由于roll是一个具有均值的数组数组，因此您可以执行嵌套的forEach循环来查找正确的数据对象，tio您将传递均值：

roll.forEach(yearArray => {
  yearArray[1].forEach(description => {
    const foundObj = data.filter(e => e.year === yearArray[0] && e.ndc_description === description[0]);
    foundObj.forEach(obj => obj.mean = description[1])
  })
});

下面是一个演示，我更改了一些值，以便您可以看到正确的方法：

const data = [{
    ndc_description: "VIAGRA 50 MG TABLET",
    new_percent_change: 10.9,
    year: "2017"
  },
  {
    ndc_description: "VIAGRA 50 MG TABLET",
    new_percent_change: 12.3,
    year: "2019"
  },
  {
    ndc_description: "VIAGRA 50 MG TABLET",
    new_percent_change: 12.9,
    year: "2017"
  },
  {
    ndc_description: "JANUVIA 100 MG TABLET",
    new_percent_change: 8.41,
    year: "2017"
  },
  {
    ndc_description: "JANUVIA 100 MG TABLET",
    new_percent_change: 5.41,
    year: "2019"
  },
  {
    ndc_description: "JANUVIA 100 MG TABLET",
    new_percent_change: 4.41,
    year: "2017"
  }
];

const roll = d3.rollups(data, v => d3.mean(v, d => d.new_percent_change), d => d.year, d => d.ndc_description);

roll.forEach(yearArray => {
  yearArray[1].forEach(description => {
    const foundObj = data.filter(e => e.year === yearArray[0] && e.ndc_description === description[0]);
    foundObj.forEach(obj => obj.mean = description[1])
  })
});

console.log(data)

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