Zod: create a schema using an existing type

Question

I have an endpoint that should get a parameter method which should comply with the Axios type Method.

How can I create a schema with Zod that validates that the value is using the type Schema?

import { Method } from 'axios';

const Schema = zod.object({
  method: zod.someHowUseTheTypeFrom(Method),
});

The type of Method from the Axios package is:

export type Method =
  | 'get' | 'GET'
  | 'delete' | 'DELETE'
  | 'head' | 'HEAD'
  | 'options' | 'OPTIONS'
  | 'post' | 'POST'
  | 'put' | 'PUT'
  | 'patch' | 'PATCH'
  | 'purge' | 'PURGE'
  | 'link' | 'LINK'
  | 'unlink' | 'UNLINK'

Answer 1

Reading your comment it sounds like you want to ensure that your schema is in sync with the Method type from axios. I would suggest doing the following:

import { z } from 'zod';
import type { Method } from 'axios';

const methods: z.ZodType<Method> = z.enum(['get', 'GET', ...]);

Which will at least enforce that the schema on the right hand side of the expression will parse valid axios Method results. Unfortunately, anything more may be out of reach unless axios also exports an array containing the strings that correspond to the values in the Method type.

The original thing that you were looking for z.something(<type here>) can't work because zod is using actual runtime objects, and types like Method don't exist at runtime. If axios exported an array containing the methods, then that would be a runtime value and you could use that (perhaps with some type casting) to generate your methods schema (more on this in a moment).

The other shortcoming to this approach is that something like this will typecheck:

const methods z.ZodType<Method> = z.enum(['get']);

The reason for that is because of how types work in TypeScript. That enum schema will only ever parse successfully for 'get' but because the literal 'get' is a subtype of the larger union type defined in Method, the resulting schema is also assignable.

So, the next option I'm going to pose feels slightly self-defeating in that it's going to require redeclaring all the values in Method, however, you can continue to use the axios Method type and you will definitely have a schema that parses all of the values in Method (ie, does not succumb to the issue mentioned above):

import { z } from "zod";
import { Method } from "axios";

const METHOD_MAP: { [K in Method]: null } = {
  get: null,
  GET: null,
  delete: null,
  DELETE: null,
  head: null,
  HEAD: null,
  options: null,
  OPTIONS: null,
  post: null,
  POST: null,
  put: null,
  PUT: null,
  patch: null,
  PATCH: null,
  purge: null,
  PURGE: null,
  link: null,
  LINK: null,
  unlink: null,
  UNLINK: null
};

const METHODS = (Object.keys(METHOD_MAP) as unknown) as readonly [
  Method,
  ...Method[]
];
const methods: z.ZodType<Method> = z.enum(METHODS);

The type assertion for METHODS is safe here because the METHODS_MAP is not exported and we know exactly what keys it has. Now, the METHOD_MAP object will cause a type error if any Method value is missing though which means the resulting schema will parse all Method values as a guarantee enforced at compile time.

Answer 2

If you want to use the type directly you can use this:

const methods = ['get','GET',...] as const;

export type Method = (typeof methods)[number];

zod.enum(methods);

You get the best of both worlds this way; having the methods in a value you can use (array), and the type that you originally wanted.

Answer 3

You can use ts-to-zod to export your typescript type into a zod schema.

Answer 4

I have found out that using z.custom<ExistingType>() is working for me, and would be appropriate for this kind of problem.