JS-Data - Proxy context issue demo (https://stackoverflow.com/posts/53197772)

ExtendedDataStore.js

import { DataStore, Mapper } from 'js-data'

class ExtendedDataStore extends DataStore {
  as(name) {
    const props = {}
    // const original = super.as(name)
    const mapper = this.getMapper(name)
    const self = this
    
    // if "this" is passed instead of "self" the downstream methods fail without straightforward Error stacktrace
    // but when using "self" as the 1st arguement, things work normal, as expected
    const proxy = new Proxy(self, {
      get: function(target, prop, receiver) {
        // first try the prop on the DataStore
        if (target[prop]) return (...args) => target[prop].apply(self, [name, ...args])
        // then try the mapper because we have custom props on mapper classes
        return mapper[prop]
      },
    })

    return proxy
  } 
}

const Store = new ExtendedDataStore()

Store.defineMapper('user')

Store.User = Store.as('user') // make a short-hand for User so we don't have to do `Store.getMapper('user').updateEmail(...)`