JavaScript range array and object with calculated keys

.js

const upperRangeBoundry = 5;

const rangeSizedArray = [...Array(upperRangeBoundry)]
// -> (5) [undefined, undefined, undefined, undefined, undefined]

const rangeValuedArray = [...Array(upperRangeBoundry).keys()]
// -> (5) [0,1,2,3,4]

[...Array(8)].map((_, index, range) => 100 * index/range.length);
// -> (8) [0, 12.5, 25, 37.5, 50, 62.5, 75, 87.5]

[...Array(8+1)].map((_, index, range) => 100 * index/(range.length-1));
// -> (9) [0, 12.5, 25, 37.5, 50, 62.5, 75, 87.5, 100]

[...Array(8+1).keys()].reduce((obj, _, index, range) => ({
  ...obj,
  [100*index/(range.length-1)]: {}
}), {});
// -> { 0: {}, 12.5: {}, 25: {}, 37.5: {}, 50: {}, 62.5: {}, 75: {}, 87.5: {}, 100: {} }

[...Array(8+1).keys()].reduce((obj, _, index, range) => ({
  ...obj,
  [`${100*index/(range.length-1)}%`]: {}
}), {});
// -> { 0%: {}, 12.5%: {}, 25%: {}, 37.5%: {}, 50%: {}, 62.5%: {}, 75%: {}, 87.5%: {}, 100%: {} }