How to use native spreadsheet functions in google apps script?

Google Apps Script is JavaScript, spreadsheets functions are not available and using a workaround like setFormula followed by getValue is not only cumbersome but really slow and inefficient. You will be better inspired to use JavaScript and Google services to manipulate data taken from a spreadsheet and write the data back in one single setValues().

https://developers.google.com/apps-script/reference/spreadsheet/range#setFormula(String)

var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getSheets()[0];

var cell = sheet.getRange("B5");
cell.setFormula("=SUM(B3:B4)");