Generating schema type from TypeScript type

toZod.ts

import * as z from 'zod';

type isAny<T> = [any extends T ? 'true' : 'false'] extends ['true']
  ? true
  : false;
type nonoptional<T> = T extends undefined ? never : T;
type nonnullable<T> = T extends null ? never : T;
type equals<X, Y> = [X] extends [Y] ? ([Y] extends [X] ? true : false) : false;

export type toZod<T> = {
  any: never;
  optional: z.ZodUnion<[toZod<nonoptional<T>>, z.ZodUndefined]>;
  nullable: z.ZodUnion<[toZod<nonnullable<T>>, z.ZodNull]>;
  array: T extends Array<infer U> ? z.ZodArray<toZod<U>> : never;
  string: z.ZodString;
  bigint: z.ZodBigInt;
  number: z.ZodNumber;
  boolean: z.ZodBoolean;
  date: z.ZodDate;
  object: z.ZodObject<{ [k in keyof T]: toZod<T[k]> }, { strict: true }, T>;
  rest: never;
}[zodKey<T>];

type zodKey<T> = isAny<T> extends true
  ? 'any'
  : equals<T, boolean> extends true //[T] extends [booleanUtil.Type]
  ? 'boolean'
  : [undefined] extends [T]
  ? 'optional'
  : [null] extends [T]
  ? 'nullable'
  : T extends any[]
  ? 'array'
  : equals<T, string> extends true
  ? 'string'
  : equals<T, bigint> extends true //[T] extends [bigintUtil.Type]
  ? 'bigint'
  : equals<T, number> extends true //[T] extends [numberUtil.Type]
  ? 'number'
  : equals<T, Date> extends true //[T] extends [dateUtil.Type]
  ? 'date'
  : T extends { [k: string]: any } //[T] extends [structUtil.Type]
  ? 'object'
  : 'rest';

  
type User = {
  name: string;
  age?: number | undefined;
  active: boolean | null;
};

// changing any method here results in a TypeError
export const User: toZod<User> = z.object({
  name: z.string().min(5), // extra validations are fine
  age: z.number().optional(),
  active: z.boolean().nullable(),
});