问如何在Javascript中动态更新背景图像大小

EN

body {
    background: url("/IMG/moebius.jpg");
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}


let changeBackground = () => {
    document.body.style.backgroundSize = "cover";
    return document.body.style.backgroundSize;
}


defInputField.addEventListener("keyup", () => {

    // document.body.style.backgroundSize = "cover";


    setTimeout(userScore, scoreInterval);
    if (splitInputWords.length < 2) {
        correct.style.display = "none";
        incorrect.style.display = "none";
        displayScore.style.display = "none";
    };

    if (totalScore > 5) {
        document.body.style.background = "url('/IMG/moebius3.jpg')";

    };

    if (totalScore > 10) {
        document.body.style.background = "url('/IMG/moebius.jpg')";

    };

    if (userScore() >= 3 || userScore() >= splitDisplayedDef().length) {
        correct.style.display = "block";
        incorrect.style.display = "none"
        displayScore.textContent = userScore();
        displayScore.style.display = "block";


    } else {
        incorrect.style.display = "none";
        correct.style.display = "none";
        displayScore.display ="none";
    }; 

});

defInputField.addEventListener("keydown", (e) => {



    if (e.keyCode === 13 && userScore() >= 1) {
            totalScore += userScore();
            totalScore--;
            currentLevel++;
            newDefButton.textContent = "Definition"
            let newWordObject = newWordObjectGenerator();
            displayNewDef.style.display = "none";
            correct.style.display = "none";
            incorrect.style.display = "none";
            displayScore.style.display = "none";    
            defInputField.value = "";
            displayNewWord.textContent = newWordObject.word;
            displayNewDef.textContent = newWordObject.definition;
            scoreTracker.textContent = "Total Score = " + " " + totalScore;
            levelTracker.textContent = "Level = " + " " + currentLevel;
            console.log(currentLevel);
            console.log(totalScore);

}});