你可以用很多方法来实现你想要的：

for...of方式

const data = [ { "level": "level3", "category": "car" }, { "level": "level1", "category": "bike" }, { "level": "level2", "category": "car" }, { "level": "level5", "category": "bike" } ];
const newObj = {};
for (const item of data) {
  const {level,category} = item;
  if (newObj[category]) {
    newObj[category].push(level);
  } else {
    newObj[category] = [level];
  }
}
console.log(newObj);

reduce方式

const data = [ { "level": "level3", "category": "car" }, { "level": "level1", "category": "bike" }, { "level": "level2", "category": "car" }, { "level": "level5", "category": "bike" } ];
const newObj = data.reduce((acc, item) => {
    const { level, category } = item;
    acc[category] = acc[category] || [];
    acc[category].push(level);
    return acc;
}, {});
console.log(newObj);

第一种方法简单易懂，第二种方法更实用。但是这两种方法都有O（n）的线性时间复杂度，选择其中一种只是个人偏好和编码风格的问题。

通过使用函数Array.prototype.reduce

const array = [{        "level": "level3",        "category": "car"    },    { "level": "level1",        "category": "bike"    },    {        "level": "level2",        "category": "car"    },    {      "level": "level5",        "category": "bike"    }],
      result = array.reduce((a, {level, category}) => {
        (a[category] = a[category] || []).push(level);
        return a;
      }, {});

console.log(result);

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