gistfile1.coffee

operations =
  '*': (a, b) -> a*b
  '+': (a, b) -> a+b

evaluate = (arr) ->

  calc = (operation, refArr) ->
    hasOperation = (index = arr.indexOf(operation)) isnt -1
    return refArr unless hasOperation
    pos = index - 1
    [a, op, b] = refArr.splice(pos, 3)
    result = operations[op](+a, +b)
    refArr.splice(pos, 0, ''+result)
    refArr

  calc('*', arr) while arr.indexOf('*') isnt -1
  calc('+', arr) while arr.indexOf('+') isnt -1

  if arr.length is 1
    results = +arr[0]
    return results unless isNaN(results)
  return undefined

a = ['10', '+', '20', '*', '3']
b = ['10', '+', '20', '*', '3', '*']
c = ['10', '20', '*', '3']

evaluate(a)
evaluate(b)
evaluate(c)