can-i-assign-jsonschema-to-my-schema-and-use-fromschema-at-the-same-time.md

Can I assign JSONSchema to my schema and use FromSchema at the same time?

json-schema-to-ts exports a JSONSchema type to help you write schemas:

import { FromSchema, JSONSchema } from "json-schema-to-ts";

const dogSchema: JSONSchema = { ... } as const;

type Dog = FromSchema<typeof dogSchema>;

However, this example will not work 🙅‍♂️

FromSchema is based on type computations. By design, it only works on "good enough" material, i.e. narrow types ({ type: "string" }) and NOT widened ones ({ type: string } which can also represent { type: "number" }). For the compiler, assigning JSONSchema to your schema "blurs" its type to pretty much the widest possible schema type 😅 That's why in this example, Dog will be equal to the unknown type.

So there is a sort of Heiseinberg's uncertainty principle at play here: You can either use FromSchema or JSONSchema, but not both at the same time.

The correct way to use them is the following:

• Define a schema with the as const statement
• Assign the JSONSchema type to it (allowing autocompletion and precise error highlighting)
• Write the schema
• Remove the type assignment
• Use FromSchema 🙌