In NativeScript, the http module doesn't currently support making network requests to the local file system. That work is intended to be done by the file reader. It's pretty simple to read a local file and parse its contents as JSON.
This is the TypeScript/Promise version of what Emil Oberg created for the same question on StackOverflow. This module should be reusable for any and all asyncronous local JSON read operations.
import * as fs from 'file-system';
var documents = fs.knownFolders.currentApp();
class FileReader {