使用Set。set是唯一项的集合：

const items = [
  {
    "id": 1,
    "brand":"Mike",
    "cycle": "C1",
    "sides": {
      "1": {"sideName":"A",
          "active": false},
      "2": {"sideName":"B",
          "active": false},
      "3": {"sideName":"C",
          "active": true}
    }
  },
  {
    "id": 2,
    "brand":"Luis",
    "cycle": "C2",
    "sides": {
      "1": {"sideName":"A",
          "active": false},
      "2": {"sideName":"B",
          "active": false},
      "3": {"sideName":"C",
          "active": true},
      "4": {"sideName":"D",
          "active": true}
    }
  },
  {
    "id": 3,
    "brand":"Lucas",
    "cycle": "Z1",
    "sides": {
      "1": {"sideName":"H",
          "active": false},
      "2": {"sideName":"U",
          "active": false},
      "3": {"sideName":"L",
          "active": true},
      "4": {"sideName":"A",
          "active": true}
    }
  },
  {
    "id": 4,
    "brand":"Edd",
    "cycle": "R1",
    "sides": {
      "1": {"sideName":"A",
          "active": false},
      "2": {"sideName":"B",
          "active": false},
      "3": {"sideName":"C",
          "active": true},
      "4": {"sideName":"Q",
          "active": true}
    }
  }
]

const uniqueSideNames = new Set();

for (const item of items) {
  for (const side of Object.values(item.sides)) {
    uniqueSideNames.add(side.sideName);
  }
}

console.log([...uniqueSideNames]);

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一种选择是首先将列表缩减为您需要的项目，然后将它们flatMap为一个扁平数组。
items.flatMap(item => Object.values(item.sides).map(side => side.sideName))的
这将给你一个给予 allsideNames的扁平列表，包括重复的。
Set数据结构是一种只允许唯一项的数据结构，因此将所有这些项添加到一个集合中，然后将它们转换回数组将为您进行重复数据删除。
这可能看起来像：

Array.from(new Set(items.flatMap(item => 
  Object.values(item.sides)
        .map(side => side.sideName)))
);

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const arr = [{
    "id": 1,
    "brand": "Mike",
    "cycle": "C1",
    "sides": {
      "1": {
        "sideName": "A",
        "active": false
      },
      "2": {
        "sideName": "B",
        "active": false
      },
      "3": {
        "sideName": "C",
        "active": true
      }
    }
  },
  {
    "id": 2,
    "brand": "Luis",
    "cycle": "C2",
    "sides": {
      "1": {
        "sideName": "A",
        "active": false
      },
      "2": {
        "sideName": "B",
        "active": false
      },
      "3": {
        "sideName": "C",
        "active": true
      },
      "4": {
        "sideName": "D",
        "active": true
      }
    }
  },
  {
    "id": 3,
    "brand": "Lucas",
    "cycle": "Z1",
    "sides": {
      "1": {
        "sideName": "H",
        "active": false
      },
      "2": {
        "sideName": "U",
        "active": false
      },
      "3": {
        "sideName": "L",
        "active": true
      },
      "4": {
        "sideName": "A",
        "active": true
      }
    }
  },
  {
    "id": 4,
    "brand": "Edd",
    "cycle": "R1",
    "sides": {
      "1": {
        "sideName": "A",
        "active": false
      },
      "2": {
        "sideName": "B",
        "active": false
      },
      "3": {
        "sideName": "C",
        "active": true
      },
      "4": {
        "sideName": "Q",
        "active": true
      }
    }
  }
]

const result = []

arr.forEach(({
  sides
}) => {
  for (const key in sides) {
    if (!result.includes(sides[key].sideName)) result.push(sides[key].sideName)
  }
})

console.log(result)

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将对象区分为它们的sideName后，可以使用new Set创建一个新数组。它设置的目的是创建没有重复值的数组。

[...new Set(items.flatMap(item=> Object.values(item.sides)).map(eachSide=> eachSide.sideName))]

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输出：

['A', 'B', 'C', 'D', 'H', 'U', 'L', 'Q']

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