that cannot be commuted, and in such cases, will throw a TypeError.

Errors

InvalidPatchOperationError

Thrown when any invalid patch operation is encountered. Invalid patch operations are outlined in sections 4.x and 5 in rfc6902. For example: non-existent path in a remove operation, array path index out of bounds, etc.

TestFailedError

Thrown when a test operation fails.

License

MIT