JavaScript function to format various value types as a string.

formatString.js

function testNumber() {
  var value = 1234.56789
  var a = format(value, {numberOfDecimalPlaces: 2})
  debugger
}

function testDate() {
  var value = new Date()
  var a = format(value)
  debugger
}

function testObject() {
  var value = {a: 1, b: 2}
  var a = format(value)
  debugger
}

/**
 * Format an object into a suitable value
 * 
 * @param {object} value
 * @param {object} options
 *   {number} numberOfDecimalPlaces [OPTIONAL, DEFAULT: 0]
 *   {string} dateFormat [OPTIONAL, DEFAULT: 'yyyy-MM-dd']
 *
 * @return {string} formattedValue
 */

function formatString(value, options) {

  var formattedValue = value
  options = options || {}

  if (typeof value === 'number') {
  
    var placeholder = '%s'
    var numberOfDecimalPlaces = 0
    
    if (options.numberOfDecimalPlaces !== undefined) {
      numberOfDecimalPlaces = options.numberOfDecimalPlaces 
    }
    
    if (numberOfDecimalPlaces > 0) {
      placeholder = '%.' + numberOfDecimalPlaces + 'f'
    }

    var multiplier = Math.pow(10, numberOfDecimalPlaces)

    formattedValue = Utilities.formatString(placeholder, Math.round(value * multiplier) / multiplier)
    
  } else if (value instanceof Date) {
  
    var timeZone = Session.getScriptTimeZone()
    var dateFormat = 'yyyy-MM-dd'
    
    if (options.dateFormat !== undefined) {
      dateFormat = options.dateFormat 
    }
    
    formattedValue = Utilities.formatDate(value, timeZone, dateFormat)
    
  } else if (typeof value === 'object') {
  
    formattedValue = JSON.stringify(value)
  }
  
  return formattedValue
}