JSON to types
A sample in, a type out.
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Example
Example: two web-shop orders turned into TypeScript interfaces, with an optional coupon
A small US online shop is typing its order API. The developer pastes two orders as a JSON array: 1041 for Megan Hughes, a mug and a T-shirt, coupon SPRING10 and gift_note null; and 1042 for James Patel, one cap, a gift note and no coupon field at all. Settings: TypeScript, name Order, indent 2 spaces.
- The JSON starts with an array, so the type describes one order and is named Order. A last line adds OrderList = Order[] for the whole response.
- Both orders are read, not just the first: coupon is in 1041 but missing from 1042, so it comes out coupon?: string. gift_note was null once and text once, so it is null | string.
- customer, each line of items and shipping are objects, so each gets its own interface named after its field: Customer, Item (items made singular) and Shipping.
- Prices 12.5, 24 and 5.99 all become number. TypeScript has no separate type for whole numbers or cents.
| Field | In the two orders | Type written |
|---|---|---|
| id | 1041, 1042 | number |
| placed_at | "2026-03-14T09:12:00Z" | string |
| customer | name and email | Customer |
| items | 2 lines, then 1 line | Item[] |
| coupon | "SPRING10", then missing | string, optional (coupon?) |
| gift_note | null, then "Happy birthday, Dad!" | null | string |
| shipping | method and cost | Shipping |
4 interfaces (Order, Customer, Item, Shipping) plus OrderList: 27 lines to paste into types.ts. Read them over: placed_at is really a date, and method is only ever ground or express, so you might tighten it to 'ground' | 'express'. Two samples cannot tell you that.
How it works
- Paste a sample response. One real record is better than a made-up one.
- Choose TypeScript, Go or Zod.
- Name the top-level type, or leave it as Root.
- Copy the result. Read it over — inference from one sample is a first draft, not a specification.
Frequently asked questions
- Does it look at every element of an array, or just the first?
- Every one. [{a:1},{a:1,b:2}] gives b as optional rather than pretending it does not exist, which is the mistake that makes generated types annoying two weeks later.
- What does it do with null?
- Widens rather than narrows. A field that was null once is a field that can be null, so in TypeScript it becomes a union and in Go a pointer. That is nearly always what the API actually does.
- Do repeated objects get one type or many?
- One. Objects with the same field names share a named type, so a list of fifty users produces one User interface rather than fifty.
- Can I trust the output?
- Trust it as a starting point. One sample cannot tell you that a field is an enum, that a number is an integer, or that a string is a date — read it over and tighten it. It saves the typing, not the thinking.
- What if my JSON starts with an array?
- The type then describes one element, named with the name you type, and every element is read, so a field missing from some of them comes out optional. In TypeScript a last line adds a list type, such as OrderList = Order[]; with Zod, validate the whole list with z.array() around the schema.
- Why is every number typed number, or float64 in Go, never integer?
- Because a sample cannot prove a number is always whole: 24 today may be 24.5 tomorrow. TypeScript and Zod write number and Go writes float64. If an id or a quantity is always whole, change it to int or int64 in Go, or add .int() in Zod.
- Is anything sent to a server?
- No. It runs in your browser, so what you paste stays on your computer.