SheetToJSON

function doGet(request) {
  var output  = ContentService.createTextOutput(),
      data    = {},
      id      = request.parameters.id,
      sheet   = request.parameters.sheet,
      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),
      data = [];

  for (var r = 0, l = rows.length; r < l; r++) {
    var row     = rows[r],
        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];  
}