ES6 Proxy - EnumBuilder

EnumBuilder.js

export default class {
    constructor() {
        this._obj;
    }
    object(obj) {
        this._obj = obj;
        return this;
    }
    keyValuePair(key, value) {
        if (!this._obj) {
            this._obj = {};
        }
        this._obj[key] = value;
        return this;
    }
    build() {
        if (!this._obj) {
            throw new ReferenceError(`Target object is not exist. _obj=${this._obj}`);
        }
        return new Proxy(this._obj, handler);
    }
}

const handler = {
    set (obj, prop, value) {
        throw TypeError('Enum is read only.');
    },
    get (obj, prop) {
        if (!(prop in obj)) {
            throw new ReferenceError(`Unknown enum key "${prop}"`);
        }
        return Reflect.get(obj, prop);
    },
    deleteProperty (obj, prop) {
        throw new TypeError('Enum is read only');
    }
}

Type.js

import EnumBuilder from './EnumBuilder';

const Type = {
  INSERT: 'insert',
  DELETE: 'delete',
  UPDATE: 'update'
}

export default new EnumBuilder().object(Type).build();