★ TypeScript · from JSON

JSON to TypeScript Converter

Paste an API response, get interfaces you can drop into your codebase. Every element of every array is read, so a key that only shows up sometimes comes out optional instead of silently required.

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Convert to

Root type name

Declare as

Strictness

Your JSON

TypeScript

3 types · 16 fields

export interface Root {
  id: number;
  email: string; // email
  "display-name": string;
  createdAt: string; // date-time
  plan: null;
  tags: string[];
  team: Team;
  invoices: Invoice[];
}

export interface Invoice {
  number: string;
  amount: number;
  paid: boolean;
  paidAt: string | null; // date-time
  note?: string;
}

export interface Team {
  id: string; // uuid
  name: string;
  website: string; // url
}

Typing APIs by hand?

I set up end-to-end type safety (DB → API → UI) so this never happens again.

Fix my types

How the TypeScript mapping works

Each JSON value is read, merged with every other sample of the same key, and then mapped to the closest TypeScript type:

JSON to TypeScript mapping
In your JSON	TypeScript
string	`string`
integer or decimal number	`number`
true / false	`boolean`
null	`null (or T \| null when other samples have a value)`
key missing in some array elements	`key?: T`
[] with items	`T[]`
[] empty	`unknown[]`
nested object	`its own named interface`
mixed values	`union, e.g. number \| string`

Worked example

A user record with a nested team, a list of invoices (one missing a note, one with a null payment date) and an awkward display-name key:

{
  "id": 1042,
  "email": "ada@example.com",
  "display-name": "Ada",
  "createdAt": "2026-11-07T09:30:00Z",
  "plan": null,
  "tags": ["beta", "founder"],
  "team": {
    "id": "7f3c9a2e-1b4d-4c8e-9f2a-6d5e4c3b2a19",
    "name": "Analytical Engines",
    "website": "https://example.com"
  },
  "invoices": [
    { "number": "INV-001", "amount": 49.5, "paid": true, "paidAt": "2026-10-01T00:00:00Z" },
    { "number": "INV-002", "amount": 49, "paid": false, "paidAt": null, "note": "Card declined" }
  ]
}

Becomes this TypeScript, generated by the same engine that runs the converter above (3 types, 16 fields):

export interface Root {
  id: number;
  email: string; // email
  "display-name": string;
  createdAt: string; // date-time
  plan: null;
  tags: string[];
  team: Team;
  invoices: Invoice[];
}

export interface Invoice {
  number: string;
  amount: number;
  paid: boolean;
  paidAt: string | null; // date-time
  note?: string;
}

export interface Team {
  id: string; // uuid
  name: string;
  website: string; // url
}

TypeScript gotchas

Optional is not nullable

key?: string means the key can be absent. key: string | null means it is always there but can be null. APIs do both, and this tool keeps them apart.

Numbers are just number

TypeScript has no integer type. If you need to tell IDs from prices at runtime, generate Zod instead and keep .int().

Types are compile-time only

Interfaces vanish after compilation, so a bad API response still gets through. Validate at the boundary with Zod if the data isn't yours.

Questions developers actually ask

Should I use interface or type?

Either works for object shapes. Interfaces can be extended and merged, types can express unions and mapped types. Pick one per codebase; the toggle switches the output.

How do I handle keys with dashes or spaces?

They are emitted as quoted property names, like "display-name": string. Access them with bracket notation: user["display-name"].

Why is a field typed as null?

Every sample of that key was null, so there is no evidence of its real type. Paste a response where it has a value and the type becomes T | null.

Is my JSON sent to a server?

No. Parsing and type generation run in your browser, so you can paste real API responses with tokens or customer data. Nothing leaves the tab.

How does it know a field is optional?

Paste an array of objects, or a response that contains one. Every element is read and merged: keys missing from some elements become optional, and keys that are sometimes null become nullable.

Other languages

Or go back to the JSON to Types hub.

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Fine print

Tools: on the house.
Engineer: by the hour.

A codebase that doesn't need this tool is the upgrade. I set up end-to-end type safety so your API, database and UI agree by construction.

I'm Himanshu, an AI and full-stack engineer. I build MVPs, AI agents and the growth plumbing around them for early-stage startups.

Hire me →My projects