How to set a relative path in javascript?

Is there a way to set a relative path name in Javascript as illustrated below?

var desired = {
“appium-version”: “1.0”,
platformName: “iOS”,
platformVersion: “8.2”,
deviceName: “iPhone 6”,
app: “{relative to here} Build/Mobile Out of the OR/Build/Products/Debug-iphonesimulator/MyApp.app”,
};