javascript - 我如何调整此功能以将两位数减少到一位数？

Question

我有一个函数，它从输入字段中获取数字，并对它们求和。但是，我希望等式继续将总和加在一起，直到结果是一个数字。我觉得我在这里的某处需要一个 if 语句，但我不确定如何编写它。我可以就如何修改此代码提出一些建议吗？

function getSum() {
    const input = document.getElementById('dateInput').value;
    var sum = 0;
    for (var i = 0; i < input.length; i++) {
        const num = parseInt(input.charAt(i));
        if (!isNaN(num)) {
            sum += num;
        }
    }
    document.getElementById("result").textContent = "Your life path number is: " + sum;
}

<div class="container">
    <div class="cell-1"><input type="date" id="dateInput"></div>
    <div class="cell-2"><h1>Blah Blah</h1></div>
    <div class="cell-3"><input type="date" id="dateInput"></div>
    <div class="cell-4" id="result"></div>
    <div class="cell-5"><button onclick="getSum()">Calculate</button></div>
    <div class="cell-6"></div>
    <div class="cell-7"></div>
    <div class="cell-8"></div>
    <div class="cell-9"></div>
</div>

Answer 1

一种选择是使用 while 循环:当计算的数字为 10 或以上时，继续迭代并计算一个新数字:

const sumNumber = num => [...String(num)].reduce(
  (a, digit) => a + Number(digit),
  0
);

function getSum(input) {
  const inputNum = Number(input);
  if (Number.isNaN(inputNum)) {
    return;
  }
  let currentSum = inputNum;
  while (currentSum >= 10) {
    currentSum = sumNumber(currentSum);
  }
  console.log(currentSum);
}
getSum(15);
getSum(1234);
getSum(999999);

如果你不喜欢while循环，你也可以使用递归:

const sumNumber = num => {
  const currentSum = [...String(num)].reduce(
    (a, digit) => a + Number(digit),
    0
  );
  return currentSum < 10 ? currentSum : sumNumber(currentSum);
}

function getSum(input) {
  const inputNum = Number(input);
  if (Number.isNaN(inputNum)) {
    return;
  }
  let currentSum = inputNum;
  console.log(sumNumber(inputNum));
}
getSum(15);
getSum(1234);
getSum(999999);

总而言之，我真的更喜欢 reduce 方法，因为它不涉及重新分配外部变量，但您可以用对 charAt< 的更多手动迭代来替换它 输入字符串的代码，如果您对此更满意的话，但它会使函数变得更长且不那么抽象:

const sumNumber = num => {
  const numStr = String(num);
  let sum = 0;
  for (var i = 0; i < numStr.length; i++) {
    const num = parseInt(numStr.charAt(i));
    if (!isNaN(num)) {
      sum += num;
    }
  }
  return sum;
};


function getSum(input) {
  const inputNum = Number(input);
  if (Number.isNaN(inputNum)) {
    return;
  }
  let currentSum = inputNum;
  while (currentSum >= 10) {

    currentSum = sumNumber(currentSum);
  }
  console.log(currentSum);
}
getSum(15);
getSum(1234);
getSum(999999);