Promisfy and bind methods that reference `this`

promisfyMethods.js

const { promisify } = require('util') // node module
/**
 * Creates Promise-returning functions from Callback-taking functions
 * and binds the function to the created object via creation, mapped
 * via methods
 * 
 * @example
 * 
 *      const obj = { get, set, del }
 *      const promisified = promisifyMethods({
 *          creation: () => obj,
 *          methods: [
 *              'get',
 *              // map obj.get to return.get
 *              'set',
 *              ['remove', 'del'],
 *              // map obj.del to return.remove
 *          ]
 *      })
 *      // {
 *      //  get: promisify(obj.get).bind(obj),
 *      //  set: promisify(obj.set).bind(obj),
 *      //  remove: promisify(obj.del).bind(obj)
 *      // }
 * 
 * @param {PromisfyMethodsConfig} config - The configuration
 * @return {Object} 
 */
const promisifyMethods = (config) => {
    const {
        creation,
        methods
    } = config
    const self = creation()
    
    return methods.reduce((a, c) => {
        // If method value is [string, string]
        // map as [outgoing, incoming]
        if (Array.isArray(c)) {
            const [outgoing, incoming] = c
            return Object.assign({}, a, {
                [outgoing]: promisify(self[incoming]).bind(self)
            })
        }

        // method is mapped one -> one
        return Object.assign({}, a, {
            [c]: promisify(self[c]).bind(self)
        })
    }, {})
}