Get rid of "Object is possibly 'undefined'" error without using non-null assertion or optional chaining

I have the code like below

export type ConditionalItemType = [
  { condition: string },
  { [key: string]: FormItemDataType }
];
export type ConditionalItemDataType = ConditionalItemType[];

export type FormItemDataType = {
  required: boolean;
  type: string;
  constraint?: TextConstraintType;
  options?: string[];
  conditional?: ConditionalItemDataType;
};

export type TextConstraintType = {
  min?: number;
  max?: number;
  format?: string;
};

const obj: FormItemDataType = {
  required: true,
  type: 'select',
  options: ['simple', 'complicated'],
  conditional: [
    [
      { condition: 'complicated' },
      {
        'How Complicated': {
          required: true,
          type: 'text',
          constraint: {
            max: 30,
          },
        },
      },
    ],
  ],
};

const func = (input: FormItemDataType) => {
  if ('conditional' in input) {
    input.conditional.filter((arrItem) => console.log(arrItem));
  } else null;
};

I am getting 'input.conditional' is possibly 'undefined' error at input.conditional.filter((arrItem) => console.log(arrItem)).

How to get rid of this error? I cannot use non-null assertion or optional chaining . I cannot change the type either.

I thought I could do this by using "Type Guards" & I assume I am doing that at if ('conditional' in input) but apparently not. Can anyone help?

Update

>Solution :

Ditch the in operator and just check if the input.conditional is set, the classic way.

const func = (input: FormItemDataType) => {
  if(input.conditional) { // 👈 tells TypeScript that input.conditional under this block must be set
    input.conditional.filter((arrItem) => console.log(arrItem));
  }
};