Testing a component that calls an external function with Jest and React Native Testing Library

Suppose I have a component that calls a function, for example:

// AComponent.js

import { Text, View } from "react-native";
import { getAString } from "./utils/function";

const AComponent = () => {
  return (
    <View>
      <Text>{getAString()}</Text>
    </View>
  );
};

export default AComponent;

and suppose getAString is in another file, and it's as simple as:

// function.js

export function getAString(): string {
  return "This comes from GetAString";
}

I want to test AComponent but with the function mocked:

var AComponent = require("../src/AComponent").default;
import { getAString } from "../src/utils/function";
import { render, screen, fireEvent } from "@testing-library/react-native";

describe("A Component's test", () => {
  it("mocking the function", async () => {
    render(<AComponent />);

    jest.spyOn(AComponent, "getAString").mockImplementation(() => {
      return "This is a mocked result";
    });

    const element = screen.getByText("This is a mocked result");
    expect(element).toBeTruthy();
  });
});

It fails with this message:

Cannot spy on the getAString property because it is not a function; undefined given instead

I guess that it expects that function to be a property of AComponent. But how could I do it then?