quicktype's type inference improves dramatically when you provide multiple JSON samples. This post explains why and how to use this feature.

The Problem with Single Samples

A single JSON sample might not represent all possible values:

// Sample 1: User is online
{
  "name": "Alice",
  "status": "online",
  "lastSeen": null
}

From this, quicktype infers lastSeen is always null. But in reality:

// Sample 2: User is offline
{
  "name": "Bob",
  "status": "offline",
  "lastSeen": "2018-03-08T10:30:00Z"
}

Multiple Samples to the Rescue

Give quicktype both samples, and it correctly infers:

interface User {
    name: string;
    status: string;
    lastSeen: string | null;
}

Using Multiple Samples

Command Line

quicktype sample1.json sample2.json sample3.json -o Types.ts

Web App

1. Go to app.quicktype.io
2. Paste your first JSON sample
3. Click "Add sample" (+ button)
4. Paste additional samples

Best Practices

• Include edge cases: Empty arrays, null values, optional fields
• Cover all variants: Different enum values, union types
• Use real data: Production API responses are ideal

How It Works

quicktype merges the type graphs from all samples, creating unions where values differ. This produces more robust types that handle real-world data variations.