Pagination buttons

arrange-pagination.js

function calculateArrangement({ pageCount = 9, currentPageNumber = 1, buttonCount = 7 } = {}) {
  buttonCount = +buttonCount;

  if (!buttonCount || buttonCount < 7) { throw Error('Must have 7 or more') }
  if (currentPageNumber > pageCount || currentPageNumber < 1) { throw Error('current page must be between 1 and page count') }

  let requiredEndsButtonCount = 1;
  let requiredCenterButtonCount = 3;
  let requiredSpareCountForGoto = 1;
  let requiredButtonCount =
    requiredEndsButtonCount +
    requiredSpareCountForGoto +
    requiredCenterButtonCount +
    requiredSpareCountForGoto +
    requiredEndsButtonCount;

  // These are addictional buttons not counting the 7 required ones
  let extraButtonCount = buttonCount - requiredButtonCount;

  // For a possibly more pleasing arrangement I add an extra value here.
  //  • addin the +1 creates: [ 1  2  …  5  6  7  8  …  11 12 ]
  //  • removing it creates:  [ 1  …  4  5  6  7  8  9  …  12 ]
  let extraEndsNavCount = Math.floor((extraButtonCount + 1) / 4);
  let extraCenterNavCount = extraButtonCount - (extraEndsNavCount * 2);

  let endsNavButtonCount   = extraEndsNavCount   + requiredEndsButtonCount;
  let centerNavButtonCount = extraCenterNavCount + requiredCenterButtonCount;

  let showStartGotoButton = false;
  let showEndGotoButton = false;
  let incrementFirstCenterNumber = 0;
  if (pageCount > buttonCount + 1) {
    if (currentPageNumber > (endsNavButtonCount + 2 + 1)) {
      showStartGotoButton = true;
    }
    if (currentPageNumber < pageCount - (endsNavButtonCount + 2)) {
      showEndGotoButton = true;
    }
  }
  // This part is shitty…
  else if (pageCount === buttonCount + 1) {
    if (currentPageNumber > pageCount / 2) {
      showStartGotoButton = true;

      // compensate for having one less goto button
      incrementFirstCenterNumber = 1;
    }
    if (currentPageNumber < (pageCount + 1) / 2) {
      showEndGotoButton = true;
    }
  }

  let startNums = [];
  let endNums = [];
  let firstCenterNumber = 1;
  if (showStartGotoButton) {
    startNums = range(endsNavButtonCount);
  } else {
    centerNavButtonCount += endsNavButtonCount + 1;
  }
  if (showEndGotoButton) {
    endNums = range(endsNavButtonCount, { from: pageCount - endsNavButtonCount + 1 });
  } else {
    centerNavButtonCount += endsNavButtonCount + 1;
  }
  centerNavButtonCount = Math.min(pageCount, centerNavButtonCount);

  if (showStartGotoButton) {
    firstCenterNumber = currentPageNumber - Math.floor((centerNavButtonCount - 1) / 2);

    // compensate for removing the end goto button
    if (!showEndGotoButton) {
      incrementFirstCenterNumber += 1;
    }
  }
  firstCenterNumber += incrementFirstCenterNumber;
  firstCenterNumber = Math.min(firstCenterNumber, pageCount - centerNavButtonCount + 1);
  let centerNumbers = range(centerNavButtonCount, { from: firstCenterNumber });

  return {
    startNums,
    centerNumbers,
    endNums,
  }

  function range(count, { from = 1 } = {}) {
    return new Array(count).fill(null).map((_, i) => i + from);
  }
}