Unable to assign a value to the HTMLInputElement's property: The input field can only be set to a filename or an empty string programmatically

Question

Unable to assign a value to the HTMLInputElement's property: The input field can only be set to a filename or an empty string programmatically

When attempting to upload an image, I encountered the error message listed in the question title:

This is my template

<input type="file" formControlName="avatar" accept=".jpg, .jpeg .svg"
   #fileInput (change)="uploadImage($event)" />

<div class="avatar-preview">
  <ng-template #selectedImage>
    <div style.backgroundImage="{{'url('+ imageUrl +')'}}"></div>
  </ng-template>
</div>
<button (click)="fileInput.click()"></button>

And this is my component ts


@ViewChild('fileInput') el: ElementRef;
imageUrl: string = "";

this.profileForm = new FormGroup({
  avatar: new FormControl(null)
});

public uploadImage(event) {
    let reader = new FileReader();
    let file = event.target.files[0];
    if (event.target.files && event.target.files[0]) {
      reader.readAsDataURL(file);
      reader.onload = () => {
        this.imageUrl = reader.result as string;
        this.profileForm.patchValue({
          avatar: reader.result
        });
      }
    }
    this.cd.markForCheck(); 
}

javascript html css angular typescript

Answer 1

Answer №1

Angular lacks a built-in ControlValueAccessor for handling file inputs. Instead, it utilizes the DefaultValueAccessor, which attempts to write the value of avatar to the control. However, this is not suitable for a FileInputControl.

To resolve this issue, you have two options:

Implement your own ControlValueAccessor and transfer the functionality from your uploadImage function to it.
Remove
```
formControlName="avatar"
```
and refrain from using reactive forms since you are already managing the events manually.

For example, you can create a directive like the one below (untested, may require adjustments):

@Directive({
  selector: 'input[type=file][formControlName]',
  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      useExisting: FileUploadAccessor,
      multi: true
    }
  ]
})
export class FileUploadAccessor implements ControlValueAccessor {
  private onChange: Function;

  @HostListener('change', ['$event.target.files']) emitFiles( event: FileList ) {
    const file = event && event.item(0);
    this.onChange(file);
  }

  constructor( private host: ElementRef<HTMLInputElement> ) {
  }

  writeValue( value: null ) {
    // clear file input
    this.host.nativeElement.value = '';
  }

  registerOnChange( fn: Function ) {
    this.onChange = fn;
  }

  registerOnTouched( fn: Function ) {
  }

}

If desired, you can also integrate your current processing logic into this directive by modifying the following section:

let reader = new FileReader();
reader.readAsDataURL(file);
reader.onload = () => {
  this.onChange(reader.result as string);
}

Answer 2

Angular lacks a built-in ControlValueAccessor for handling file inputs. Instead, it utilizes the DefaultValueAccessor, which attempts to write the value of avatar to the control. However, this is not suitable for a FileInputControl.

To resolve this issue, you have two options:

Implement your own ControlValueAccessor and transfer the functionality from your uploadImage function to it.
Remove
```
formControlName="avatar"
```
and refrain from using reactive forms since you are already managing the events manually.

For example, you can create a directive like the one below (untested, may require adjustments):

@Directive({
  selector: 'input[type=file][formControlName]',
  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      useExisting: FileUploadAccessor,
      multi: true
    }
  ]
})
export class FileUploadAccessor implements ControlValueAccessor {
  private onChange: Function;

  @HostListener('change', ['$event.target.files']) emitFiles( event: FileList ) {
    const file = event && event.item(0);
    this.onChange(file);
  }

  constructor( private host: ElementRef<HTMLInputElement> ) {
  }

  writeValue( value: null ) {
    // clear file input
    this.host.nativeElement.value = '';
  }

  registerOnChange( fn: Function ) {
    this.onChange = fn;
  }

  registerOnTouched( fn: Function ) {
  }

}

If desired, you can also integrate your current processing logic into this directive by modifying the following section:

let reader = new FileReader();
reader.readAsDataURL(file);
reader.onload = () => {
  this.onChange(reader.result as string);
}

Unable to assign a value to the HTMLInputElement's property: The input field can only be set to a filename or an empty string programmatically

Answer №1

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