如何在使用d3.group分组后获取键名数组？

已通过d3.group按sport字段对运动员数据完成分组：

athletes = [
  {name: "Floyd Mayweather", sport: "Boxing", nation: "United States", earnings: 285},
  {name: "Lionel Messi", sport: "Soccer", nation: "Argentina", earnings: 111},
  {name: "Cristiano Ronaldo", sport: "Soccer", nation: "Portugal", earnings: 108},
  {name: "Conor McGregor", sport: "MMA", nation: "Ireland", earnings: 99},
  {name: "Neymar", sport: "Soccer", nation: "Brazil", earnings: 90},
  {name: "LeBron James", sport: "Basketball", nation: "United States",  earnings: 85.5},
  {name: "Roger Federer", sport: "Tennis", nation: "Switzerland", earnings: 77.2},
  {name: "Stephen Curry", sport: "Basketball", nation: "United States", earnings: 76.9},
  {name: "Matt Ryan", sport: "Football", nation: "United States", earnings: 67.3},
  {name: "Matthew Stafford", sport: "Football", nation: "United States", earnings: 59.5}
]

athletesBySport = d3.group(athletes, d => d.sport)

需要提取所有分组键（运动项目）组成如下格式的数组（顺序无关）：

['Boxing', 'Soccer', 'MMA', 'Basketball', 'Tennis', 'Football']

方法1：利用Map的keys()方法 + Array.from() #

d3.group返回的是标准的Map对象，Map自带keys()方法可获取所有键的迭代器，通过Array.from()将其转换为数组：

const sportsArray = Array.from(athletesBySport.keys());

方法2：利用扩展运算符... #

扩展运算符可直接展开可迭代对象（如Map的键迭代器），快速生成数组：

const sportsArray = [...athletesBySport.keys()];

方法3：直接从原数组提取去重（无需依赖分组结果） #

如果不需要保留分组后的Map，也可以直接从原始athletes数组中提取并去重sport字段：

const sportsArray = [...new Set(athletes.map(d => d.sport))];

内容的提问来源于stack exchange，提问作者jtr13