JSON Merge Patch and JSON Patch: Update APIs Without Full Replace

The Partial Update Problem

REST APIs often need to update only some fields of a resource. Sending the full object on every update is wasteful and risks overwriting fields the client did not intend to change. Two standards solve this: JSON Merge Patch and JSON Patch.

JSON Merge Patch (RFC 7396)

Send only the fields you want to change. Null means delete the field.

// Original resource
{
  "title": "Hello World",
  "content": "My first post",
  "tags": ["intro", "beginner"],
  "published": false
}

// Merge Patch request body
{
  "published": true,
  "tags": null
}

// Result after merge
{
  "title": "Hello World",
  "content": "My first post",
  "published": true
  // tags deleted because value was null
}

Applying Merge Patch in JavaScript

function applyMergePatch(target, patch) {
  if (typeof patch !== 'object' || patch === null) return patch;

  const result = { ...target };
  for (const [key, value] of Object.entries(patch)) {
    if (value === null) {
      delete result[key];
    } else if (typeof value === 'object' && !Array.isArray(value)) {
      result[key] = applyMergePatch(result[key] || {}, value);
    } else {
      result[key] = value;
    }
  }
  return result;
}

JSON Patch (RFC 6902)

JSON Patch is more powerful — an array of operations:

// JSON Patch operations
[
  { "op": "replace", "path": "/published", "value": true },
  { "op": "add", "path": "/tags/-", "value": "new-tag" },
  { "op": "remove", "path": "/draft" },
  { "op": "move", "path": "/archive/title", "from": "/title" },
  { "op": "copy", "path": "/backup", "from": "/content" },
  { "op": "test", "path": "/version", "value": 3 }
]

Applying JSON Patch in JavaScript

// npm install fast-json-patch
const jsonpatch = require('fast-json-patch');

const document = { name: 'Alice', age: 30 };
const patch = [
  { op: 'replace', path: '/age', value: 31 },
  { op: 'add', path: '/email', value: 'alice@example.com' }
];

const result = jsonpatch.applyPatch(document, patch).newDocument;
console.log(result);
// { name: 'Alice', age: 31, email: 'alice@example.com' }

Which to Use

FAQ