A column filter based on a cell's value in Google Apps Scripting

filter.js

/** 
 * Basically, we only want this to run when you edit something in this spreadsheet.
 *
 * The "onEdit" function is a reserved function that is triggered whenever you edit
 * anything in the sheet.
 * 
 * Save this, then make sure you've selected "onEdit" as the function to run. Whenever
 * you edit the filterCell value, it should automatically run this function without
 * you having this window open.
 *
 * @param {event} Google Apps Scripting event object
 */
function onEdit(e) {
  // Documentation for "e": https://developers.google.com/apps-script/guides/triggers/events
  // Basically we only care if the cell edited was the "filter" cell
  
  var filterCell = "L4";
  var changedCell = e.range.getA1Notation();
  var changedSheet = e.source.getActiveSheet();
  
  // Now, we check to make sure the cell that was edited was the cell in question (filterCell)
  if (changedCell == filterCell) {
    // It is, so we get the value and pass it onto filterSubjects() so we don't have to look it up again later
    // This is good engineering: filterSubjects() has all the context it needs to operate. Don't need to 
    // look anything up
    var filterCellValue = e.range.getValue();
    return filterSubjects(changedSheet, filterCellValue);
  } else {
    return;
  }
}


/**
 * This is the main function that runs, given a sheet, what to hide and what to show.
 * You'll want to edit the cases here and duplicate as neccessary.
 *
 * @param {Sheet} A Google Spreadsheets Sheet object
 * @param {String} The value of the filtered cell to act upon
 */
function filterSubjects(sheet, filterCellValue) {
  
  // First we hide all the subject columns.
  // Assuming all sheets are laid out the same...if not you'd need additional logic here
  sheet.hideColumns(13,11);
  
  // Next, to make this work no matter how someone puts in the filter, just make the
  // comparison case-insensitive by making everything lowercase.
  filterCellValue = filterCellValue.toLowerCase();
  
  // Here is your switch/case statement. Basically it tries to match exactly each case.
  // If it does not, it goes to the default, which is to show everything.
  switch (filterCellValue) {
    case "english":
      sheet.showColumns(13);
      break;
    case "maths":
      sheet.showColumns(14);
      break;
    default:
      // Just in case something invalid was entered, show everything
      sheet.showColumns(13,11);
      break;
  }
  
  // We done!
  return;
}