Treat a Google Sheet like a JSON API

gas_sheet_as_json.js

/**
 * create a Google Sheet then go to Tools > Script Editor
 * Paste this code into the editor. Save. 
 * Publish > Deploy as Web App
 * Set new version, publish as me, who has access - anyone, even anon.
 * GET to the URL, add on end ?sheet=[sheet name]
 * Sheet name is the sheet name, manage appropriately (no spaces or symbols to keep it simple)
 * Request returns JSON representation of the sheet.
 */

/**
 * doGet is an installed trigger that responds to a GET request 
 * to the deployed app URL (Publish -> Deploy as web app -> execute as 
 * Me and who has access is Everyone, even anon). 
 * We are going to use the ?sheet=[sheet name] query params on the end of
 * the script URL to tell the script which sheet to return (it only returns 
 * one sheet per request, not all of them at once).
 * Response is JSON structured as an array of items under the "items"
 * key.
 */
function doGet(e) {
  return ContentService
    .createTextOutput(JSON.stringify(getData(e.parameter.sheet)))
    .setMimeType(ContentService.MimeType.JSON); 
}

/**
 * This function takes the requested sheet and gets the values 
 * in the sheet as an array of arrays with the first row being 
 * the header. The entire array is then sent to a helper which 
 * will convert the array to an appropriate object.
 */
function getData(sheet) {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getSheetByName(sheet);
  var range = (sheet) ? sheet.getDataRange() : null;
  var values = (sheet) ? range.getValues() : null;

  return (sheet) ? objectify(values) : null;
}

/**
 * Our sheet data is retrieved as an array of arrays, but 
 * what we really want to send back to the client is JSON. 
 * This helper takes the array and converts it to an object 
 * by using arr[0] as the keys and each subsequent arr item 
 * as the values. We'll deal with converting to JSON when 
 * we actually send the object as a GET response later.
 * Note that this func will err if the arr sent has a length 
 * of less than 2 - make sure to do error checking on your 
 * arr before you call this func.
 */
function objectify(arr) {
  if (arr.length > 1) {
    var header = arr.shift();
    var arrItems = [];
    arr.forEach(function(item, itemIndex) {
      var  obj = {};
      obj['index'] = itemIndex;
      item.forEach(function(i, index) {
        obj[header[index]] = i;
      });
      arrItems.push(obj); 
    });
    return {items: arrItems};
  } else {
    return null;
  }
  
}