JSON to TypeScript Interface Generator — Free Online, No Upload

The fastest way to turn a JSON payload into a TypeScript interface is to paste it into a browser-based generator and copy the output. Our free tool does exactly that — no signup, no file upload, no install. Paste on the left, copy the interface on the right.

This matters when you're integrating with an API you don't own. The docs might list the shape of a response somewhere, but what you actually get back from production is the source of truth. Copy a real response into the tool and you have a typed interface that matches production exactly — not an aspirational schema that's already drifted from reality.

Paste JSON, Get a TypeScript Interface

Drop any valid JSON object into the left panel. The generator infers types from the values you give it and produces a Root interface plus any nested interfaces it needs for sub-objects.

Example input:

{
  "id": 42,
  "email": "[email protected]",
  "active": true,
  "roles": ["admin", "editor"],
  "profile": { "firstName": "Alex", "lastName": "Kim" }
}

Example output:

interface Profile {
  firstName: string;
  lastName: string;
}

interface Root {
  id: number;
  email: string;
  active: boolean;
  roles: string[];
  profile: Profile;
}

Notice the nested profile object gets lifted into its own named interface. That's the convention most TypeScript codebases follow — it keeps the top-level interface readable and makes the nested shape reusable elsewhere.

How TypeScript Types Are Inferred From JSON

JSON has six value types. The generator maps each to the closest TypeScript equivalent:

JSON Value	TypeScript Type	Notes
"hello"	string	All JSON strings map to string
42, 3.14	number	TypeScript has one number type for int and float
true, false	boolean	Direct map
null	null	See the optional-field section below
[1, 2, 3]	number[]	Array element type inferred from first element
{ a: 1 }	Named interface	Extracted as its own type

Two edge cases to know about. First, JSON numbers don't distinguish between integer and float — TypeScript's number covers both, so the generator never needs to guess. Second, JSON has no Date type — dates come through as ISO strings, so the generator types them as string. You can change them to Date manually if your parsing layer converts them.

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Optional Fields, Null Values, and Mixed Arrays

Real API responses are messy. A field that's always present in the docs might be null half the time in production, or missing entirely for certain user states. TypeScript handles both, but the two cases look different in the interface.

Missing fields become optional with a ? — meaning the property may not exist on the object at all:

interface User {
  id: number;
  nickname?: string;  // may be missing
}

Null fields become a union with null — meaning the property is always there, but its value can be null:

interface User {
  id: number;
  avatar: string | null;  // always present, sometimes null
}

The distinction matters for your consuming code. user.nickname requires a presence check; user.avatar requires a null check. If your API is inconsistent about which pattern it uses, paste a representative sample and the generator will infer based on what it sees. For responses where a field could be either, paste multiple examples or edit the output to field?: string | null.

Mixed arrays like [1, "two", true] become union types: (number | string | boolean)[]. This is almost always a sign that you want a tagged variant rather than a raw array — but the generator will type it honestly.

Your JSON Never Leaves Your Browser

This is a real consideration. API responses from internal systems often contain emails, names, IDs, or entire customer records — things you absolutely do not want to paste into a random online converter that logs your input.

Our tool runs the conversion in JavaScript inside the browser tab. There's no upload endpoint, no server roundtrip, no logging pipeline. You can open DevTools, go to the Network tab, paste your JSON, and watch zero requests go out. If you want to be paranoid, disconnect from the internet after the page loads — the tool still works.

For enterprise environments where even pasting sensitive data into a browser tab gives compliance hives, the same tool can be saved as a local HTML file and run from disk. The entire generator is a single page of JavaScript.

When to Use This vs a Schema-Based Tool

There are two families of JSON-to-TypeScript tools. This tool belongs to the first.

Sample-based (this tool): You give it real JSON. It infers the types. Fast, accurate for the shape you pasted, but it only knows what your sample shows.

Schema-based (like json-schema-to-typescript): You give it a JSON Schema document. It generates types that match the schema. More complete — the schema can describe unions, enums, required fields, and validation rules that a single sample can't — but you need to have a schema in the first place.

Most day-to-day work is sample-based. You hit an API in Postman, copy the response, and want a type. For that, our tool is immediate. For generating types from OpenAPI or JSON Schema documents, we cover that workflow in Swagger and OpenAPI JSON to TypeScript.

If you need runtime validation — not just compile-time types — what you want is a Zod schema, not an interface. Our JSON to Zod converter does that. We've written about which to pick based on whether your validation needs are compile-time or runtime.

Frequently Asked Questions

Does this tool upload my JSON anywhere?

No. Conversion runs in JavaScript inside your browser tab. There are no network requests to any server during the conversion step. You can verify this in DevTools Network tab.

Can it handle deeply nested JSON?

Yes. Nested objects are extracted into their own named interfaces. Depth is effectively unlimited — the generator walks the entire tree.

What happens with empty arrays like []?

An empty array gets typed as any[] since there are no elements to infer from. Paste a non-empty sample if you want a specific element type.

Does it generate classes instead of interfaces?

No — interfaces only. TypeScript interfaces and types cover almost all type-definition needs without the runtime weight of classes. If you need a class, copy the interface and wrap it.

Can I convert TypeScript interfaces back to JSON?

Not directly — that would require generating example data from the type. Some tools do this (faker.js with type definitions), but our generator only handles JSON to interface.