为什么 Javascript 在 Switch 语句中返回 "undefined"？

Question

我正在做一个 Javascript 练习来学习 Switch 语句，但我找不到错误在哪里（实际上我不明白为什么它返回“未定义”）。我试过：

var myNumber = document.querySelector(".myNumber").value;

但没用。我试过插入一个 case 0，但没用。

  
    var printNumber = document.getElementById("numresult").innerHTML = printNumber;
    var myNumber = document.querySelector(".myNumber").textContent;
    
    switch(myNumber) {
        case 1:
            printNumber = "You got 1";
            break;
         case 2:
            printNumber = "You got 2";
            break;           
        case 3:
            printNumber = "You got 3";
            break;
        case 4:
            printNumber = "You got 4";
            break;
        case 5:
            printNumber = "You got 5";
            break;
        case 6:
            printNumber = "You got 6";
            break;
        case 7:
            printNumber = "You got 7";
            break;
        case 8:
            printNumber = "You got 8";
            break;
        case 9:
            printNumber = "You got 9";
            break;
        default:
            printNumber = "Oops";
    }

<p class="myNumber">1</p>
<p id="numresult"></p>

Answer 1

您正在比较字符串和数字。

value returns 一个字符串和 switch 严格比较 ===.

最后，您需要为元素赋值。

document.getElementById("numresult").innerHTML = printNumber;

var printNumber,
    myNumber = +document.querySelector(".myNumber").textContent;
    //         ^ unary plus to convert string to number

switch (myNumber) {
    case 1:
        printNumber = "You got 1";
        break;
    case 2:
        printNumber = "You got 2";
        break;
    case 3:
        printNumber = "You got 3";
        break;
    case 4:
        printNumber = "You got 4";
        break;
    case 5:
        printNumber = "You got 5";
        break;
    case 6:
        printNumber = "You got 6";
        break;
    case 7:
        printNumber = "You got 7";
        break;
    case 8:
        printNumber = "You got 8";
        break;
    case 9:
        printNumber = "You got 9";
        break;
    default:
        printNumber = "Oops";
}

document.getElementById("numresult").innerHTML = printNumber;

<p class="myNumber">1</p>
<p id="numresult"></p>

为什么 Javascript 在 Switch 语句中返回 "undefined"？

Why is Javascript returning "undefined" in Switch statement?

javascript

switch-statement