Merge JSON files online, instantly.

Merge JSON files online — free, fast, and private

MergeJSON is a free online tool to merge JSON files directly in your browser. Whether you need to combine two configuration files, stitch together several API responses, or consolidate translation catalogs, our JSON merger lets you combine multiple JSON files into one clean, valid document in seconds. There is no signup, no software to install, and no upload — because every byte is processed locally, the tool is both the fastest and the most private way to merge JSON files online.

Most people who search for how to combine JSON files end up hand-editing them in a text editor, copying braces around and hoping they did not break the syntax. MergeJSON removes that risk entirely. Drop in your files, choose how they should combine, and download the result. It is a genuinely free way to merge JSON files with no usage limits and no catch.

How to combine JSON files in three steps

Learning how to combine JSON files with our tool takes under a minute:

1. Add your JSON. Drag and drop two or more .json files into the merge tool, or switch to the code editor and paste your snippets to merge JSON without creating files at all.
2. Choose your options. Pick a deep or shallow merge, select an array strategy, and decide which file wins when keys conflict — full control over how you combine JSON data.
3. Merge and export. Press merge, then copy the output or download it as merged.json or merged.min.json.

Merge JSON objects with complete control

Combining data is rarely as simple as gluing two files together. When you merge JSON objects, nested structures and arrays need real handling. That is why MergeJSON exposes the options other tools hide: a deep merge recursively combines nested objects so nothing is lost, while a shallow merge replaces top-level keys. Arrays can be concatenated, de-duplicated into a union, replaced, or merged by a key such as id — so you can combine JSON records intelligently rather than just appending them. You can also sort keys alphabetically and remove duplicate array items for a clean, diff-friendly result.

Common reasons to combine JSON data

Developers reach for a tool to merge JSON in all kinds of situations. Backend engineers layer a base configuration with environment-specific overrides. Frontend teams combine JSON data from several feature-flag files. API developers merge paginated or multi-endpoint responses into a single dataset. Localization engineers consolidate dozens of language files — en.json, fr.json, de.json — into one catalog. QA engineers assemble test fixtures from smaller fragments, and data analysts combine JSON files before loading them into a pipeline. Whatever the source, the goal is the same: turn many JSON documents into one correct, predictable result without losing data along the way.

Merge JSON in JavaScript, Python, or the terminal

If you would rather script the merge, here are the standard approaches — along with the traps each one hides. The tool above exists because these traps are easy to miss.

JavaScript — the shallow-merge trap. Spread and Object.assign are what most people reach for, and both are shallow:

const a = { db: { host: "localhost", port: 5432 } };
const b = { db: { port: 6543 } };

const merged = { ...a, ...b };
// { db: { port: 6543 } }   ← host is GONE

The entire db object was replaced, not combined — host silently vanished. That is the single most common JSON merge bug. A deep merge walks into nested objects instead:

const deepMerge = (a, b) => {
  const out = { ...a };
  for (const [k, v] of Object.entries(b)) {
    out[k] = v && typeof v === "object" && !Array.isArray(v) &&
             a[k] && typeof a[k] === "object" && !Array.isArray(a[k])
      ? deepMerge(a[k], v)
      : v;
  }
  return out;
};
// { db: { host: "localhost", port: 6543 } }   ← correct

Python — dict.update() and | are shallow for the same reason:

import json
from collections.abc import Mapping

def deep_merge(a, b):
    out = dict(a)
    for k, v in b.items():
        if isinstance(v, Mapping) and isinstance(out.get(k), Mapping):
            out[k] = deep_merge(out[k], v)
        else:
            out[k] = v
    return out

merged = deep_merge(json.load(open("base.json")), json.load(open("prod.json")))

Command line (jq) — note the two different operators:

jq -s '.[0] * .[1]' base.json prod.json > merged.json   # * = recursive (deep)
jq -s '.[0] + .[1]' base.json prod.json > merged.json   # + = shallow
jq -s 'reduce .[] as $f ({}; . * $f)' *.json            # deep-merge every file

None of these de-duplicate arrays, merge array items by id, or let you choose which file wins per conflict — which is exactly what the merger above does, with no script to maintain.

Common JSON merge problems (and fixes)

“My nested config got wiped out.” You did a shallow merge. Turn on deep merge (the default here) so nested objects combine level by level instead of replacing each other.

“My arrays doubled up.” The default array strategy is concatenate, which appends. Switch to union to keep only unique items, or merge by key to match objects on their id and combine them instead of duplicating them.

“The wrong file won a conflict.” Precedence follows file order. Drag the files to reorder them, or switch from last-wins to first-wins.

“One of my files will not parse.” The tool reports the exact line and column. Fix it with the JSON repair tool — trailing commas and single quotes are the usual culprits — or check it with the JSON validator.

“I need to see what actually changed.” Compare the before and after with the JSON diff tool, which ignores key order and whitespace so you only see real differences.

Why choose our JSON merger?

MergeJSON does more than glue files together — it lets you combine multiple JSON files into one in a single step. Unlike many sites that quietly send your data to a server, MergeJSON runs entirely client-side — so you can merge JSON files free without ever exposing API keys, customer records, or internal configuration. Each input is validated as you type, with the exact line and column of any syntax error surfaced before you merge, so you never end up with broken output. It works on any device with a modern browser, handles large files limited only by your machine's memory, and even keeps working offline once the page has loaded.

Ready to combine JSON files online? Open the merge tool and drop in your first two files, read the full guide to how it works, or explore the JSON guides blog for tutorials on merging, validating, and working with JSON.