Recursive FormBuilder which returns an AbstractControl

createFormObject.ts

import { AbstractControl, FormArray, FormControl, FormGroup } from '@angular/forms';

/**
 * Creates a nested FormGroup, FormArray, or FormControl with validators 
 * from a given object
 */
export const createFormObject = (initial: any): AbstractControl => {
  if (Array.isArray(initial)) {
    // detemine if array contains ValidatorFn or ValidatorFn[]
    // eg: use same syntax as formBuilder
    const isFn = obj => typeof obj === 'function';
    const hasValidators = initial.some(isFn) || 
      initial.filter(Array.isArray).some(isFn);

    if (hasValidators)
      return new FormControl(...initial);
    else return new FormArray(
      initial.map(val => this.createFormObject(val))
    );
  }

  // return a FormGroup for objects
  else if (typeof initial === 'object' && initial !== null)
    return new FormGroup(
      Object.keys(initial).reduce((acc, key) => ({
        ...acc,
        [key]: this.createFormObject(initial[key])
      }), {})
    );

  // otherwise, return a FormControl
  return new FormControl(initial);
}