Avoiding Double Parsing of JSON API Responses

A common mistake when working with APIs in JavaScript is attempting to parse JSON data that’s already been parsed, which leads to errors and unexpected behavior. Here’s how to properly handle API responses to avoid this issue.

The Core Problem

Problematic code that causes errors:

fetch('https://api.example.com/data')
  .then(response => {
    const parsedData = response.json(); // First parse (returns Promise)
    const data = JSON.parse(parsedData); // ❌ Attempting second parse
    return data;
  })

Or with async/await:

const response = await fetch('/api/data');
const data = JSON.parse(await response.json()); // ❌ Double parsing

Correct Implementation

1. Proper Fetch API Usage

fetch('https://api.example.com/data')
  .then(response => response.json()) // ✅ Single parse
  .then(data => {
    console.log(data); // Already JavaScript object
  });

2. Async/Await Version

async function getData() {
  const response = await fetch('/api/data');
  const data = await response.json(); // ✅ Single parse
  return data; // Ready to use as object
}

Common Scenarios and Solutions

❌ Double Parsing with Axios

axios.get('/api/data')
  .then(response => {
    const data = JSON.parse(response.data); // ❌ response.data is already parsed
  });

✅ Fix: Use Axios data directly

axios.get('/api/data')
  .then(response => {
    const data = response.data; // ✅ Already parsed
  });

❌ Parsing Already-Parsed Local Storage

const user = JSON.parse(localStorage.getItem('user')); // First parse
const name = JSON.parse(user).name; // ❌ Double parse

✅ Fix: Use the first parsed result

const user = JSON.parse(localStorage.getItem('user')); // Single parse
const name = user.name; // ✅ Access property directly

How to Check if Data is Already Parsed

Use this helper function to safely handle data that might be either a string or object:

function safeJsonParse(data) {
  if (typeof data === 'string') {
    try {
      return JSON.parse(data);
    } catch (e) {
      return data; // Return original if not valid JSON
    }
  }
  return data; // Already parsed
}

// Usage
const apiData = await response.json();
const finalData = safeJsonParse(apiData); // Safe for either case

Debugging Double-Parsing Issues

Check the data type:

   console.log(typeof data); // Should be "object" for parsed JSON

Inspect the data structure:

   console.log(data);

Look for error messages:

SyntaxError: Unexpected token o in JSON at position 1 (typical when parsing an object)

Best Practices

Understand your API client:

fetch() requires .json()
Axios parses automatically
jQuery depends on settings

Check documentation for your HTTP client library
Avoid unnecessary parsing – check type first
Use TypeScript for better type safety:

   interface UserData {
     id: number;
     name: string;
   }

   const response = await fetch('/api/user');
   const data: UserData = await response.json();

Key Takeaways

Modern HTTP clients often parse JSON automatically
Never assume you need to parse – check the data first
Double parsing leads to errors and performance overhead
Type checking (typeof) helps identify already-parsed data
Consistent error handling prevents crashes from invalid JSON

Proper handling of JSON data ensures your application works reliably and avoids common pitfalls when processing API responses. Always verify whether your data is already in object form before attempting to parse it.

Using JSON.parse() on an already parsed API response