1
function format(num) {
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  let [int, fraction] = String(num).split(".");
3

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  int = int.split("");
5

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  let intStr = "";
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  // 反转数组，从逆序开始，才能算准确千位
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  int.reverse().forEach((str, idx) => {
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    if (idx !== 0 && idx % 3 === 0) {
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      // 要从前加字符串，生成的字符串才是正序的
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      intStr = str + "," + intStr;
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    } else {
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      // 要从前加字符串，生成的字符串才是正序的
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      intStr = str + intStr;
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    }
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  });
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  if (fraction.length) {
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    return intStr + "." + fraction;
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  }
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  return intStr;
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}