---
title: "API action validator"
---

## Overview

**API action validator** — is an array of functions (think middlewares) that is used to make sure that data sent by client is valid.

## Examples

```typescript
import { z } from 'zod';

import { AppKoaContext, Next } from 'types';
import { EMAIL_REGEX, PASSWORD_REGEX } from 'app-constants';

import { userService } from 'resources/user';

import { validateMiddleware } from 'middlewares';

const schema = z.object({
  firstName: z.string().min(1, 'Please enter fist name.').max(100),
  lastName: z.string().min(1, 'Please enter last name.').max(100),
  email: z.string().regex(EMAIL_REGEX, 'Email format is incorrect.'),
  password: z.string().regex(PASSWORD_REGEX, 'The password format is incorrect'),
});

type ValidatedData = z.infer<typeof schema>;

async function validator(ctx: AppKoaContext<ValidatedData>, next: Next) {
  const { email } = ctx.validatedData;

  const isUserExists = await userService.exists({ email });

  ctx.assertClientError(!isUserExists, {
    email: 'User with this email is already registered',
  });

  await next();
}

async function handler(ctx: AppKoaContext<ValidatedData>) {
  // ...action code
}

export default (router: AppRouter) => {
  router.post('/sign-up', validateMiddleware(schema), validator, handler);
};
```

To pass data from the `validator` to the `handler`, utilize the `ctx.validatedData` object:

``` typescript
import { z } from 'zod';

import { AppKoaContext, AppRouter, Next, User } from 'types';
import { EMAIL_REGEX, PASSWORD_REGEX } from 'app-constants';

import { userService } from 'resources/user';

import { validateMiddleware } from 'middlewares';
import { securityUtil } from 'utils';

const schema = z.object({
  email: z.string().regex(EMAIL_REGEX, 'Email format is incorrect.'),
  password: z.string().regex(PASSWORD_REGEX, 'The password format is incorrect'),
});

interface ValidatedData extends z.infer<typeof schema> {
  user: User;
}

async function validator(ctx: AppKoaContext<ValidatedData>, next: Next) {
  const { email, password } = ctx.validatedData;

  const user = await userService.findOne({ email });

  ctx.assertClientError(user && user.passwordHash, {
    credentials: 'The email or password you have entered is invalid',
  });

  const isPasswordMatch = await securityUtil.compareTextWithHash(password, user.passwordHash);

  ctx.assertClientError(isPasswordMatch, {
    credentials: 'The email or password you have entered is invalid',
  });

  ctx.validatedData.user = user;
  await next();
}

async function handler(ctx: AppKoaContext<ValidatedData>) {
  const { user } = ctx.validatedData;

  // ...action code
}

export default (router: AppRouter) => {
  router.post('/sign-in', validateMiddleware(schema), validator, handler);
};
```
