JSON to TypeScript Converter
Generate TypeScript interfaces and type definitions from JSON data automatically. Converting API responses to TypeScript types enables compile-time type checking, IDE autocomplete, and better code documentation. This tool analyzes JSON structure and produces accurate TypeScript definitions instantly.
Why Convert JSON to TypeScript
TypeScript's type system catches errors at compile time rather than runtime. When working with external APIs, manually creating type definitions is tedious and error-prone. Automated conversion ensures your types accurately match the actual data structure, reducing bugs and improving developer experience.
Benefits of proper typing:
- Compile-time error detection - Catch typos and wrong assumptions early
- IDE autocomplete - Code faster with intelligent suggestions
- Self-documenting code - Types describe data shape
- Refactoring confidence - TypeScript catches breaking changes
Conversion Example
Input JSON:
``json
{
"id": 123,
"name": "John Doe",
"email": "john@example.com",
"roles": ["admin", "user"],
"settings": {
"theme": "dark",
"notifications": true
}
}
`
Output TypeScript:
`typescript
interface User {
id: number;
name: string;
email: string;
roles: string[];
settings: Settings;
}
interface Settings {
theme: string;
notifications: boolean;
}
`
Type Inference Rules
| JSON Value | TypeScript Type | Notes |
|---|---|---|
| "string" | string | All text values |
| 123 | number | Integers and floats |
| true / false | boolean | Boolean literals |
| null | null | Explicit null |
| [...] | Type[] | Array of inferred type |
| {} | Interface | Nested object type |
Interface vs Type Alias
| Feature | Interface | Type |
|---|---|---|
| Object shapes | Preferred | Supported |
| Extension | extends | & intersection |
| Declaration merging | Yes | No |
| Union types | No | Yes |
For JSON conversion, interfaces are generally preferred because they can be extended and merged.Use this converter to quickly generate TypeScript types from your API responses and maintain type safety throughout your application.
Numbers Are the Recurring Problem
A JSON number is an IEEE 754 double. Three consequences that bite in production:
| Value | What happens |
|---|---|
| Integers above 2⁵³ | Silently lose precision — send IDs as strings |
| Money as a float | 0.1 + 0.2 = 0.30000000000000004 |
| Leading zeros | 007 is invalid JSON; "007" is a string |
NaN and Infinity | Not valid JSON at all |
Twitter hit the first one publicly: 64-bit tweet IDs arrived in JavaScript rounded, so the
API began sending an id_str alongside every id.Keys, Order and Duplicates
Objects are formally unordered, though every JavaScript engine preserves insertion order for
string keys — with one exception: integer-like keys sort numerically and come first.
`javascript
JSON.stringify({ b: 1, 2: 2, a: 3 }); // {"2":2,"b":1,"a":3}
`
Duplicate keys are not an error in the spec, and JSON.parse keeps the last one. Two
parsers can legitimately disagree about which value wins, which has been the basis of real
request-smuggling attacks.
Rules That Prevent Most JSON Bugs
| Rule | Why |
|---|---|
| No trailing commas | {"a": 1,} is invalid JSON, though JavaScript accepts it in object literals |
| Double quotes only | {'a': 1} is not JSON |
| Keys must be quoted | {a: 1} is a JavaScript object, not JSON |
| No comments | There is no comment syntax; JSON5 and JSONC are different formats |
No undefined, NaN, Infinity | JSON.stringify silently drops or nulls them |
| Numbers are IEEE 754 doubles | Integers above 2⁵³ lose precision — send them as strings |
Parsing Safely
`javascript
try {
const data = JSON.parse(text);
} catch (error) {
// The message names the character offset, which is the fastest way
// to find an unescaped quote in a large document.
console.error(error.message);
}
`
JSON.parse` throws on invalid input rather than returning null, so it always belongs in a try/catch when the source is a file, a request body or a clipboard paste.