SheetAsJSON - expose a Google spreadsheet (with a header row) as JSON - See https://qmacro.org/2013/10/04/sheetasjson-google-spreadsheet-data-as-json/ for a writeup

gistfile1.js

function doGet(request) {
  var output = ContentService.createTextOutput();
  var data = {};
  var id = request.parameters.id;
  var sheet = request.parameters.sheet;
  var ss = SpreadsheetApp.openById(id);
  data["records"] = readData_(ss, sheet);
  var callback = request.parameters.callback;
  if (callback == undefined) {
    output.setContent(JSON.stringify(data));
  }
  else {
    output.setContent(callback + "(" + JSON.stringify(data) + ")");
  }
  output.setMimeType(ContentService.MimeType.JSON);
  return output;
}


function readData_(ss, sheetname, properties) {

  if (typeof properties == "undefined") {
    properties = getHeaderRow_(ss, sheetname);
    properties = properties.map(function(p) { return p.replace(/\s+/g, '_'); });
  }
  
  var rows = getDataRows_(ss, sheetname);
  var data = [];
  for (var r = 0, l = rows.length; r < l; r++) {
    var row = rows[r];
    var record = {};
    for (var p in properties) {
      record[properties[p]] = row[p];
    }
    data.push(record);
  }
  return data;
}



function getDataRows_(ss, sheetname) {

  var sh = ss.getSheetByName(sheetname);
  return sh.getRange(2, 1, sh.getLastRow() - 1, sh.getLastColumn()).getValues();

}


function getHeaderRow_(ss, sheetname) {

  var sh = ss.getSheetByName(sheetname);
  return sh.getRange(1, 1, 1, sh.getLastColumn()).getValues()[0];

}