Jest mock testing with input

How can I write a jest test for this function?

const input = require('prompt-sync')();

export function choices(): void {
    const choice = input("Choose a letter");
    if (choice === "a") {
        console.log("Airplane");
    } if (choice === "b") {
        console.log("Balloon");
    } else {
        console.log("Neither");
    }

}

I know that I should use mocking. But I do not know how to do it when the input is being stored inside of the function, and not as a parameter. I do not want to change this.

You can use jest.mock to mock your prompt-sync module. And since that module is returning a function that returns your input, Jest.mock would have something like:

const mockInput = jest.fn();
jest.mock("prompt-sync", () => () => mockInput);

Which should be included before the import in your test file.

Full example:

const mockInput = jest.fn();
jest.mock("prompt-sync", () => () => mockInput);

import { choices } from "../path/to/file";

describe("test", () => {
  let consoleSpy: jest.SpyInstance;

  beforeEach(() => {
    consoleSpy = jest.spyOn(console, "log");
  });

  afterEach(() => {
    jest.resetAllMocks();
  });

  it("should call the input", () => {
    choices();

    expect(mockInput).toHaveBeenCalledWith("Choose a letter");
  });

  it("should call A", () => {
    mockInput.mockReturnValue("a");

    choices();

    expect(consoleSpy).toHaveBeenCalledWith("Airplane");
  });
});

However I advice refactoring your code to make it easily testable