How to Filter User Input in Angular While Maintaining ngModel?

[Lomanu4]

Introduction


Filtering user input in Angular can be a bit tricky, especially when you want to enforce a specific format through user input validations. In this article, we'll explore how to filter user input in an HTML text input while utilizing Angular's reactive forms or template-driven forms. Let's address a common scenario where you want to ensure a specific format for a tracking number, for instance, ensuring it matches the pattern AB123456789CD.

Why Input Filtering Is Necessary


User input filtering is important for several reasons, including preventing invalid data submission, improving data integrity, and enhancing user experience. In scenarios like tracking numbers, ensuring the format is kept clean can prevent errors when processing such inputs.

Filtering User Input with Native JavaScript


Before diving into Angular, let’s review how input filtering is typically handled using native HTML and JavaScript. The example below demonstrates a basic form that captures a tracking number:

<form>
<label for="tracking">Tracking Number:</label>
<input
type="text"
id="tracking"
name="tracking"
pattern="^[A-Z]{2}\d{9}[A-Z]{2}$"
title="Format must be like AB123456789CD"
required
minlength="13"
maxlength="13"
/>
</form>

<script>
const input = document.getElementById('tracking');

input.addEventListener('input', () => {
console.log('input fired');

// Remove non-alphanumeric chars and force uppercase
input.value = input.value.replace(/[^a-zA-Z0-9]/g, '').toUpperCase();
});
</script>


This approach successfully removes non-alphanumeric characters and converts input to uppercase. This works seamlessly with traditional JavaScript.

Filtering User Input with Angular


Now, let’s shift our focus to Angular. The goal is to replicate similar behavior while managing user input and state through Angular's ngModel. Initially, your Angular code may look something like this:

@Component({
selector: 'app-root',
template: `
<form>
<label for="tracking">Tracking Number:</label>
<input
type="text"
id="tracking"
name="tracking"
pattern="^[A-Z]{2}\d{9}[A-Z]{2}$"
title="Format must be like AB123456789CD"
required
minlength="13"
maxlength="13"
[ngModel]="trackingNumber"
(ngModelChange)="onTrackingChange($event)"
/>
</form>
`,
imports: [FormsModule],
})
export class App {
trackingNumber = '';

onTrackingChange(value: string) {
console.log('input fired');

// Remove non-alphanumeric characters and force uppercase
this.trackingNumber = value.replace(/[^a-zA-Z0-9]/g, '').toUpperCase();
}
}

Understanding The Problem


The problem arises because Angular is designed to effectively manage state. When the modified input, which goes through filtering, does not change the ngModel value, Angular does not recognize this as a change, and subsequently, it does not update the input field. Thus, the filtered input won't reflect in the UI.

Solution: Forcing Angular to Recognize Changes


To overcome this issue, we need Angular to acknowledge changes manually. One effective solution involves the use of the NgModel directive in combination with Angular’s ChangeDetectorRef to force change detection.

Step-by-Step Solution


Let’s enhance our example with this approach:

1. 
   Inject ChangeDetectorRef in your component:
   
   import { ChangeDetectorRef } from '@angular/core';
2. 
   Modify Constructor to inject ChangeDetectorRef:
   
   constructor(private cdr: ChangeDetectorRef) {}
3. 
   Update onTrackingChange to detect changes:
   
   onTrackingChange(value: string) {
   console.log('input fired');
   // Remove non-alphanumeric characters and force uppercase
   this.trackingNumber = value.replace(/[^a-zA-Z0-9]/g, '').toUpperCase();
   // Manually trigger change detection
   this.cdr.detectChanges();
   }
4. 
   Final Code Implementation:
   
   import { Component, ChangeDetectorRef } from '@angular/core';
   import { FormsModule } from '@angular/forms';
   
   @Component({
   selector: 'app-root',
   template: `
   <form>
   <label for="tracking">Tracking Number:</label>
   <input
   type="text"
   id="tracking"
   name="tracking"
   pattern="^[A-Z]{2}\d{9}[A-Z]{2}$"
   title="Format must be like AB123456789CD"
   required
   minlength="13"
   maxlength="13"
   [ngModel]="trackingNumber"
   (ngModelChange)="onTrackingChange($event)"
   />
   </form>
   `,
   imports: [FormsModule],
   })
   export class App {
   trackingNumber = '';
   
   constructor(private cdr: ChangeDetectorRef) {}
   
   onTrackingChange(value: string) {
   console.log('input fired');
   // Remove non-alphanumeric characters and force uppercase
   this.trackingNumber = value.replace(/[^a-zA-Z0-9]/g, '').toUpperCase();
   // Manually trigger change detection
   this.cdr.detectChanges();
   }
   }

This method will ensure that any changes made to the trackingNumber are reflected in the user interface correctly.

Frequently Asked Questions

1. Why isn't my input value updating in Angular?


The input value may not be updating due to the nature of Angular's binding methods not recognizing the changes if the value remains the same after filtering. Using ChangeDetectorRef to manually trigger a change can solve this issue.

2. How does ngModel react to changes?


ngModel listens for changes in input fields and binds them to model properties. If no changes are detected, Angular does not trigger an update to the UI.

3. Can I apply similar filtering logic in reactive forms?


, the same principle applies to reactive forms; you would manage the FormControl value in a similar manner while using changeDetection to ensure updates are reflected appropriately.

Conclusion


By employing Angular's mechanisms effectively, you can create a robust user experience when managing input fields. In this post, we tackled the intricacies of input filtering in detail, ensuring users can provide valid tracking numbers without inconsistencies. With a manual change detection approach, you can ensure your application remains responsive and user-friendly. Happy coding!

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