Google sheets Apps Script: Border rows in order to keep columns with the same value visually separate

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I want to use a script that will partition cells with equivalent column F values like the above example.

function borderByTask() {
  var ss = SpreadsheetApp.getActive();
  var sh = ss.getActiveSheet();
  var dataStartRow = 4;
  var lastRow = sh.getLastRow();
  var range_input = ss.getRange("F4:F").getValues();
  var data = range_input.filter(String); //eliminate empty cells
  //clear previous bordering
  sh.getRange('A4:S').setBorder(false, false, false, false /*right*/, false /*vertical*/, false /*horizontal*/, "black" /*color*/, SpreadsheetApp.BorderStyle.None);//Clear all borders
  Logger.log(data.length);

    for (var i = (dataStartRow - 1); i < data.length; i++) {
      if (data[i][0] != data[i+1][0]) {   // index 2 = 3rd column = C
      Logger.log(data[i][0]);
      //Border the bottom of the cells if task is different below
      //setBorder(top, left, bottom, right, vertical, horizontal, color, style)
      sh.getRange(i, 2, 1, 18).setBorder(false, false, true, false /*right*/, false /*vertical*/, true /*horizontal*/, "black" /*color*/, SpreadsheetApp.BorderStyle.SOLID_THIN);
      }
    }
}

**My problem is:

• TypeError: Cannot read properties of undefined (reading '0')
  borderByTask @ buttonFunctions.gs:85
• Borders do not get placed on appropriate rows**

Although I'm not sure whether I could correctly understand your expected result, is the following sample script your expected result?

Sample script:

function borderByTask() {
  var sheet = SpreadsheetApp.getActive().getActiveSheet();
  sheet.getRange('A4:S').setBorder(false, false, false, false /*right*/, false /*vertical*/, false /*horizontal*/, "black" /*color*/, SpreadsheetApp.BorderStyle.None);//Clear all borders
  var lastRow = sheet.getLastRow();
  var values = sheet.getRange("F4:F" + lastRow).getDisplayValues();
  var rows = [...values.reduce((m, [f], i) => m.set(f, i + 4), new Map()).values()];
  rows.forEach(e => {
    if (e == lastRow) return;
    sheet.getRange(`A${e}:S${e}`).setBorder(null, false, true, false /*right*/, false /*vertical*/, true /*horizontal*/, "black" /*color*/, SpreadsheetApp.BorderStyle.SOLID_THIN);
  });
}

Testing:

When this script is run, the following result is obtained.

References:

• reduce()

• Map