输出一年的所有日期

Question

我想列出格式为mmddhh

的所有日期和小时

Q1：为什么我在输出之前得到“未定义”？我该如何解决？

var m, d, h, month, day, hour, output;
for (m = 1; m <= 12; m++) {
    month = addZero(m).toString();
    for (d = 1; d <= 31; d++) {
        day = addZero(d).toString();
        for (h = 1; h <= 24; h++) {
            hour = addZero(h).toString();
            output += month + day + hour + "<br>";
        }
    }
}
document.getElementById("result").innerHTML = output;

function addZero(z) {
    var z
    if (z < 10)
        return "0" + z;
    else
        return z;
}

<p id="result"></p>

Q2：我尝试使用d循环播放if (m = "01" || "03" || "05" || "07" || "08" || "10" || "12") 31次，否则循环播放30次。但是它失败了，我怎么能这样做？

    var m, d, h, month, day, hour, output;
    for (m = 1; m <= 12; m++) {
        month = addZero(m).toString();
        if (m = "01" || "03" || "05" || "07" || "08" || "10" || "12") {
            for (d = 1; d <= 31; d++) {
                day = addZero(d).toString();
                for (h = 1; h <= 24; h++) {
                    hour = addZero(h).toString();
                    output += month + day + hour + "<br>";
                }
            }
        } else {
            for (d = 1; d <= 30; d++) {
                day = addZero(d).toString();
                for (h = 1; h <= 24; h++) {
                    hour = addZero(h).toString();
                    output += month + day + hour + "<br>";
                }
            }
        }
    }
    document.getElementById("result").innerHTML = output;


    function addZero(z) {
        var z
        if (z < 10)
            return "0" + z;
        else
            return z;
    }

<p id="result"></p>

Answer 1

您已声明一个名为output的变量，但未使用值初始化，因此其值未定义。

然后，当你说output += month + day + hour + "<br>";它真的是output = undefined + month + day + hour + "<br>";时，你就会在undefined

的开头找到output

＆＃13;

＆＃13;

var m, d, h, month, day, hour, output = "";
for (m = 1; m <= 12; m++) {
  month = addZero(m).toString();
  for (d = 1; d <= 31; d++) {
    day = addZero(d).toString();
    for (h = 1; h <= 24; h++) {
      hour = addZero(h).toString();
      output += month + day + hour + "<br>";
    }
  }
}
document.getElementById("result").innerHTML = output;

function addZero(z) {
  var z
  if (z < 10)
    return "0" + z;
  else
    return z;
}

＆＃13;

<p id="result"></p>

＆＃13;

＆＃13;

＆＃13;