I have a very simple js-program in a file called Task2.js, where one function processInput(date) is used to process the user input. I have written some basic tests for it, but most of them failed with unexpected messages. I wrote all those tests in the file Task2.test.js. When I move the tests that fail from Task2.test.js to Task2.js and run jest again, all of them pass. I have no idea why they work in one file, but not the other.

Task2.js:

function processInput(date){
    if(date.trim() == "quit"){
        rl.close();
        throw new Error("User requested quit");
    }

    // Trim trailing spaces and then split into three variables
    try{
        // Regex that matches any year starting with 1-9, any month between 1-12, and any day between 1-31
        if(!(/^\s*[1-9]\d*\s+([1-9]|1[0-2])\s+([1-9]|[1-2][0-9]|3[01])\s*$/.test(date))){
            console.log("Invalid input, please try again");
            return false;
        };
        [y, m, d] = date.trim().split(/\s+/);
    } catch(error){
        console.log("Invalid input, please try again");
        return false;
    }
    return [y, m, d];
}

Some tests in Task2.test.js:

const processInput = require("./Task2");

test("TestQuit", () => {
    expect(() => processInput("quit")).toThrow();
});

test("TestInvalidInput", () => {
    expect(processInput(" 2021   13 13  ")).toBeFalsy();
});

Here's what those tests return when run in Task2.test.js:

 TestQuit

    expect(received).toThrow()
    Received function did not throw

 TestInvalidInput

    expect(received).toBeFalsy()
    Received: " 2021   13 13   undefined NaN"