Deep copy of Angular Reactive Form?

Question

I'm trying to build a function that will produce a copy of a given FormGroup. I started with:

function copyForm(form: FormGroup): FormGroup {
  const copy = new FormGroup({});
  for (let key of Object.keys(form.value)) {
    const control = form.controls[key];

    /* Copy the data from the control into a new control */
    const copyControl = new FormControl({[key]: control.value});

    copy.addControl(key, copyControl);
 }

But that doesn't work if there is a FormArray or FormGroup. This one might work if it were recursive, but I couldn't get a good handle on it.

I also attempted to solve it with

function copyForm(form: FormGroup): FormGroup {
  const copy = new FormGroup({});
  for (let key of Object.keys(form.value)) {
    const control = form.controls[key];
    const copyControl = new FormControl({...control.value});
    copy.addControl(key, copyControl);
  }
  return copy;

}

But that didn't work for double-nested FormGroups, any FormArrays or regular FormControls...

I also tried:

function copyForm(form: FormGroup): FormGroup {
  const copy = new FormGroup(Object.assign({}, form.value));
  return copy;
}

But that gives me the error:

ERROR TypeError: control.setParent is not a function

I'm stumped.

Answer 1

This is the deep copy function I came up with which also retains the associated validator / async validator functions and disabled status of each AbstractControl.

/**
 * Deep clones the given AbstractControl, preserving values, validators, async validators, and disabled status.
 * @param control AbstractControl
 * @returns AbstractControl
 */
export function cloneAbstractControl<T extends AbstractControl>(control: T): T {
  let newControl: T;

  if (control instanceof FormGroup) {
    const formGroup = new FormGroup({}, control.validator, control.asyncValidator);
    const controls = control.controls;

    Object.keys(controls).forEach(key => {
      formGroup.addControl(key, cloneAbstractControl(controls[key]));
    })

    newControl = formGroup as any;
  }
  else if (control instanceof FormArray) {
    const formArray = new FormArray([], control.validator, control.asyncValidator);

    control.controls.forEach(formControl => formArray.push(cloneAbstractControl(formControl)))

    newControl = formArray as any;
  }
  else if (control instanceof FormControl) {
    newControl = new FormControl(control.value, control.validator, control.asyncValidator) as any;
  }
  else {
    throw new Error('Error: unexpected control value');
  }

  if (control.disabled) newControl.disable({emitEvent: false});

  return newControl;
}

Answer 2

If you have simple FormGroups that contain only FormControls (i.e. not FormGroups or FormArrays), and you know their structure in advance, then this is a simple solution:

Note: by using initaliseFormGroup for initalising the original FormGroup, you will match the validators for each control.

initaliseFormGroup(){
    return new FormGroup({
        x : new FormControl('', Validators.required),
        y : new FormControl('', Validators.minLength(10))
    });
}

cloneFormGroup(oldForm: FormGroup){
    let newForm = this.initaliseFormGroup()
    newForm.patchValue(oldForm.value);
    return newForm;
}

If you have more complex forms (with child FormGroups), or want to clone them dynamically without knowing their structure in advance, then the other answers will better suited.