patchValue of Select with value not in options keeps the form valid
Explanation of the problem
When using the patchValue function to set a value to a Select box in Angular, a behavior inconsistency occurs. If the value being set does not exist in the Select options, the model is still updated with that value, and the field is considered valid. This behavior differs from the expected behavior, where setting a value that is not present in the options should either not set the value at all or set an empty value. To ensure that the issue has not already been reported, it is recommended to search GitHub for similar issues or pull requests before submitting a bug report. The following code snippet demonstrates the current behavior:
this.form.patchValue({
selectField: 'd' // Value that doesn't exist in the options
});
To reproduce the problem, a reactive form with a Select box and the Validators.required validation can be set up. The Select box should have several options available, such as “a,” “b,” and “c.” Then, using the patchValue function, set the value of the Select box to a value that is not present in the options, for example, “d.” The issue can be observed in the behavior of the form. The following Plunkr example provides a minimal reproduction of the problem: Plunkr Link.
In the described use case, the motivation for changing the behavior of the patchValue function is to handle the import of previously saved form data. When users import their data, the current form needs to be patched with their supplied data. However, if the available values in a Select box have changed over time and the user has old data that includes values not present in the options, the field and the form remain valid after the import. This leads to a situation where no data is displayed in the Select box. It is unclear whether this behavior should be considered a bug or a feature request. The issue is encountered in an environment using Angular version 4.0.0-beta.6, running on CentOS 6.8 and the Chrome 56 browser.
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Problem solution for: patchValue of Select with value not in options keeps the form valid
To solve the issue described, the behavior of the patchValue function in Angular can be modified to align with the expected behavior. One approach is to implement custom validation logic that checks if the value being set exists in the Select options. If the value is not found, the field can be marked as invalid.
Here’s an example of how this can be implemented:
- Modify the form control for the Select box to include custom validation logic:
import { ValidatorFn, AbstractControl } from '@angular/forms';
function validateSelectOption(options: string[]): ValidatorFn {
return (control: AbstractControl): { [key: string]: any } | null => {
const value = control.value;
if (options.indexOf(value) === -1) {
return { invalidOption: true };
}
return null;
};
}
// Inside the form creation
this.form = new FormGroup({
selectField: new FormControl('', validateSelectOption(['a', 'b', 'c']))
});
- Update the patchValue function to trigger the custom validation:
const valueToSet = 'd';
this.form.get('selectField')?.setValue(valueToSet);
this.form.get('selectField')?.updateValueAndValidity();
By implementing the custom validation logic, the Select box will correctly handle cases where the value being set is not present in the options. The field will be marked as invalid, ensuring that the behavior aligns with the expected behavior.
Problems with angular
- Problem: “Cannot read property ‘length’ of undefined” error when using the
async
pipe with aBehaviorSubject
. Description: Users have reported an error when using theasync
pipe with aBehaviorSubject
. The error message is “Cannot read property ‘length’ of undefined”. This error occurs when theBehaviorSubject
has not yet emitted any values and theasync
pipe tries to access thelength
property of the value. Solution: One solution is to initialize theBehaviorSubject
with a default value. For example:
private dataSubject = new BehaviorSubject<any[]>([]);
data$ = this.dataSubject.asObservable();
Then, in the component, check if the value is undefined before accessing its length property:
<ng-container *ngIf="data$ | async as data">
<div *ngIf="data.length > 0">
// Display data here
</div>
</ng-container>
- Problem: Error “zone.js has detected that ZoneAwarePromise
(window|global).Promise
has been overwritten” when running Angular tests. Description: When running Angular tests, users have reported an error message that says “zone.js has detected that ZoneAwarePromise(window|global).Promise
has been overwritten”. This error occurs when a third-party library, such as a polyfill, overwrites the globalPromise
object. Solution: One solution is to use thezone.js
library to patch the globalPromise
object before running the tests. Add the following line to thetest.ts
file:
import 'zone.js/testing';
Then, in the beforeEach
function of the test, patch the Promise
object:
beforeEach(() => {
(window as any)['Promise'] = Promise;
});
- Problem: Error “Maximum call stack size exceeded” when using
ng serve
. Description: Some users have reported an error message that says “Maximum call stack size exceeded” when running theng serve
command. This error occurs when there is a circular dependency in the application. Solution: One solution is to use the Angular dependency injection system to break the circular dependency. For example, ifComponentA
depends onComponentB
andComponentB
depends onComponentA
, create a new service and move the shared logic to the service. Then, inject the service into both components. For example:
@Injectable({ providedIn: 'root' })
export class SharedService {
// Shared logic here
}
@Component({...})
export class ComponentA {
constructor(private sharedService: SharedService) {}
// Use the shared service here
}
@Component({...})
export class ComponentB {
constructor(private sharedService: SharedService) {}
// Use the shared service here
}
A brief introduction to angular
Angular is a popular open-source web application framework used to build dynamic single-page applications. The Angular framework is written in TypeScript, a superset of JavaScript that allows for static typing and other features. The primary goal of Angular is to provide a robust and efficient framework for building web applications that can be easily maintained and scaled. Angular is known for its powerful features such as two-way data binding, dependency injection, and an extensive library of built-in directives and services.
One of the main benefits of Angular is its component-based architecture, where each component represents a specific feature or view in the application. Components are reusable and can be composed to create complex UIs. Angular also supports reactive programming and provides a robust set of APIs for working with observables, which allows for easier handling of asynchronous operations. The framework has a steep learning curve, but once mastered, developers can build complex and scalable web applications in a relatively short time. Overall, Angular is a versatile framework that offers a lot of power and flexibility to developers.
Most popular use cases for angular
-
- Building dynamic and scalable web applications: Angular provides a comprehensive framework for building modern web applications that are scalable, maintainable, and easy to develop. It offers a wide range of tools and features that make it easy to create dynamic and interactive web applications that can handle large amounts of data and complex workflows.
- Creating reusable UI components: Angular’s component-based architecture allows developers to create reusable UI components that can be easily integrated into different parts of an application. This feature makes it easier to maintain and update applications over time, as changes made to a component are automatically propagated to all instances of that component in the application.
Here’s an example code block for creating a reusable component in Angular:
import { Component, Input } from '@angular/core';
@Component({
selector: 'app-button',
template: `
<button [class]="classes">
{{label}}
</button>
`
})
export class ButtonComponent {
@Input() label: string;
@Input() classes: string;
}
- Developing cross-platform applications: With Angular, developers can build applications that can run on multiple platforms, including web, mobile, and desktop. This is made possible by using technologies such as Angular Universal for server-side rendering, and frameworks like Ionic for mobile app development. By leveraging these tools, developers can create a single codebase that can be used to build applications for multiple platforms.
Here’s an example code block for building a cross-platform application with Angular and Ionic:
# Install Ionic CLI
npm install -g @ionic/cli
# Create a new Ionic app with Angular
ionic start myApp --type=angular
# Add a new page to the app
ionic generate page myPage
# Build and run the app on a mobile device
ionic capacitor add ios
ionic capacitor copy ios
ionic capacitor open ios
It’s Really not that Complicated.
You can actually understand what’s going on inside your live applications.