ignoringFields.mjs

import { z } from "zod";

/**
 * Approach 1:
 *
 * Errors with `"Unrecognized key(s) in object: 'readonlyProperty'"`
 */

const s1 = z
  .object({
    property: z.string(),
  })
  // This does the work...
  .strict();

/**
 * @typedef {z.infer<typeof s1>} S1T
 */

const s1_r1 = s1.safeParse({
  property: "hello world!",
  readonlyProperty: "fail",
});
if (!s1_r1.success) {
  console.error("Approach 1:", s1_r1.error);
} else {
  console.log("Approach 1:", s1_r1.data);
}

console.log("\n========\n");

const s1_r2 = s1.safeParse({
  property: "hello world!",
});
if (!s1_r2.success) {
  console.error("Approach 1:", s1_r2.error);
} else {
  console.log("Approach 1:", s1_r2.data);
}

console.log("\n========\n");

/**
 * Approach 2:
 *
 * Errors with `"Expected never, received string"`
 */

const s2_inner = z.object({
  property: z.string(),
});

const s2 = z.intersection(
  s2_inner,
  z
    .object({
      readonlyProperty: z.never(),
    })
    .partial()
);

/**
 * @typedef {z.infer<typeof s2>} S2T
 * @typedef {S2T['readonlyProperty']} ReadonlyPropertyType
 */

const s2_r1 = s2.safeParse({
  property: "hello world!",
  readonlyProperty: "noway",
});
if (!s2_r1.success) {
  console.error("Approach 2:", s2_r1.error);
} else {
  console.log("Approach 2:", s2_r1.data);
}

console.log("\n========\n");

const s2_r2 = s2.safeParse({
  property: "hello world!",
});
if (!s2_r2.success) {
  console.error("Approach 2:", s2_r2.error);
} else {
  console.log("Approach 2:", s2_r2.data);
}

(Approach 2 doesn't work since there is no way of validating that a property doesn't exist and that if it is set is an error. We've unfortunately allowed it to be undefined above.)