JSON Diffing & Merging — Algorithms, Libraries & Conflict Resolution

1import { compare, applyPatch } from 'fast-json-patch';
2
3const original = {
4  name: 'Alice',
5  age: 30,
6  roles: ['user'],
7};
8
9const modified = {
10  name: 'Alice',
11  age: 31,
12  roles: ['user', 'admin'],
13  email: '[email protected]',
14};
15
16// Generate the patch
17const patch = compare(original, modified);
18console.log(JSON.stringify(patch, null, 2));
19// [
20//   { "op": "replace", "path": "/age", "value": 31 },
21//   { "op": "add", "path": "/roles/1", "value": "admin" },
22//   { "op": "add", "path": "/email", "value": "[email protected]" }
23// ]
24
25// Apply the patch to recreate the modified version
26const result = applyPatch(structuredClone(original), patch);
27console.log(result.newDocument);

1function deepMerge(target, source, options = {}) {
2  const { arrayStrategy = 'replace' } = options;
3  const result = { ...target };
4
5  for (const [key, sourceValue] of Object.entries(source)) {
6    const targetValue = result[key];
7
8    if (Array.isArray(sourceValue) && Array.isArray(targetValue)) {
9      switch (arrayStrategy) {
10        case 'replace':
11          result[key] = sourceValue;
12          break;
13        case 'concat':
14          result[key] = [...targetValue, ...sourceValue];
15          break;
16        case 'union':
17          result[key] = [...new Set([...targetValue, ...sourceValue])];
18          break;
19      }
20    } else if (
21      typeof sourceValue === 'object' && sourceValue !== null &&
22      typeof targetValue === 'object' && targetValue !== null &&
23      !Array.isArray(sourceValue)
24    ) {
25      result[key] = deepMerge(targetValue, sourceValue, options);
26    } else {
27      result[key] = sourceValue;
28    }
29  }
30
31  return result;
32}
33
34// Example: merge config overlays
35const defaults = { db: { host: 'localhost', port: 5432 }, debug: false };
36const production = { db: { host: 'prod-db.example.com' }, debug: false };
37const result = deepMerge(defaults, production);
38// { db: { host: 'prod-db.example.com', port: 5432 }, debug: false }

1import deepmerge from 'deepmerge';
2
3const defaultConfig = {
4  db: { host: 'localhost', port: 5432, pool: 10 },
5  cache: { ttl: 3600, maxSize: 1000 },
6  features: { darkMode: false, beta: false },
7};
8
9const envConfig = {
10  db: { host: 'prod-db.internal', pool: 50 },
11  features: { darkMode: true },
12};
13
14const runtimeOverride = {
15  features: { beta: true },
16};
17
18// Merge in order of increasing priority
19const finalConfig = deepmerge.all([
20  defaultConfig,
21  envConfig,
22  runtimeOverride,
23]);
24// Result: db.host = 'prod-db.internal', db.port = 5432, db.pool = 50,
25//         features.darkMode = true, features.beta = true

1import { compare } from 'fast-json-patch';
2import { readFileSync, writeFileSync } from 'fs';
3
4function snapshotTest(testName, actualResponse) {
5  const snapshotPath = `__snapshots__/${testName}.json`;
6
7  try {
8    const expected = JSON.parse(readFileSync(snapshotPath, 'utf-8'));
9    const diff = compare(expected, actualResponse);
10
11    if (diff.length > 0) {
12      console.error(`Snapshot mismatch for "${testName}":`);
13      console.error(JSON.stringify(diff, null, 2));
14      throw new Error(`${diff.length} differences found`);
15    }
16  } catch (err) {
17    if (err.code === 'ENOENT') {
18      // First run: create snapshot
19      writeFileSync(snapshotPath, JSON.stringify(actualResponse, null, 2));
20      console.log(`Snapshot created: ${snapshotPath}`);
21    } else {
22      throw err;
23    }
24  }
25}