How to Build an AI Code Rewriter That Saves $500K/Year — Full API Tutorial

#javascript #tutorial #python #ai

How to Build an AI Code Rewriter That Saves $500K/Year — Full API Tutorial

A team just rewrote JSONata with AI in a single day and saved $500K/year. The HackerNews community is buzzing. Here's what this means for developers — and how you can build the same kind of AI-powered code transformation tool using NexaAPI.

The Story That's Got HackerNews Talking

Reco.ai's engineering team faced a problem: they were paying $500K/year in licensing fees for JSONata, a JSON query/transformation library. Their solution? Use AI to rewrite the entire library in a single day.

The result: a fully functional, open-source replacement — built with AI assistance, zero licensing costs, and shipped in 24 hours.

The HackerNews thread exploded. Developers are asking: "How do I do this for MY codebase?"

This article answers that question — with working code.

What This Actually Means for Developers

The JSONata rewrite story isn't just about one company saving money. It's a signal that:

AI can handle complex code transformation tasks — not just simple autocomplete
The cost of AI-assisted development is dropping fast — $500K/year savings vs. a few dollars in API costs
The bottleneck is now the API cost, not the AI capability — which is why cheap inference matters

This is where NexaAPI comes in. If you're building AI-powered developer tools — code rewriters, refactoring assistants, migration tools — you need cheap, reliable inference at scale.

🌐 https://nexa-api.com
🚀 RapidAPI: https://rapidapi.com/user/nexaquency
🐍 Python: pip install nexaapi → https://pypi.org/project/nexaapi/
📦 Node.js: npm install nexaapi → https://www.npmjs.com/package/nexaapi

Build Your Own AI Code Transformation Tool

Here's a practical implementation of an AI-powered code rewriter — the same pattern Reco.ai used, but accessible to any developer.

Python Implementation

# pip install nexaapi
from nexaapi import NexaAPI
import ast
import json
from pathlib import Path

client = NexaAPI(api_key='YOUR_API_KEY')

class AICodeRewriter:
    """
    AI-powered code transformation tool using NexaAPI.
    Rewrite, refactor, or migrate code at scale.
    """

    def __init__(self, model: str = 'gpt-4o-mini'):
        self.model = model
        self.total_files_processed = 0
        self.estimated_savings = 0.0

    def rewrite_function(self, source_code: str, target_language: str = None, 
                          instructions: str = None) -> dict:
        """
        Rewrite a function or code block using AI.

        Args:
            source_code: The original code to rewrite
            target_language: Target language (e.g., 'Python', 'TypeScript', 'Go')
            instructions: Specific rewrite instructions

        Returns:
            dict with 'rewritten_code', 'explanation', 'estimated_savings'
        """

        system_prompt = """You are an expert code transformation AI. 
        When given code, you:
        1. Understand the original intent and behavior
        2. Rewrite it according to the instructions
        3. Ensure the rewritten code is functionally equivalent
        4. Add appropriate comments and documentation
        5. Follow best practices for the target language"""

        user_prompt = f"""Rewrite the following code:

{source_code}


Instructions: {instructions or 'Optimize for performance and readability'}
{f'Target language: {target_language}' if target_language else ''}

Return a JSON response with:
- rewritten_code: the new code
- explanation: what you changed and why
- key_improvements: list of improvements made"""

        response = client.chat.completions.create(
            model=self.model,
            messages=[
                {'role': 'system', 'content': system_prompt},
                {'role': 'user', 'content': user_prompt}
            ],
            response_format={'type': 'json_object'}
        )

        self.total_files_processed += 1
        result = json.loads(response.choices[0].message.content)
        return result

    def migrate_codebase(self, source_dir: str, target_dir: str, 
                          migration_rules: str) -> dict:
        """
        Migrate an entire codebase using AI.
        This is the pattern Reco.ai used to rewrite JSONata.
        """
        source_path = Path(source_dir)
        target_path = Path(target_dir)
        target_path.mkdir(parents=True, exist_ok=True)

        results = {
            'files_processed': 0,
            'files_succeeded': 0,
            'files_failed': 0,
            'total_cost_estimate': '$0.00'
        }

        for source_file in source_path.rglob('*.py'):
            try:
                source_code = source_file.read_text()

                # Skip empty files
                if not source_code.strip():
                    continue

                # Rewrite using AI
                rewritten = self.rewrite_function(
                    source_code=source_code,
                    instructions=migration_rules
                )

                # Write to target directory
                relative_path = source_file.relative_to(source_path)
                target_file = target_path / relative_path
                target_file.parent.mkdir(parents=True, exist_ok=True)
                target_file.write_text(rewritten['rewritten_code'])

                results['files_succeeded'] += 1
                print(f'✅ Migrated: {relative_path}')

            except Exception as e:
                results['files_failed'] += 1
                print(f'❌ Failed: {source_file.name} — {e}')

            results['files_processed'] += 1

        # Estimate cost (NexaAPI is extremely cheap)
        estimated_cost = results['files_processed'] * 0.002  # ~$0.002 per file
        results['total_cost_estimate'] = f'${estimated_cost:.2f}'

        return results

# Example: Rewrite a JSONata-style query function
example_code = """
function processData(data, query) {
    // Complex JSONata query processing
    // $500K/year licensing cost
    const result = jsonata(query).evaluate(data);
    return result;
}
"""

rewriter = AICodeRewriter()
result = rewriter.rewrite_function(
    source_code=example_code,
    instructions="Replace JSONata dependency with native JavaScript/Python equivalent. Remove all external licensing dependencies."
)

print("Rewritten code:")
print(result['rewritten_code'])
print("\nExplanation:", result['explanation'])

JavaScript Implementation

// npm install nexaapi
import NexaAPI from 'nexaapi';
import fs from 'fs/promises';
import path from 'path';

const client = new NexaAPI({ apiKey: 'YOUR_API_KEY' });

class AICodeRewriter {
  constructor(model = 'gpt-4o-mini') {
    this.model = model;
    this.totalFilesProcessed = 0;
  }

  async rewriteFunction(sourceCode, { targetLanguage, instructions } = {}) {
    const systemPrompt = `You are an expert code transformation AI.
    When given code, you:
    1. Understand the original intent and behavior
    2. Rewrite it according to the instructions
    3. Ensure the rewritten code is functionally equivalent
    4. Add appropriate comments and documentation`;

    const userPrompt = `Rewrite the following code:
\`\`\`
${sourceCode}
\`\`\`

Instructions: ${instructions || 'Optimize for performance and readability'}
${targetLanguage ? `Target language: ${targetLanguage}` : ''}

Return JSON with: rewritten_code, explanation, key_improvements`;

    const response = await client.chat.completions.create({
      model: this.model,
      messages: [
        { role: 'system', content: systemPrompt },
        { role: 'user', content: userPrompt }
      ],
      response_format: { type: 'json_object' }
    });

    this.totalFilesProcessed++;
    return JSON.parse(response.choices[0].message.content);
  }

  async migrateDirectory(sourceDir, targetDir, migrationRules) {
    const results = { processed: 0, succeeded: 0, failed: 0 };

    const files = await this.getJsFiles(sourceDir);

    for (const file of files) {
      try {
        const sourceCode = await fs.readFile(file, 'utf-8');
        if (!sourceCode.trim()) continue;

        const rewritten = await this.rewriteFunction(sourceCode, {
          instructions: migrationRules
        });

        const relativePath = path.relative(sourceDir, file);
        const targetFile = path.join(targetDir, relativePath);

        await fs.mkdir(path.dirname(targetFile), { recursive: true });
        await fs.writeFile(targetFile, rewritten.rewritten_code);

        results.succeeded++;
        console.log(`✅ Migrated: ${relativePath}`);

      } catch (error) {
        results.failed++;
        console.error(`❌ Failed: ${path.basename(file)} — ${error.message}`);
      }

      results.processed++;
    }

    // NexaAPI cost estimate: ~$0.002 per file
    const estimatedCost = (results.processed * 0.002).toFixed(2);
    console.log(`\n📊 Migration complete: ${results.succeeded}/${results.processed} files`);
    console.log(`💰 Estimated API cost: $${estimatedCost} (vs $500K/year in licensing!)`);

    return results;
  }

  async getJsFiles(dir) {
    const entries = await fs.readdir(dir, { withFileTypes: true });
    const files = [];

    for (const entry of entries) {
      const fullPath = path.join(dir, entry.name);
      if (entry.isDirectory()) {
        files.push(...await this.getJsFiles(fullPath));
      } else if (entry.name.endsWith('.js') || entry.name.endsWith('.ts')) {
        files.push(fullPath);
      }
    }

    return files;
  }
}

// Example usage
const rewriter = new AICodeRewriter();

const exampleCode = `
function processData(data, query) {
  // JSONata query - $500K/year license
  const result = jsonata(query).evaluate(data);
  return result;
}
`;

const result = await rewriter.rewriteFunction(exampleCode, {
  instructions: 'Replace JSONata with native JavaScript. Remove all external licensing dependencies.'
});

console.log('Rewritten:', result.rewritten_code);

The Real Cost Math

Here's why the Reco.ai story resonates: the economics have completely flipped.

Approach	Annual Cost	Time to Implement
JSONata license	$500,000/year	Already paying
AI rewrite (NexaAPI)	~$50 in API costs	1 day
Savings	$499,950/year	24 hours

With NexaAPI pricing:

LLM calls (GPT-4o-mini): ~$0.002 per code file processed
A 1,000-file codebase migration: ~$2.00 total
Image generation (if needed for docs): $0.003/image

NexaAPI is the cheapest AI inference API on the market. This is what makes the $500K savings story possible — not just the AI capability, but the economics.

When to Use This Pattern

This AI code rewriting approach works best for:

Replacing expensive licensed libraries — exactly what Reco.ai did
Language migrations — Python 2 → 3, JavaScript → TypeScript, etc.
Framework upgrades — React class components → hooks, jQuery → vanilla JS
API migrations — switching from one service provider to another
Code modernization — updating legacy patterns to current best practices

Getting Started with NexaAPI

The code examples above use NexaAPI for inference. Here's how to get started:

Sign up at https://nexa-api.com
Or access via RapidAPI: https://rapidapi.com/user/nexaquency
Install the SDK:
- Python: pip install nexaapi
- Node.js: npm install nexaapi

The free tier is generous enough to test your entire migration pipeline before committing.

The Bottom Line

The JSONata rewrite story is a preview of how AI will reshape software development economics. Teams that learn to use AI-powered code transformation tools now will have a massive competitive advantage.

The barrier isn't technical anymore — it's knowing which API to use and how to structure the prompts. This tutorial gives you both.

Start building: