Use this script to build a javascript object array from a Google Spreadsheet.

GAS-spreadsheet-object-builder.js

//display custom menu
function onOpen() {
  var ui = SpreadsheetApp.getUi();
  ui.createMenu('Export to JSON')
    .addItem('Get Sheet Header', 'getSheetHeader')
    .addItem('Get Sheet Data', 'getSheetData')
    .addItem('Download Spreadsheet as JSON', 'doGet')
    .addToUi();
}

//Declare gloabl variables
var sheet = SpreadsheetApp.getActiveSheet();
var allSheetData = sheet.getDataRange().getValues();  //get all spreadsheet data
var filename = SpreadsheetApp.getActive().getName();
var headerRow = allSheetData[0];

//Get the Header Row of current Sheet
function getSheetHeader() {
  return sheetHeader; //return the header row of the spreadsheet
}

//Get data of current sheet
function getSheetData(){
     
  var data = [];  //create an empty array to hold spreadsheet data
  
  //loop through each row in the spreadsheet
  for (var currentRow = 1; currentRow < allSheetData.length; currentRow++) {  
    
    //Create an object to hold the data from the current row
    var obj   = {};
   
    //loop through each column in the current row of the spreadshset
    for(var rowColumn=0;rowColumn<headerRow.length;rowColumn++){
      obj[headerRow[rowColumn]]=allSheetData[currentRow][rowColumn];
    }
    
    data.push(obj);
    
  }
 
  return data;
}


function doGet() {
  return ContentService.createTextOutput(JSON.stringify(getSheetData())).setMimeType(ContentService.MimeType.JSON);}