Generate TypeScript types from JSON

Turn any JSON file into TypeScript interfaces or Zod schemas in one command — with automatic semantic inference and enum detection.

Installation

npm install -D snaptype

Requires Node.js 20+. ESM only.

Basic usage

Pass any JSON file to from-json and snaptype infers the TypeScript type from the actual values.

snaptype from-json <file> [options]
# TypeScript interfaces (default)
snaptype from-json users.json -o src/types/user.ts

# Zod schema only
snaptype from-json users.json --zod -o src/schemas/user.ts

# Both TypeScript and Zod at once
snaptype from-json users.json --zod --ts -o src/types/user.ts

Example

Given a JSON file with this structure:

Download users.json
users.json
[
  {
    "id": 1,
    "email": "alice@example.com",
    "role": "admin",
    "createdAt": "2024-01-15T10:30:00.000Z"
  },
  {
    "id": 2,
    "email": "bob@example.com",
    "role": "user",
    "createdAt": "2024-03-22T08:00:00.000Z"
  }
]

snaptype generates:

src/types/user.ts
export interface User {
  readonly id: number;
  email: string;       // email
  role: "admin" | "user";
  createdAt: string;   // ISO 8601
}

With --zod:

src/schemas/user.ts
import { z } from 'zod';

export const UserSchema = z.object({
  id: z.number(),
  email: z.email(),
  role: z.enum(["admin", "user"]),
  createdAt: z.iso.datetime(),
}).readonly();

export type User = z.infer<typeof UserSchema>;

Automatic semantic inference

snaptype inspects the actual values in your JSON — not just the types. Email addresses become z.email(), ISO dates become z.iso.datetime(), URLs become z.url() — automatically. String fields with few distinct values are promoted to union literals (enum detection).

Options

FlagDescription
--name <name>Override the root type name (default: derived from filename)
--zodGenerate a Zod schema instead of (or alongside) TypeScript interfaces
--tsAlso emit a TypeScript file when --zod is set
-o, --output <path>Output file path (default: stdout)
-n, --naming <convention>Property naming: camel (default), pascal, snake
--readonlyAdd readonly to all properties
--emit <kind>Declaration style: interface (default) or type
-c, --config <path>Path to .snaptyperc config file

Pro features Pro

Multi-file merging

Pass multiple JSON files and snaptype merges them into a single unified type. Fields that appear in some files but not all become optional — giving you the most accurate type when a real-world API returns slightly different shapes.

snaptype from-json sample1.json sample2.json sample3.json -o src/types/user.ts

Watch mode

Automatically regenerate types whenever the source file changes. Useful during development when your JSON schema is still evolving.

snaptype from-json users.json --watch -o src/types/user.ts
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