Encountering the perplexing error “Expected validator to return Promise or Observable” can be a significant hurdle when developing applications, particularly within frameworks like Angular that heavily rely on asynchronous operations and reactive programming. This error typically arises when a custom validator, intended to perform asynchronous validation, isn’t correctly structured to return either a Promise or an Observable, which are the standard mechanisms for handling asynchronous results in these environments. Understanding the underlying reasons for this error, and knowing how to properly implement asynchronous validators, is crucial for ensuring robust and reliable data validation within your applications. A correctly implemented validator provides a smoother user experience and prevents unexpected application behavior. Let’s delve into the common causes, solutions, and best practices for avoiding this frustrating issue.
Understanding Asynchronous Validators
Asynchronous validators play a vital role in modern web applications, especially when dealing with scenarios where validation requires interaction with a server or a database. Unlike synchronous validators, which perform validation immediately and return a boolean value, asynchronous validators handle validation processes that may take some time to complete. This is essential when, for example, checking if a username is already taken in a database, or verifying a credit card number against a payment gateway. The key difference lies in how the validator communicates its result: it returns a Promise or an Observable, both of which are designed to handle asynchronous operations. These constructs allow the application to continue executing other tasks while the validation is in progress, and then react accordingly when the validation result is available. The “Expected validator to return Promise or Observable” error signals a mismatch between what the validator is supposed to return (an asynchronous signal) and what it actually returns (something else, or nothing). Properly implementing asynchronous validators ensures a non-blocking, responsive user interface.
Choosing between Promises and Observables often depends on the specific needs of your application and your familiarity with reactive programming. Promises are simpler to use for single asynchronous operations, as they represent a one-time completion or failure. Observables, on the other hand, are more powerful for handling streams of data or multiple asynchronous events. They offer more flexibility in terms of cancellation, transformation, and error handling. According to the Angular documentation, both Promises and Observables are acceptable return types for asynchronous validators Angular AsyncValidatorFn. The critical point is ensuring that your validator consistently returns one of these types to avoid the dreaded error. Failing to do so can lead to unpredictable behavior and difficult-to-debug issues. Consider the complexity of the validation logic when deciding which approach best suits your needs.
The syntax for implementing an asynchronous validator involves defining a function that takes an AbstractControl (representing the form control being validated) as input and returns a Promise or Observable that resolves to either null (if the control is valid) or a validation error object (if the control is invalid). It is crucial to handle potential errors within the asynchronous operation using .catch() for Promises or .pipe(catchError()) for Observables to prevent unhandled rejections or exceptions that could crash the application. For example, if the server is unavailable during a username availability check, the validator should gracefully handle the error and return an appropriate validation message to the user, rather than throwing an unhandled exception. This robust error handling is a hallmark of well-designed asynchronous validators.
Common Causes of the Error
Several factors can contribute to the “Expected validator to return Promise or Observable” error. One common mistake is forgetting to return a Promise or Observable at all. This often happens when the validator logic is complex, and a conditional statement prevents the asynchronous operation from being initiated in certain cases. For example, if the input field is empty, the validator might skip the asynchronous validation step and implicitly return undefined, which is not a valid return type. Another frequent issue is returning a synchronous value instead of an asynchronous one. This can occur if the developer accidentally mixes synchronous and asynchronous validation logic, or if they incorrectly assume that a particular operation is synchronous when it actually involves an asynchronous call. This leads to the validator returning a value immediately, before the asynchronous operation has completed, resulting in the error.
Incorrectly handling asynchronous operations within the validator can also lead to this error. If you’re using Promises, failing to chain a .then() or .catch() block can prevent the validator from returning the final validation result. Similarly, if you’re using Observables, not subscribing to the Observable or not using the .pipe() operator to transform the emitted values can cause the validator to behave unexpectedly. Furthermore, type mismatches can be a source of confusion. If the validator is declared to return a specific type (e.g., Promise
Here’s an example of a common mistake. Let’s say you have an asynchronous validator that checks if an email is already registered: typescript static emailTakenValidator(control: AbstractControl): ValidationErrors | null { // Incorrect: Doesn’t return anything in some cases if (control.value === ‘’) { return null; // Synchronous valid } this.emailCheckService.isEmailTaken(control.value).then(isTaken => { if (isTaken) { return { ’emailTaken’: true }; // This doesn’t return from the validator function! } else { return null; // This also doesn’t return from the validator function! } }); } The above code will likely cause the error because the return statements inside the .then() block only return from the anonymous function within the .then() callback, not from the emailTakenValidator function itself. The validator function implicitly returns undefined in this case.
Solutions and Best Practices
To resolve the “Expected validator to return Promise or Observable” error, you must ensure that your asynchronous validator consistently returns a Promise or Observable. If you’re using Promises, explicitly return the Promise returned by the asynchronous operation. For example: typescript static emailTakenValidator(control: AbstractControl): Promise
When working with Observables, make sure to subscribe to the Observable and return it from the validator. Use the .pipe() operator to transform the emitted values and handle errors. A common pattern is to use the map() operator to transform the result of the asynchronous operation into a validation error object or null, and the catchError() operator to handle potential errors: typescript import { of } from ‘rxjs’; import { map, catchError } from ‘rxjs/operators’; static emailTakenValidator(control: AbstractControl): Observable
Here are some additional best practices to keep in mind:
- Always explicitly return a Promise or Observable from your asynchronous validator.
- Use strict type checking to ensure that the return type matches the expected type.
- Handle errors gracefully using .catch() for Promises and .pipe(catchError()) for Observables.
- Avoid mixing synchronous and asynchronous validation logic.
- Consider using a linting tool to automatically detect potential errors in your code.
FAQ: Asynchronous Validators
- What is an asynchronous validator?
- An asynchronous validator is a function that validates a form control's value asynchronously, typically by making a server-side request. It returns a Promise or Observable that resolves to either null (if the control is valid) or a validation error object (if the control is invalid).
- Why use asynchronous validators instead of synchronous validators?
- Asynchronous validators are necessary when validation requires interaction with a remote server or database, such as checking if a username is already taken. Synchronous validators are not suitable for these scenarios because they would block the user interface while waiting for the server to respond.
- What's the difference between a Promise and an Observable in the context of asynchronous validators?
- Both Promises and Observables can be used to represent asynchronous operations. Promises are simpler for single asynchronous operations, while Observables are more powerful for handling streams of data or multiple asynchronous events. Observables also offer better support for cancellation and error handling.
- How do I handle errors in an asynchronous validator?
- Use .catch() for Promises and .pipe(catchError()) for Observables to handle potential errors. Return a valid value (e.g., null) or a specific error object to indicate that the validation failed due to an error.
- Ensure consistent return types.
- Use robust error handling.
- Leverage RxJS operators for Observables.
- Identify the Form Control Needing Validation
- Create the Asynchronous Validator Function
- Return a Promise or Observable from the Validator
- Handle Potential Errors Within the Asynchronous Operation
- Attach the Validator to the Form Control
The “Expected validator to return Promise or Observable” error can be a frustrating roadblock, but with a solid understanding of asynchronous validation principles and best practices, it becomes a manageable challenge. Remember to always explicitly return a Promise or Observable, handle errors gracefully, and leverage the power of RxJS operators when working with Observables. By consistently applying these techniques, you can ensure that your applications are robust, reliable, and provide a seamless user experience. If you’re eager to learn more about advanced form handling techniques and building complex reactive forms, consider exploring resources on reactive forms in Angular, advanced RxJS patterns, and custom validator implementations. Don’t let validation errors hold you back – empower yourself with the knowledge and tools to build exceptional applications.
Question & Answer :
I’m trying to do a custom validation on Angular 5 but I’m facing the following error
Expected validator to return Promise or Observable
I just want to return an error to the form if the value doesn’t match the required, here’s my code:
This is the component where my form is
constructor(fb: FormBuilder, private cadastroService:CadastroService) { this.signUp = fb.group({ "name": ["", Validators.compose([Validators.required, Validators.minLength(2)])], "email": ["", Validators.compose([Validators.required, Validators.email])], "phone": ["", Validators.compose([Validators.required, Validators.minLength(5)])], "cpf": ["", Validators.required, ValidateCpf] }) }
This code is in the file with the validation I want to implement:
import { AbstractControl } from '@angular/forms'; export function ValidateCpf(control: AbstractControl){ if (control.value == 13445) { return {errorCpf: true} } return null; }
Does that type of validation only work with observables or can I do it without being a promise or observable?
It means that you have to add multiple validators in array
. Example:
With Error
profileFormGroup = { budget: [null, Validators.required, Validators.min(1)] };
Above one throws error that validator to return Promise or Observable
Fix:
profileFormGroup = { budget: [null, [Validators.required, Validators.min(1)]] };
Explanation:
In angular Reactive form validation done by using in-built validators which could given in array in 2nd postion, when multiple validators used.
FIELD_KEY: [INITIAL_VALUE, [LIST_OF_VALIDATORS]]
