In Angular 2 (and newer versions), you can modify a form control value based on another form control by subscribing to changes in the source form control and updating the target form control accordingly. This can be achieved using reactive forms and the valueChanges observable provided by Angular's FormControl class.

Here's an example of how to do it:

  1. Create the Form Group and Form Controls: First, create a form group with the form controls you need. For this example, let's say we have two form controls: sourceControl and targetControl.

    typescript
    import { Component } from '@angular/core'; import { FormBuilder, FormGroup } from '@angular/forms'; @Component({ selector: 'app-my-form', templateUrl: './my-form.component.html', }) export class MyFormComponent { myForm: FormGroup; constructor(private formBuilder: FormBuilder) { this.myForm = this.formBuilder.group({ sourceControl: [''], targetControl: [''], }); // Subscribe to changes in the sourceControl this.myForm.get('sourceControl').valueChanges.subscribe(newValue => { // Modify the targetControl value based on the sourceControl value this.updateTargetControlValue(newValue); }); } updateTargetControlValue(newValue: any): void { // You can modify the targetControl value here based on the newValue // For example, you can add a prefix or perform any custom logic // before updating the targetControl value this.myForm.get('targetControl').setValue(newValue + '_modified'); } }
  2. Bind Form Controls to HTML: In the HTML template, bind the form controls to the form inputs using the formControlName directive.

    html
    <form [formGroup]="myForm"> <input type="text" formControlName="sourceControl" /> <input type="text" formControlName="targetControl" /> </form>

In this example, when the user types into the sourceControl, the valueChanges observable is triggered. The updateTargetControlValue() method is then called, modifying the targetControl value based on the newValue from the sourceControl. The updated value is then reflected in the targetControl input field.

By using reactive forms and subscribing to valueChanges, you can easily implement dynamic form control interactions in Angular. This approach allows you to perform custom logic based on the values of multiple form controls, giving you full control over how the form behaves and updates in real-time.

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