1、proxy

let dictionary = {
  'Hello': 'Hola',
  'Bye': 'Adiós'
};

dictionary = new Proxy(dictionary, {
  get(target, phrase) { // intercept reading a property from dictionary
    if (phrase in target) { // if we have it in the dictionary
      return target[phrase]; // return the translation
    } else {
      // otherwise, return the non-translated phrase
      return phrase;
    }
  }
});

// Look up arbitrary phrases in the dictionary!
// At worst, they're not translated.
alert( dictionary['Hello'] ); // Hola
alert( dictionary['Welcome to Proxy']); // Welcome to Proxy (no translation)

2、Reflect.get

const negativeArray = els =>
    new Proxy(els, {
        get: (target, propKey, receiver) =>
            Reflect.get(
                target,
                +propKey < 0 ? String(target.length + +propKey) : propKey,
                receiver
            )
    });
debugger
const unicorn = negativeArray(["京", "程", "一", "灯"]);
debugger
unicorn[-1]; 

const negativeArray = els =>
    new Proxy(els, {
        get: (target, propKey, receiver) =>{
          if(+propKey < 0 ){
            return target[String(target.length + +propKey)]
          }else{
            return target[propKey]
          }
        }
    });
debugger
const unicorn = negativeArray(["京", "程", "一", "灯"]);
debugger
unicorn[-1];
unicorn[3];

2、get + set +deleteProperty

let user = {
  name: "John",
  _password: "***"
};

user = new Proxy(user, {
  get(target, prop) {
    if (prop.startsWith('_')) {
      throw new Error("Access denied");
    }
    let value = target[prop];
    return (typeof value === 'function') ? value.bind(target) : value; // (*)
  },
  set(target, prop, val) { // to intercept property writing
    if (prop.startsWith('_')) {
      throw new Error("Access denied");
    } else {
      target[prop] = val;
      return true;
    }
  },
  deleteProperty(target, prop) { // to intercept property deletion
    if (prop.startsWith('_')) {
      throw new Error("Access denied");
    } else {
      delete target[prop];
      return true;
    }
  },
  ownKeys(target) { // to intercept property list
    return Object.keys(target).filter(key => !key.startsWith('_'));
  }
});

debugger
// "get" doesn't allow to read _password
try {
  alert(user._password); // Error: Access denied
} catch(e) { alert(e.message); }

// "set" doesn't allow to write _password
try {
  user._password = "test"; // Error: Access denied
} catch(e) { alert(e.message); }

// "deleteProperty" doesn't allow to delete _password
try {
  delete user._password; // Error: Access denied
} catch(e) { alert(e.message); }

// "ownKeys" filters out _password
for(let key in user) alert(key); // name

javascript.info/proxy