How can I convert all my user inputs to the upper case letter using JavaScript?

I have a JavaScript function that checks the user input and will compare it against the database.

function checkUserInput() {
    var userInput = document.getElementById("textInput").value;
    var stringToCheckAgainst = random_images_array[num].split('.');

    if (userInput == stringToCheckAgainst[0]) {

        document.getElementById("row1").innerHTML = "<p>" + txt.fontsize(5) + "</p>";
        document.close();

    } else {
        //user has inputted an incorrect string
        document.getElementById("row1").innerHTML = "<p>" + txt1.fontsize(5) + "</p>";
        document.close();
    }
}

My problem is at the moment this function is case sensitive which means if the user types in “apple” it will return false statements as a fact that all of my objects are named with uppercase letters. How can I make my function not case sensitive to works with both lower case and uppercase letters no matter how the user is typing in?