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"Make me a JS function that validates an API response using Zod and fixes missing fields with default values."

Before diving into the code, let’s understand what we’re trying to solve here. Sometimes, APIs don’t return exactly what we expect. Maybe a field is missing, or an extra field sneaks in. This is where Zod helps. It allows us to validate data and set default values for missing fields.

Imagine you expect this API response:

{ "a": "nice", "b": "good" }

But instead, you get this:

{ "a": "nice", "c": "something else" }

Field b is missing, and we have an extra field c we didn’t ask for. The solution? Zod.

Let’s break this down. The goal is to ensure that:

• If b is missing, we add it with a default value.
• If extra fields like c show up, we ignore them.

Here’s the plan: we’ll define a schema that requires a, allows b with a default value, and strictly ignores anything else.

import { z } from 'zod';
 
// Define the schema
const ResponseSchema = z.object({
  a: z.string(),  // 'a' must exist
  b: z.string().default("default value")  // 'b' gets a default if missing
}).strict();  // Prevents any extra fields
 
// Example API response with missing 'b' and extra 'c'
const apiResponse = { a: "nice", c: "something else" };
 
// Validate and parse the response
const parsedResponse = ResponseSchema.parse(apiResponse);
 
console.log(parsedResponse); // Output: { a: "nice", b: "default value" }

Explanation:

• The .default() method sets a default value for b if it’s missing.
• .strict() ensures that only the fields a and b are allowed. Any extras, like c, will throw an error.

If you don’t mind extra fields, you can remove the .strict() part and let Zod just focus on the required fields.

const LooseResponseSchema = z.object({
  a: z.string(),
  b: z.string().default("default value")
});
 
// This will ignore 'c' and fix the missing 'b'
const looseParsedResponse = LooseResponseSchema.parse(apiResponse);
 
console.log(looseParsedResponse); // Output: { a: "nice", b: "default value" }

Now, even if the API returns extra fields like c, Zod will fix the missing b and just move on.

With Zod, you can easily guard your API responses by:

1. Ensuring required fields are present.
2. Setting default values for missing fields.
3. Handling extra fields by either ignoring or rejecting them.

That’s it! You’ve got a reliable way to clean up your API responses and keep things consistent.