use javascriptcore in swift
import JavaScriptCore
let context = JSContext()
context.evaluateScript("var a = 5;")
context.evaluateScript("var b = function(elem){return elem + 2}")
let result = context.evaluateScript("b(a)")
let str = String(result)
try str.writeToFile("foo.txt", atomically: true, encoding: NSUTF8StringEncoding)