JSON Schema や Ajv と TypeScript の型を紐づけるときの考え方や技術

import Ajv from "ajv";
const ajv = new Ajv();

const schema = {
  type: "object",
  properties: {
    foo: { type: "string" },
    bar: { type: "string" },
    baz: { type: "boolean" },
  },
  required: ["foo"],
  additionalProperties: false,
};

const data = { foo: 1, bar: "abc" };

const validate = ajv.compile(schema);
if (validate(data)) {
  // data is MyData here
  console.log(data.bar);
} else {
  console.log(validate.errors);
}

import Ajv, { JSONSchemaType } from "ajv";
const ajv = new Ajv();

interface MyData {
  foo: number;
  bar?: string;
}

const schema: JSONSchemaType<MyData> = {
  type: "object",
  properties: {
    foo: { type: "integer" },
    bar: { type: "string", nullable: true },
  },
  required: ["foo"],
  additionalProperties: false,
};

// validate is a type guard for MyData - type is inferred from schema type
const validate = ajv.compile(schema);

// or, if you did not use type annotation for the schema,
// type parameter can be used to make it type guard:
// const validate = ajv.compile<MyData>(schema)

const data = {
  foo: "1",
  bar: "abc",
};

if (validate(data)) {
  // data is MyData here
  console.log(data.foo);
} else {
  console.log(validate.errors);
}

const schema: UncheckedJSONSchemaType<MyData, false>
Type '{ type: "object"; properties: { foo: { type: "integer"; }; bar: { type: "string"; nullable: true; }; }; required: "foo"[]; additionalProperties: false; }' is not assignable to type 'UncheckedJSONSchemaType<MyData, false>'.
  The types of 'properties.foo' are incompatible between these types.
    Type '{ type: "integer"; }' is not assignable to type '{ $ref: string; } | (UncheckedJSONSchemaType<boolean, false> & { nullable?: false | undefined; const?: boolean | undefined; enum?: readonly boolean[] | undefined; default?: boolean | undefined; })'.
      Type '{ type: "integer"; }' is not assignable to type '{ type: "boolean"; } & { allOf?: readonly UncheckedPartialSchema<boolean>[] | undefined; anyOf?: readonly UncheckedPartialSchema<boolean>[] | undefined; ... 4 more ...; not?: UncheckedPartialSchema<...> | undefined; } & { ...; } & { ...; }'.
        Type '{ type: "integer"; }' is not assignable to type '{ type: "boolean"; }'.
          Types of property 'type' are incompatible.
            Type '"integer"' is not assignable to type '"boolean"'.ts(2322)

import { Static, Type } from "@sinclair/typebox";

const T = Type.Object({
  // const T = {
  x: Type.Number(), //   type: 'object',
  y: Type.Number(), //   required: ['x', 'y', 'z'],
  z: Type.Number(), //   properties: {
});
//     x: { type: 'number' },
//     y: { type: 'number' },
//     z: { type: 'number' }
//   }
// }

type T = Static<typeof T>;
// type T = {
//   x: number,
//   y: number,
//   z: number
// }

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

const dogSchema = {
  type: "object",
  properties: {
    name: { type: "string" },
    age: { type: "integer" },
    hobbies: { type: "array", items: { type: "string" } },
    favoriteFood: { enum: ["pizza", "taco", "fries"] },
  },
  required: ["name", "age"],
} as const;

type Dog = FromSchema<typeof dogSchema>;