Javascript native config object using proxy and eventEmitter

proxyEmitter.js

import { EventEmitter } from 'events';

let listener = new EventEmitter();
let config = new Proxy(filteredEnvVars(), { get, set });

export default Object.assign(config, { listener });

function filteredEnvVars () {
    return Object.keys(process.env)
        .filter(removeNPMVars)
        .reduce(assignToObject, {});
}

function removeNPMVars (key) {
    return !key.includes('npm_');
}

function assignToObject (obj, key) {
    return Object.assign(obj, { [key]: process.env[key] });
}

function get (target, key) {
    return target[key];
}

function set (target, key, val) {
    validate(key, val, 'set');
    listener.emit(`${key}.set`, val);
    return Object.assign(target, { [key]: val });
}

function validate (key, val, action) {
    if (val === undefined) {
        throw new Error(`Invalid attempt to ${action} "${key}" to undefined.`);
    }
}