Demystifying Validation Errors: A Guide to "Expected validator to return Promise or Observable" in Angular

• This error occurs in Angular's reactive forms validation mechanism.
• You're using validators to define rules for form controls, ensuring user input adheres to specific criteria.

The Issue:

• Angular expects validators to return either a Promise or an Observable.
• These asynchronous mechanisms allow Angular to handle validation that might take time, such as checking data against a backend server.

Why It Happens:

• You've likely defined a custom validator function that doesn't explicitly return a Promise or an Observable.
• It might just perform some logic and not return anything, or it might return a simple value like true or false.

Resolving It:

1. Wrap Synchronous Validation Logic in a Promise.resolve():

   If your validation logic is synchronous (doesn't involve asynchronous operations), wrap the result in a Promise.resolve():

   customValidator(control: FormControl) {
     // Your validation logic here
     if (/* condition met */) {
       return Promise.resolve(null); // Valid
     } else {
       return Promise.resolve({ someError: true }); // Invalid, return an error object
     }
   }
   

2. Use an Observable for Asynchronous Validation:

   If you need to perform asynchronous checks (e.g., contacting a server), create an Observable using operators like map and catchError:

   import { of } from 'rxjs';
   import { catchError, map } from 'rxjs/operators';
   
   customValidator(control: FormControl) {
     const httpRequest = this.http.get(/* your API endpoint */); // Replace with your HTTP request
     return httpRequest.pipe(
       map(response => (response.isValid ? null : { someError: true })), // Process response
       catchError(() => of({ someError: true })) // Handle errors
     );
   }
   

3. Ensure Correct Usage of Built-in Validators:

   • Built-in validators like Validators.required, Validators.minLength, etc., already return null for valid input and an error object for invalid input.
   • Use them directly without needing to wrap them in Promise.resolve().

Example Usage:

import { FormBuilder, FormGroup, Validators } from '@angular/forms';

// ...

myForm = this.formBuilder.group({
  username: ['', [Validators.required, customValidator]],
  email: ['', [Validators.required, Validators.email]]
});

import { FormControl } from '@angular/forms';

function usernameValidator(control: FormControl) {
  if (control.value.length < 5) {
    return Promise.resolve({ usernameTooShort: true }); // Invalid, return error object
  }
  // No errors, username is valid
  return Promise.resolve(null);
}

const myForm = new FormBuilder().group({
  username: ['', usernameValidator]
});

In this example, the usernameValidator checks if the username is at least 5 characters long. If it's not, it returns a Promise.resolve() with an error object indicating "usernameTooShort". Otherwise, it returns Promise.resolve(null) to signal validity.

Asynchronous Validation with Observable:

import { FormControl } from '@angular/forms';
import { of } from 'rxjs';
import { catchError, map } from 'rxjs/operators';

function uniqueEmailValidator(control: FormControl) {
  // Simulate an asynchronous HTTP request to check email uniqueness
  const httpRequest = of({ isUnique: true }).pipe( // Replace with your actual HTTP request
    delay(1000), // Simulate delay for demonstration
    map(response => (response.isUnique ? null : { emailNotUnique: true })),
    catchError(() => of({ emailNotUnique: true })) // Handle errors
  );

  return httpRequest;
}

const myForm = new FormBuilder().group({
  email: ['', [Validators.required, uniqueEmailValidator]]
});

Here, the uniqueEmailValidator simulates an asynchronous check for email uniqueness. It creates an Observable using of() and delay() to represent an HTTP request (replace this with your actual backend interaction). It then uses map to process the response and catchError to handle errors, returning appropriate error objects for invalid emails.

Remember to replace the simulated of() with your actual HTTP request logic using a service like HttpClient in Angular.

Using Built-in Validators:

import { FormBuilder, FormGroup, Validators } from '@angular/forms';

const myForm = new FormBuilder().group({
  name: ['', Validators.required],
  email: ['', [Validators.required, Validators.email]]
});

Built-in validators like Validators.required and Validators.email already handle synchronous validation and return the necessary structures. You can use them directly without needing custom wrappers.

• Angular provides a rich set of built-in validators like Validators.required, Validators.minLength, Validators.email, etc. These cover many common validation scenarios and already return the correct structures (null for valid, error object for invalid).
• Use them whenever possible to simplify your code and avoid the need for custom validators entirely.

Custom Validators with Arrow Functions (Angular 6+):

• If necessary, you can create custom validators using concise arrow functions (introduced in Angular 6 and later). These functions implicitly return the validation result, eliminating the explicit return statement.

customValidator = (control: FormControl) => {
  if (/* validation logic */) {
    return { someError: true }; // Invalid
  }
  return null; // Valid
};

Custom Validators with Conditional Logic:

• You can structure your custom validators with conditional logic to determine validity and return the appropriate result (null for valid, error object for invalid):

customValidator(control: FormControl) {
  if (/* validation logic */) {
    return { someError: true };
  }
  // No errors, return null
  return null;
}

Custom Validators with async Keyword (Advanced):

• For complex asynchronous validation logic like handling multiple asynchronous operations, consider using the async keyword (requires TypeScript). This allows you to write asynchronous functions within your validator:

async customValidator(control: FormControl) {
  const response1 = await this.http.get(/* endpoint 1 */);
  const response2 = await this.http.get(/* endpoint 2 */);
  // Process responses and return validation result
}