form_array_nested_arrays.ts

class MyClass {

  constructor(private readonly fb: FormBuilder) { }

  /**
   * @desc makes an form array out of deeply nested array of objects
   */
  foo(arr = []) {
    return arr.map((val) => {
      const newVal = JSON.parse(JSON.stringify(val));
      Object.keys(newVal).forEach((key) => {
        const keyVal = newVal(key);
        newVal[key] = Array.isArray(keyVal) ? 
          this.fb.array(this.foo(keyVal)) : (typeof(keyVal) === 'object' ? this.fb.group(keyVal) : keyVal);  
      });
      return newVal;
    });
  }

}