How to group by multiple keys at the same time using D3?

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This works, but I was wondering if there was a better way than creating a string with a and b and later splitting it:

const data = [
    { a: 10, b: 20, c: 30, d: 40 },
    { a: 10, b: 20, c: 31, d: 41 },
    { a: 12, b: 22, c: 32, d: 42 }
];

d3.rollups(
    data,
    x => ({
      c: x.map(d => d.c),
      d: x.map(d => d.d)
    }),
    d => `${d.a} ${d.b}`
  )
  .map(([key, values]) => {
    const [a, b] = key.split(' ');
    return {a, b, ...values};
  });

// OUTPUT
// [
//   {a: "10", b: "20", c: [30, 31], d: [40, 41]},
//   {a: "12", b: "22", c: [32], d: [42]}
// ]

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