Set private object fields via JS proxy

set-private-field-via-proxy.js

// https://betterprogramming.pub/everything-you-should-know-about-javascript-proxy-67576f2e069e
function setPrivateField(obj, prefix = "_"){
  return new Proxy(obj, {
    has: (obj, prop) => {
      if(typeof prop === "string" && prop.startsWith(prefix)){
        return false
      }
      return prop in obj
    },
    ownKeys: obj => {
      return Reflect.ownKeys(obj).filter(
        prop => typeof prop !== "string" || !prop.startsWith(prefix)
      )
    },
    get: (obj, prop) => {
      if(typeof prop === "string" && prop.startsWith(prefix)){
        return undefined
      }
      return obj[prop]
    }
  });
}