gistfile1.js

function evaluate(ast){ 
  return ast.reduce(function(result, a){ return compute(a, result) }, []) 
}

function parse(input){ 
  return input.trim().split(/\s+/) 
}

function compute(a, stack) {
  return a === '+'?  concat(stack, pop(stack) + pop(stack))
  :      a === '*'?  concat(stack, pop(stack) * pop(stack))
  :      a === '-'?  concat(stack, pop(stack) - pop(stack))
  :      a === '/'?  concat(stack, pop(stack) / pop(stack))
  :                  stack.concat([a])
}

function concat(stack, value){
  return stack.concat(value)
}

function pop(stack) {
  if (stack.length === 0)  throw new Error("Not enough parameters")
  return Number(stack.pop())
}