您可以使用Array#flatMap来构造最终数组。

function createArr(initial, count, fill) {
  const each = Math.floor(count / initial.length) - 1, rem = count % initial.length;
  return initial.flatMap((x, i) => [x, ...Array(each + (i < rem)).fill(fill)]);
}
console.log(createArr(['hello', 'there', 'world'], 15, null));
console.log(createArr(['hello', 'there', 'world'], 4, null));
console.log(createArr(['hello', 'there', 'world'], 3, null));

诀窍在于找出元素的位置，如果元素的数量不适合输出数组，则每个索引将被前一个元素向右推1，以平均分配开销：

function fillPadded(arr, count, fillWith){
  const filled = Array(count).fill(fillWith)
  const step = Math.floor(count/arr.length)
  const offs = count%arr.length
  for(let i = 0; i < arr.length; i++){
    const pos = i*step + (i <= offs ? i : offs) // or Math.min(i, offs)
    filled[pos] = arr[i]
  }
  return filled
}

console.log(JSON.stringify(fillPadded(['hello', 'there', 'world', 'foo'], 9, null)))
console.log(JSON.stringify(fillPadded(['hello', 'there', 'world', 'foo'], 10, null)))
console.log(JSON.stringify(fillPadded(['hello', 'there', 'world', 'foo'], 11, null)))
console.log(JSON.stringify(fillPadded(['hello', 'there', 'world'], 15, null)))

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