Sure! Here's a more complete example that demonstrates how to bind a hidden field to a component and update its value based on the value of another form field:

HTML template:

html
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<form [formGroup]="myForm" (ngSubmit)="onSubmit()">
  <div>
    <label>First Name: <input type="text" formControlName="firstName"></label>
  </div>
  <div>
    <label>Last Name: <input type="text" formControlName="lastName"></label>
  </div>
  <div>
    <label>Email Address: <input type="email" formControlName="email"></label>
  </div>
  <div>
    <label>Phone Number: <input type="text" formControlName="phoneNumber"></label>
  </div>
  <div>
    <input type="hidden" name="fullName" formControlName="fullName" [hidden]="!myForm.controls['fullName'].value">
    <p *ngIf="myForm.controls['fullName'].value">Full Name: {{ myForm.controls['fullName'].value }}</p>
  </div>
  <button type="submit">Submit</button>
</form>
Component:

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

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.css']
})
export class MyComponent {

  myForm: FormGroup;

  constructor(private fb: FormBuilder) {
    this.myForm = this.fb.group({
      firstName: ['', Validators.required],
      lastName: ['', Validators.required],
      email: ['', [Validators.required, Validators.email]],
      phoneNumber: ['', Validators.required],
      fullName: ['']
    });

    // Listen for changes to the 'firstName', 'lastName', 'email', and 'phoneNumber' controls
    this.myForm.controls['firstName'].valueChanges.subscribe(value => {
      this.updateFullName();
    });
    this.myForm.controls['lastName'].valueChanges.subscribe(value => {
      this.updateFullName();
    });
    this.myForm.controls['email'].valueChanges.subscribe(value => {
      this.updateFullName();
    });
    this.myForm.controls['phoneNumber'].valueChanges.subscribe(value => {
      this.updateFullName();
    });
  }

  onSubmit() {
    console.log(this.myForm.value);
  }

  private updateFullName() {
    // Concatenate the values of the 'firstName', 'lastName', 'email', and 'phoneNumber' controls
    const fullName = `${this.myForm.controls['firstName'].value} ${this.myForm.controls['lastName'].value} (${this.myForm.controls['email'].value}, ${this.myForm.controls['phoneNumber'].value})`;

    // Update the value of the hidden field
    this.myForm.controls['fullName'].setValue(fullName);
  }

}
In this example, we added subscriptions to the valueChanges observables of the 'firstName', 'lastName', 'email', and 'phoneNumber' controls that call a private updateFullName() method. This method concatenates the values of these controls and sets the value of the hidden field (fullName) based on the concatenated value.

We also added an *ngIf directive to the HTML template to conditionally show the full name when the value of the hidden field is truthy.

I hope this helps! Let me know if you have any further questions.





any way to do that ?
Yes, there are multiple ways to achieve this behavior in Angular. One way is to use the (ngModelChange) directive on the input field that triggers the update, and then update the value of the hidden field in the component. Here's an example:

HTML template:

html
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<input type="text" [(ngModel)]="firstName" (ngModelChange)="updateFullName()">
<input type="hidden" [value]="fullName">
<p *ngIf="fullName">Full Name: {{ fullName }}</p>
Component:

typescript
Copy code
import { Component } from '@angular/core';

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.css']
})
export class MyComponent {

  firstName: string = '';
  lastName: string = '';
  email: string = '';
  phoneNumber: string = '';
  fullName: string = '';

  updateFullName() {
    // Concatenate the values of the 'firstName', 'lastName', 'email', and 'phoneNumber' fields
    this.fullName = `${this.firstName} ${this.lastName} (${this.email}, ${this.phoneNumber})`;
  }

}
In this example, we used two-way data binding with [(ngModel)] to bind the value of the firstName field to a variable in the component. We also added the (ngModelChange) directive to trigger the updateFullName() method in the component whenever the value of firstName changes. The updateFullName() method concatenates the values of the fields and sets the value of the fullName variable in the component. Finally, we used the [value] directive to bind the value of the hidden field to the fullName variable, and added an *ngIf directive to conditionally show the full name when the fullName variable is truthy.

I hope this helps! Let me know if you have any further questions.





wich one is the popular chose ?
The approach I mentioned earlier, which uses two-way data binding with (ngModel) and (ngModelChange), is a popular choice for binding hidden fields in Angular. However, there are other ways to achieve the same behavior, such as using reactive forms and updating the value of the hidden field using the patchValue() or setValue() methods of the form control. The choice ultimately depends on your specific use case and preferences.