What does deep merge mean?

Nested objects are merged recursively — the second object's keys override the first's, but keys that exist only in the first are preserved at every level. Shallow merge would replace entire nested objects wholesale.

How does it handle arrays?

Arrays are replaced, not concatenated. If the second object has a shorter array at a given key, you get the shorter array. Switch to a custom merge strategy if you need concat behavior.

Is this useful for config layering?

Very — it's how dotenv overrides, Kubernetes overlay files, and Next.js environment config work. Merge base + environment overrides to get the effective config.

Does null in the override remove the key?

No — null is a valid value and replaces the original. If you want to remove a key, use JSON Patch with a 'remove' operation.

JSON Merge / Deep Merge | DevTools Surf

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Example: JSON Merge / Deep Merge

A worked example, rendered from real sample data. Sign in to run the tool on your own input.

Input

{"name":"Alice","settings":{"theme":"dark"}}
---
{"email":"alice@test.com","settings":{"lang":"en"}}

Output

{
  "name": "Alice",
  "settings": {
    "theme": "dark",
    "lang": "en"
  },
  "email": "alice@test.com"
}

About JSON Merge / Deep Merge

JSON Merge / Deep Merge preview - JSON tool

Merge JSON documents with deep, shallow or replace strategies and a conflict report. Part of the DevTools Surf developer suite. Browse more tools in the JSON collection.

Use Cases

DevOps engineers combining environment-specific config overrides
Backend developers merging default and user settings objects
CI/CD pipelines assembling config from multiple partial files
Data engineers consolidating JSON records from different sources

Tips

Preview the merge result before applying to production configs
Check array handling — deep merge can concatenate or replace arrays
Use this to combine partial config overrides with defaults

Fun Facts

Deep merge is not part of the JSON specification — RFC 7396 (JSON Merge Patch) was published in October 2014 to standardize shallow merge semantics.
JavaScript's Object.assign() only performs shallow merging, which led to lodash's _.merge() becoming one of the most downloaded npm functions, with over 40 million weekly downloads.
The difference between deep merge and shallow merge has caused countless production bugs — Kubernetes kubectl apply uses a strategic three-way merge to handle this complexity.

FAQ

What does deep merge mean?: Nested objects are merged recursively — the second object's keys override the first's, but keys that exist only in the first are preserved at every level. Shallow merge would replace entire nested objects wholesale.
How does it handle arrays?: Arrays are replaced, not concatenated. If the second object has a shorter array at a given key, you get the shorter array. Switch to a custom merge strategy if you need concat behavior.
Is this useful for config layering?: Very — it's how dotenv overrides, Kubernetes overlay files, and Next.js environment config work. Merge base + environment overrides to get the effective config.
Does null in the override remove the key?: No — null is a valid value and replaces the original. If you want to remove a key, use JSON Patch with a 'remove' operation.

Related JSON Tools

JSON Formatter JSON Validator JSON Viewer JSON Stringify JSON Patch JSON Diff JSONPath Tester JSON Sort Keys