Flatten JSON in JavaScript

JavaScript flatten JSON searches usually come from a practical problem: a nested object is hard to compare, log, test, or map into a table. You may have an API response in a React component, a Node.js script preparing fixtures, or a test snapshot where user.profile.email is easier to assert than response.user.profile.email.

This page lets you preview the flattened output before writing a helper. It uses the same engine as the Flatten JSON tool, with examples focused on JavaScript object paths and array behavior.

JavaScript flatten JSON example

Input object:

const input = {
  user: {
    id: 7,
    profile: {
      email: "ada@example.com",
      active: true
    }
  }
};

Flat output:

{
  "user.id": 7,
  "user.profile.email": "ada@example.com",
  "user.profile.active": true
}

That shape is convenient for assertions, logs, CSV headers, and simple key-value inspection.

A simple JavaScript flatten helper

function flattenObject(value, prefix = "", output = {}) {
  if (value === null || typeof value !== "object" || Array.isArray(value)) {
    output[prefix] = value;
    return output;
  }

  for (const [key, child] of Object.entries(value)) {
    const path = prefix ? `${prefix}.${key}` : key;
    flattenObject(child, path, output);
  }

  return output;
}

That snippet keeps arrays intact. If you need indexed paths such as items.0.sku, handle arrays separately and recurse through each index. Preview that behavior with Flatten JSON Array before adding it to production code.

JavaScript edge cases

• null is a JSON value, not an object to recurse into.
• Arrays may need to stay arrays for APIs but expand for CSV.
• Dotted source keys can collide with dotted path output.
• Date, Map, Set, and class instances are not JSON values until serialized.
• Property order is usually not a durable contract for downstream systems.

Online tool vs a JS library

Use the online flattener when you need to inspect a payload quickly, create a fixture, debug a webhook, or confirm a path convention. Use a JavaScript library when flattening is part of application logic, batch processing, or a production API.

The online page is still useful before installing a package because it makes the expected output concrete. Once you know the delimiter, array mode, and null behavior, the implementation choice is much less ambiguous.

Next JavaScript workflows

After flattening, export with JSON to CSV, compare payloads with JSON Diff, or rebuild nested structure with Unflatten JSON. For generic examples, return to the main JSON flattener.

Related flatten JSON tools

• JSON flattener - run the same flatten engine with general examples.
• Flatten JSON Array - inspect indexed array path output.
• Flatten JSON for CSV - prepare path maps for spreadsheet export.
• Flatten API Response JSON - debug nested payloads before coding.