Flattening and Unflattening JSON: A Practical Guide

Flattening JSON converts nested objects into dot-notation keys. This is essential for databases, spreadsheets, and certain API formats.

What is JSON Flattening?

Nested JSON:

{

  "user": {
    "name": "Alice",
    "address": {
      "city": "New York",
      "zip": "10001"
    }
  },
  "tags": ["admin", "dev"]
}

Flattened:

{

  "user.name": "Alice",
  "user.address.city": "New York",
  "user.address.zip": "10001",
  "tags.0": "admin",
  "tags.1": "dev"
}

Flatten Implementation

function flatten(obj, prefix = '') {

  const result = {};
  for (const [key, value] of Object.entries(obj)) {
    const newKey = prefix ? ${prefix}.${key} : key;
    if (value && typeof value === 'object' && !Array.isArray(value)) {
      Object.assign(result, flatten(value, newKey));
    } else {
      result[newKey] = value;
    }
  }
  return result;
}

Unflatten Implementation

function unflatten(obj) {

  const result = {};
  for (const [key, value] of Object.entries(obj)) {
    const keys = key.split('.');
    let current = result;
    for (let i = 0; i < keys.length - 1; i++) {
      current[keys[i]] = current[keys[i]] || {};
      current = current[keys[i]];
    }
    current[keys[keys.length - 1]] = value;
  }
  return result;
}

Use Cases

Handling Arrays

Arrays can use numeric indices or bracket notation:

// Index notation: "items.0.name"

// Bracket notation: "items[0].name"

Choose one convention and stick with it.

Use our JSON to CSV converter to export flattened JSON to spreadsheets.