JSON Merge

JSON Merge Tool

The JSON Merge tool combines two JSON documents into a single output using flexible strategies. It is ideal when you need to overlay configuration files, merge API responses, or compose objects from multiple sources. You can choose how conflicts are resolved, how arrays are handled, and whether to keep or overwrite existing values. The merge runs fully in the browser, so your data remains private.

Merging JSON is common in modern apps: configuration layering, feature flags, environment overrides, and form submissions often require combining inputs with defaults. A good merge strategy prevents accidental data loss. This tool lets you experiment with strategies and immediately inspect the merged output before using it in production.

Merge strategies explained

• Prefer Right: Right-side values overwrite left-side values when conflicts occur.
• Preserve Left: Left-side values are kept, and right-side values fill only missing fields.
• Concat Arrays: Arrays are joined by appending right-side items to left-side arrays.

How to use the JSON merge tool

1. Paste the base JSON in the left editor.
2. Paste the JSON to merge in the right editor.
3. Select a merge strategy that matches your use case.
4. Click Merge and copy or download the output.

Example: configuration overlay

Left (base) JSON:

{
  "theme": "light",
  "limits": { "max": 100, "timeout": 20 },
  "tags": ["alpha"]
}

Right (override) JSON:

{
  "limits": { "timeout": 10 },
  "tags": ["beta"],
  "features": { "newUI": true }
}

Prefer Right output:

{
  "theme": "light",
  "limits": { "timeout": 10 },
  "tags": ["beta"],
  "features": { "newUI": true }
}

Example: preserve left

With Preserve Left, base values remain unchanged and only missing keys are filled from the right. This is ideal for default configuration files.

Common errors and fixes

• Invalid JSON input: Merging requires valid JSON on both sides. Validate first with JSON Validator.
• Unexpected overwrite: If the right side replaces a value you wanted to keep, switch to Preserve Left.
• Array handling confusion: Arrays are either concatenated or overridden depending on strategy. Choose the behavior that matches your needs.
• Conflicting types: If one side has an object and the other has a string, the merge may overwrite. Normalize data types before merging.
• Large JSON performance: Very large merges can be slower in the browser. Try smaller chunks or pre-filter the JSON.

Best practices for clean merges

• Use Preserve Left for defaults and Prefer Right for overrides.
• Keep arrays consistent in shape when concatenating.
• Validate the merged output with JSON Schema Validator if you rely on strict structure.
• Use JSON Compare to review changes between base and merged output.
• Store merged results as formatted JSON for easier debugging.

Common merge scenarios

Environment overrides: Start with a base config in the left editor and apply environment-specific overrides on the right. Prefer Right ensures the environment values take effect.

Feature flags: Merge a flag set into a base user profile. Preserve Left keeps existing profile data and fills missing keys.

Multi-source APIs: Combine responses from two services into one object without manually copying fields.

Merge checklist

• Validate both JSON inputs before merging.
• Choose a strategy that matches your business rules.
• Review the merged output and compare with the base.
• Save or export the merged JSON for reuse.

Example: array concatenation

Left JSON:

{ "tags": ["alpha", "beta"] }

Right JSON:

{ "tags": ["release"] }

Concat Arrays output:

{ "tags": ["alpha", "beta", "release"] }

Handling conflicts safely

If a key exists in both inputs but the values are different types (e.g., object vs string), a merge strategy will choose one side. Decide in advance which side should win to avoid silent data loss. When strict structure matters, validate the merged result with a schema and compare it to the base JSON to confirm expected changes.

Shallow vs deep merge (conceptual)

A shallow merge replaces an entire object when keys overlap, while a deep merge combines nested fields. This tool focuses on practical, predictable strategies for configuration overlays. If you need deep merge behavior for nested objects, use Prefer Right with carefully structured overrides and validate the merged output to ensure it matches expectations.

When you want strict deep merge behavior, consider normalizing your data into flatter structures before merging. This can reduce ambiguity and make the results easier to reason about.

If you need to preserve provenance, keep a copy of the original left and right inputs and store the merged output with a timestamp. This makes it easier to audit changes later and to revert if the merge introduces unexpected values.

When sharing merged JSON with others, format it for readability and include a short note describing the strategy you used. That context prevents confusion and reduces accidental misinterpretation of the merged result.

This small step saves time in reviews and keeps teams aligned on merge behavior.

It also reduces back-and-forth when troubleshooting configuration issues.

FAQs

Is JSON Merge the same as JSON Merge Patch?
No. JSON Merge Patch is a specific RFC format for partial updates. This tool provides flexible strategies; see JSON Merge Patch for the RFC behavior.

Does it preserve the left JSON?
Yes. The left input remains unchanged; the merged result appears in the output pane.

Can I merge arrays by index?
This tool focuses on preserve/override/concat strategies. Use the strategy that best matches your array behavior.

Is my data uploaded?
No. Everything runs locally in your browser.

What happens if keys overlap?
Overlap rules depend on strategy. Prefer Right overwrites, Preserve Left keeps the original value.

Can I merge more than two JSON files?
Yes. Merge iteratively: use one output as the next base input.

Does this handle nested objects?
Yes. Nested objects are merged according to the selected strategy.

Why did an array get replaced?
In Prefer Right, arrays are overwritten. Use Concat Arrays to combine items.

Can I merge JSON that contains null values?
Yes. Nulls are treated as explicit values and may overwrite existing values depending on strategy.

What if I need conditional merging?
Perform merges in stages: split your JSON into smaller parts, merge selectively, then combine results.

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